Re: missing 'xor' Boolean operator
greg wrote: Terry Reedy wrote: In Math and Python, abc means ab and bc, not (ab)c or a(bc). != is a comparison operator like , Although Python extends the chaining principle to !=, this is somewhat questionable, because a b and b c implies a c, but a != b and b != c does not imply a != c. I'm not sure I've ever seen a mathematician write a != b != c, but if I did, I would tend to think he meant to say that none of a, b, c are equal to any other. That's not what it means in Python, though. However, == is transitive, and a == b == c is quite common. It would hardly do to have different rules for !=. Either we have a uniform rule for a compare_op b compare_ob c, as we do, or we have several fussy rules that would be hard to remember. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 27, 1:53 am, Delaney, Timothy (Tim) tdela...@avaya.com wrote: Mark Dickinson wrote: Since the 'and' and 'or' already return objects (and objects evaluate to true or false), then 'xor' should behave likewise, IMO. I expect that would be the case if it were ever added to the language. I'm not so sure. Did you ever wonder why the any() and all() functions introduced in 2.5 return a boolean rather than returning one of their arguments? (I did, and I'm still not sure what the answer is.) Consider the case of any() and all() operating on an empty iterable. What type should they return? It is impossible in the case of any() and all() to always return one of the elements due to this edge case. Yes, of course; the alternative implementation I was thinking of was the one that I implemented eons ago for my own pre-2.5 code, where I defined any and all roughly as: any([x1, x2, x3, ...]) - False or x1 or x2 or x3 or ... all([x1, x2, x3, ...]) - True and x1 and x2 and x3 and ... At the time this seemed like the obvious choice, so I was a bit surprised when it was chosen to always return a bool instead in the official versions. Now that I'm older and wise^H^H^H^H, well, maybe just older, the pure bool version seems cleaner and less error-prone, even if it's mildly inconsistent with the behaviour of and and or. Mark -- http://mail.python.org/mailman/listinfo/python-list
RE: missing 'xor' Boolean operator
Mark Dickinson wrote: Since the 'and' and 'or' already return objects (and objects evaluate to true or false), then 'xor' should behave likewise, IMO. I expect that would be the case if it were ever added to the language. I'm not so sure. Did you ever wonder why the any() and all() functions introduced in 2.5 return a boolean rather than returning one of their arguments? (I did, and I'm still not sure what the answer is.) Consider the case of any() and all() operating on an empty iterable. What type should they return? It is impossible in the case of any() and all() to always return one of the elements due to this edge case. Similarly, it is impossible in all cases for a boolean xor to return one of the arguments - if both arguments evaluate to a true (Something) value, xor must return a false (Nothing) result. Tim Delaney -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Terry Reedy wrote: In Math and Python, abc means ab and bc, not (ab)c or a(bc). != is a comparison operator like , Although Python extends the chaining principle to !=, this is somewhat questionable, because a b and b c implies a c, but a != b and b != c does not imply a != c. I'm not sure I've ever seen a mathematician write a != b != c, but if I did, I would tend to think he meant to say that none of a, b, c are equal to any other. That's not what it means in Python, though. -- Greg -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
In article mailman.3164.1247670958.8015.python-l...@python.org, Jean-Michel Pichavant jeanmic...@sequans.com wrote: Christian Heimes wrote: Chris Rebert wrote: Using the xor bitwise operator is also an option: bool(x) ^ bool(y) I prefer something like: bool(a) + bool(b) == 1 It works even for multiple tests (super xor): if bool(a) + bool(b) + bool(c) + bool(d) != 1: raise ValueError(Exactly one of a, b, c and d must be true) Christian While everyone's trying to tell the OP how to workaround the missing xor operator, nobody answered the question why is there no xor operator ?. If the question was Why is there no 'or' operator ?, would because A or B = not(not A and not B) be a proper answer ? No. I think it is because and/or can be extended to be sensible in a context where objects can be used. (What others have expressed as having short-circuit evaluation. So sce indeed is the underlying reason that and/or can be extended sensibly to objects.) Remains whether we need an xor that only works and requires that both operands are booleans. That one we have already! It is called != . (a!=b)!=c and a!=(b!=c) are the same for booleans, so can indeed be expressed a!=b!=c (associativy of xor) JM Groetjes Albert -- -- Albert van der Horst, UTRECHT,THE NETHERLANDS Economic growth -- being exponential -- ultimately falters. alb...@spearc.xs4all.nl =n http://home.hccnet.nl/a.w.m.van.der.horst -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Albert van der Horst wrote: Remains whether we need an xor that only works and requires that both operands are booleans. That one we have already! It is called != . (a!=b)!=c and a!=(b!=c) are the same for booleans, so can indeed be expressed a!=b!=c (associativy of xor) Not in Python a,b,c = True, False, True (a!=b)!=c False a!=(b!=c) False a!=b!=c True (a!=b) and (b!=c) True In Math and Python, abc means ab and bc, not (ab)c or a(bc). != is a comparison operator like , not an arithmetic operator like + (or logical xor). If one has 0/1 or True/False values, we have arithmetic xor already, ^, which works as expected. (a^b)^c False a^b^c False Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 20, 11:34 pm, Ethan Furman et...@stoneleaf.us wrote: Dr. Phillip M. Feldman wrote: Suppose that 'xor' returns the value that is true when only one value is true, and False otherwise. This definition of xor doesn't have the standard associative property, that is, (a xor b) xor c will not necessarily equal a xor (b xor c) To see that this is the case, let a= 1, b= 2, and c= 3. (a xor b) xor c yields 3, while a xor (b xor c) yields 1. So, I'd prefer an xor operator that simply returns True or False. Phillip You are, of course, free to write your version however it makes sense to you and your team. :) Since the 'and' and 'or' already return objects (and objects evaluate to true or false), then 'xor' should behave likewise, IMO. I expect that would be the case if it were ever added to the language. I'm not so sure. Did you ever wonder why the any() and all() functions introduced in 2.5 return a boolean rather than returning one of their arguments? (I did, and I'm still not sure what the answer is.) Mark -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Mark Dickinson wrote: On Jul 20, 11:34 pm, Ethan Furman et...@stoneleaf.us wrote: Dr. Phillip M. Feldman wrote: Suppose that 'xor' returns the value that is true when only one value is true, and False otherwise. This definition of xor doesn't have the standard associative property, that is, (a xor b) xor c will not necessarily equal a xor (b xor c) To see that this is the case, let a= 1, b= 2, and c= 3. (a xor b) xor c yields 3, while a xor (b xor c) yields 1. So, I'd prefer an xor operator that simply returns True or False. Phillip You are, of course, free to write your version however it makes sense to you and your team. :) Since the 'and' and 'or' already return objects (and objects evaluate to true or false), then 'xor' should behave likewise, IMO. I expect that would be the case if it were ever added to the language. I'm not so sure. Did you ever wonder why the any() and all() functions introduced in 2.5 return a boolean rather than returning one of their arguments? (I did, and I'm still not sure what the answer is.) Mark Very good question -- I likewise do not know the answer. I will only observe that any() and all() are functions, while 'and' and 'or' are not. If one wanted the object-returning behavior one could string together 'or's or 'and's instead. ~Ethan~ -- Thinking out loud here -- YMMV. :) -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
[fixed for bottom-posting] Dr. Phillip M. Feldman wrote: MRAB-2 wrote: snip What values should 'xor' return? IMHO, if only one of the values is true then it should return that value, otherwise it should return False. 1 xor 0 = 1 0 xor 2 = 2 1 xor 2 = False 0 xor 0 = False This is because it's a Boolean operator, so it should fall back to Boolean values when necessary, like 'not': not 0 = True not 1 = False Also: x and y and z = (x and y) and z x or y or z = (x or y) or z therefore: x xor y xor z = (x xor y) xor z Suppose that 'xor' returns the value that is true when only one value is true, and False otherwise. This definition of xor doesn't have the standard associative property, that is, (a xor b) xor c will not necessarily equal a xor (b xor c) To see that this is the case, let a= 1, b= 2, and c= 3. (a xor b) xor c yields 3, while a xor (b xor c) yields 1. So, I'd prefer an xor operator that simply returns True or False. Phillip You are, of course, free to write your version however it makes sense to you and your team. :) Since the 'and' and 'or' already return objects (and objects evaluate to true or false), then 'xor' should behave likewise, IMO. I expect that would be the case if it were ever added to the language. ~Ethan~ -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 17, 12:06 pm, Jean-Michel Pichavant jeanmic...@sequans.com wrote: I was saying that using boolean operators with object instead of boolean values is error prone, I agree with this to some extent. After all, Python conditional expressions were eventually introduced in response to buggy uses of the 'a and b or c' idiom. See PEP 308, and: http://mail.python.org/pipermail/python-dev/2005-September/056546.html In my own code, I'm finding myself increasingly using conditional expressions where I would once have used 'and' or 'or': daysInAdvance = int(inputVar) if inputVar is not None else 0 -- Mark -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Luis Alberto Zarrabeitia Gomez wrote: Quoting Jean-Michel Pichavant jeanmic...@sequans.com: Emile van Sebille wrote: On 7/16/2009 7:04 AM Unknown said... On 2009-07-16, Emile van Sebille em...@fenx.com wrote: daysInAdvance = int(inputVar) or 25 I don't get it. That doesn't work right when inputVar == 0. Aah, but you didn't get to define right. :) For that particular example 0 is not a valid response. When I was talking about such error prone form of boolean operations, I didn't expect to be right so quickly :p What do you mean by being right so quickly, and error prone in this context? I would also ask Unknown why he believes that int(intputVar) or 25 doesn't work right when inputVar == 0. The only false value that int() may return is zero, so the or 25 clause is there only for that case. I can't see then how you think that is an error. I was saying that using boolean operators with object instead of boolean values is error prone, cause no language behaves he same way, and all behaviors are conventions difficult to figure out without diving deeply into the documentation (or being explained as it happened to me). I think the initialization trick is an error, because I don't want foo(0) to set daysInAdvance to 25. I'll want it to set the attribute to 0, cause 0 is a valid integer. 0 is a valid integer content, None wouldn't be a valid integer content. JM -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Thu, 16 Jul 2009 15:53:45 +0200, Jean-Michel Pichavant wrote: Python has extended the algebra definition of or and and top any type, but it is so unintuitive (I'm no LISP programmer). I disagree. The Something/Nothing dichotomy is so intuitive to me that I would hate to go back to a language that only accepted booleans as arguments to `if`. I think than using the short-circuiting mechanism of bool operators along with the python tricks is just error prone and may result in bug difficult to spot, unless you are very aware of all python boolean mechanisms. In other words, if you don't know how Python behaves, you will make mistakes. Of course you will. That applies to *anything* -- if you don't know how it works, you will make mistakes. Given three result codes, where 0 means no error and an arbitrary non- zero integer means some error, it is simple and easy to write: failed = result_1 or result_2 or result_3 The equivalent: failed = (result_1 != 0) or (result_2 != 0) or (result_3 != 0) # or if you prefer: succeeded = (result_1 == 0) and (result_2 == 0) and (result_3 == 0) are longer and more difficult to read and easier to get wrong. Even worse are tricks like this: failed = (result_1 + result_2 + result_3) != 0 This obscures the fact that the result codes are flags and makes it seem like (flag + flag) is meaningful. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Friday 17 July 2009 07:06:26 am Jean-Michel Pichavant wrote: I was saying that using boolean operators with object instead of boolean values is error prone, cause no language behaves he same way, I don't know of many languages that actively promote the duck typing concept, are as highly dynamic as python, have the metaclass concept, treats almost all language concepts as first class citizens (including functions and classes), and so on. And don't get me started on assignment (which I consider very natural, by the way, but apparently most of the popular languages have pretty unnatural assignments). It is error prone if you are expecting the result to be a bool instead of just behaving like one (it is certainly unexpected, but you shouldn't be expecting to get an instance of certain class anyway, for most of python's operations). And it is confusing if you are reading the int(inputVar) or 25 line and have no idea of what it means (but once you know it, it becomes very readable, almost plain english). and all behaviors are conventions difficult to figure out without diving deeply into the documentation (or being explained as it happened to me). That happens with many new concepts. Welcome to python, I guess... if you are willing to shake some of your expectations from previous programming languages, you will enjoy it. My [little] experience teaching python tells me that duck typing, non-enforced encapsulation and everything is an object are the hardest to accept for the C# folk at my faculty, but once past it, they (the few of them who don't leave the course after that) really enjoy the language. I think the initialization trick is an error, because I don't want foo(0) to set daysInAdvance to 25. I'll want it to set the attribute to 0, cause 0 is a valid integer. 0 is a valid integer content, None wouldn't be a valid integer content. Well, that wouldn't be a problem with or, but with the programmer. The exact same behaviour could be obtained with if int(inputValue) == 0: inputValue = 25 and no or involved. However, using only inputValue = inputValue or 25 could have been an error if you only wanted 25 in case inputValue is None. (the or trick implies certain trust in that the object you have in hand has a reasonable definition of truth value). -- Luis Zarrabeitia (aka Kyrie) Fac. de Matemática y Computación, UH. http://profesores.matcom.uh.cu/~kyrie -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Steven D'Aprano wrote: On Thu, 16 Jul 2009 15:53:45 +0200, Jean-Michel Pichavant wrote: Given three result codes, where 0 means no error and an arbitrary non- zero integer means some error, it is simple and easy to write: failed = result_1 or result_2 or result_3 The equivalent: failed = (result_1 != 0) or (result_2 != 0) or (result_3 != 0) # or if you prefer: succeeded = (result_1 == 0) and (result_2 == 0) and (result_3 == 0) [snip] This is, I guess, where we disagree. I find the second proposal less error prone, and universally understandable unlike the first one. It may be verbose, it may look even lame to some people, but in the end this is perfectly reliable, because you manipulate only False or True within the boolean operations. The first form does not clearly show what is the failed criteria. It just happens by coincidence that in this case the failed criteria matches the Nothingness of result_1, result_2, result_3. What if results may be 'OK' or 'KO'. failed = result_1 or result_2 or result_3 won't work. failed = (result_1 =='KO') or (result_2 =='KO') or (result_3 =='KO') is lame but reliable. JM -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Jean-Michel Pichavant wrote: Steven D'Aprano wrote: On Thu, 16 Jul 2009 15:53:45 +0200, Jean-Michel Pichavant wrote: Given three result codes, where 0 means no error and an arbitrary non- zero integer means some error, it is simple and easy to write: failed = result_1 or result_2 or result_3 The equivalent: failed = (result_1 != 0) or (result_2 != 0) or (result_3 != 0) # or if you prefer: succeeded = (result_1 == 0) and (result_2 == 0) and (result_3 == 0) [snip] This is, I guess, where we disagree. I find the second proposal less error prone, and universally understandable unlike the first one. Careful! The very few (if any) things in this world that can be considered universally understandable do *not* include any programming language! :) It may be verbose, it may look even lame to some people, but in the end this is perfectly reliable, because you manipulate only False or True within the boolean operations. The first form does not clearly show what is the failed criteria. It just happens by coincidence that in this case the failed criteria matches the Nothingness of result_1, result_2, result_3. What if results may be 'OK' or 'KO'. As the programmer, particularly a Python programmer, you should be taking advantage of Python's strengths, of which this is one. If, as you say, some function uses a something value to indicate an undesirable result, then you have to use something akin to your last statement. ~Ethan~ failed = result_1 or result_2 or result_3 won't work. failed = (result_1 =='KO') or (result_2 =='KO') or (result_3 =='KO') is lame but reliable. JM -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Fri, 17 Jul 2009 16:34:57 +0200, Jean-Michel Pichavant wrote:
Steven D'Aprano wrote:
On Thu, 16 Jul 2009 15:53:45 +0200, Jean-Michel Pichavant wrote:
Given three result codes, where 0 means no error and an arbitrary
non- zero integer means some error, it is simple and easy to write:
failed = result_1 or result_2 or result_3
The equivalent:
failed = (result_1 != 0) or (result_2 != 0) or (result_3 != 0) # or if
you prefer:
succeeded = (result_1 == 0) and (result_2 == 0) and (result_3 == 0)
[snip]
This is, I guess, where we disagree. I find the second proposal less
error prone, and universally understandable unlike the first one. It may
be verbose, it may look even lame to some people, but in the end this is
perfectly reliable, because you manipulate only False or True within the
boolean operations.
Because it is verbose, it is more error-prone. The more code you have to
write, the more opportunities you have to make mistakes.
(This holds up to a point, beyond which being more and more terse also
leads to more errors.)
Boolean algebra is notorious for programmer errors. The more complicated
the boolean expression, the more often programmers get it wrong. The more
book-keeping they have to do, the easier it is to get it wrong. All those
(result != 0) comparisons are mere book-keeping, they don't add anything
to the code except to force the flag to be True or False.
The first form does not clearly show what is the failed criteria.
Of course it does: at least one of the three result codes is non-zero. As
a bonus, failed will be assigned the first non-zero result code (if any).
Your preferred form, on the other hand, folds all the possible error
codes into True, throwing away useful information:
result_1 = 0 # success
result_2 = 0
result_3 = 15 # failure
failure = result_1 or result_2 or result_3
failure
15
failure = (result_1 != 0) or (result_2 != 0) or (result_3 != 0)
failure
True
It
just happens by coincidence that in this case the failed criteria
matches the Nothingness of result_1, result_2, result_3. What if results
may be 'OK' or 'KO'.
Obviously the test you write depends on the data you have to work with.
Do you think that's surprising?
failed = result_1 or result_2 or result_3 won't work.
Not in the example you gave, no. Although if I had to work with lots and
lots of strings of the form 'OK' and 'KO', I'd consider sub-classing
string:
class OKString(str):
... def __nonzero__(self):
... if self == 'OK': return True
... elif self == 'KO': return False
... else:
... raise ValueError('invalid result code')
...
a = OKString('OK')
b = OKString('KO')
c = OKString('KO')
if a and b and c:
... print Success!
... else: print Failed!
...
Failed!
(I wouldn't bother if I only needed one or two such tests, but if I had
lots of them, I'd consider it.)
failed = (result_1 =='KO') or (result_2 =='KO') or (result_3 =='KO') is
lame but reliable.
Yes, it's reliably complicated, reliably easy to get wrong, reliably
harder to read, and it reliably throws away information.
But apart from those disadvantages, it does the job you want it to do.
--
Steven
--
http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On 7/17/2009 7:34 AM Jean-Michel Pichavant said... Steven D'Aprano wrote: On Thu, 16 Jul 2009 15:53:45 +0200, Jean-Michel Pichavant wrote: Given three result codes, where 0 means no error and an arbitrary non- zero integer means some error, it is simple and easy to write: failed = result_1 or result_2 or result_3 The equivalent: failed = (result_1 != 0) or (result_2 != 0) or (result_3 != 0) # or if you prefer: succeeded = (result_1 == 0) and (result_2 == 0) and (result_3 == 0) [snip] This is, I guess, where we disagree. I find the second proposal less error prone, and universally understandable unlike the first one. It may be verbose, it may look even lame to some people, but in the end this is perfectly reliable, because you manipulate only False or True within the boolean operations. The first form does not clearly show what is the failed criteria. It just happens by coincidence No -- it happens by design because the premise is 'where 0 means no error and an arbitrary non-zero integer means some error'. that in this case the failed criteria matches the Nothingness of result_1, result_2, result_3. What if results may be 'OK' or 'KO'. Which by definition won't happen for the example cited... failed = result_1 or result_2 or result_3 won't work. ... so you certainly wouldn't write a test that couldn't properly determine success or failure. failed = (result_1 =='KO') or (result_2 =='KO') or (result_3 =='KO') is lame but reliable. In this case I'd write something that meets the specs of the problem your addressing... failed = 'KO' in (result_1,result_2,result_3) Emile -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Suppose that 'xor' returns the value that is true when only one value is true, and False otherwise. This definition of xor doesn't have the standard associative property, that is, (a xor b) xor c will not necessarily equal a xor (b xor c) To see that this is the case, let a= 1, b= 2, and c= 3. (a xor b) xor c yields 3, while a xor (b xor c) yields 1. So, I'd prefer an xor operator that simply returns True or False. Phillip MRAB-2 wrote: snip What values should 'xor' return? IMHO, if only one of the values is true then it should return that value, otherwise it should return False. 1 xor 0 = 1 0 xor 2 = 2 1 xor 2 = False 0 xor 0 = False This is because it's a Boolean operator, so it should fall back to Boolean values when necessary, like 'not': not 0 = True not 1 = False Also: x and y and z = (x and y) and z x or y or z = (x or y) or z therefore: x xor y xor z = (x xor y) xor z -- http://mail.python.org/mailman/listinfo/python-list -- View this message in context: http://www.nabble.com/missing-%27xor%27-Boolean-operator-tp24485116p24543805.html Sent from the Python - python-list mailing list archive at Nabble.com. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Hrvoje Niksic hnik...@x..s.org wrote: Note that in Python A or B is in fact not equivalent to not(not A and not B). De Morgan would turn in his grave. - Hendrik -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Thu, 16 Jul 2009 09:43:53 +0200, Hendrik van Rooyen wrote: Hrvoje Niksic hnik...@x..s.org wrote: Note that in Python A or B is in fact not equivalent to not(not A and not B). De Morgan would turn in his grave. No he wouldn't. Python isn't Boolean algebra, and there is no requirement to limit Python's operators to the semantics of Boolean algebra. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Nobody wrote: On Wed, 15 Jul 2009 21:05:16 +0200, Jean-Michel Pichavant wrote: So if I resume: - not 'foo' = False - 'foo' or 'foo' = 'foo' I may be missing something, but honestly, Guido must have smoked some heavy stuff to write such logic, has he ? Several languages (e.g. Lisp, Bourne shell) behave the same way, i.e. or returns the first element which is considered true while and returns the last element provided that all preceding elements are considered true. [snip] Ok then, why or does not return True, if the first element is considered True ? Why returning the element itself. Any reason for that ? Because it's confusing, maybe people used to that logic find it obvious, but I really don't. JM -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 15, 8:32 pm, Paul Rubin http://phr...@nospam.invalid wrote: Among other things, that uses quadratic time! Why do you want to keep popping items from that list instead of iterating through it anyway? Anyway, I think you wrote something close to this: ... Very true! I didn't think about the problems with pop(). I was using it as a shortcut for pulling off the first operand. I forgot that if you start with an initial operand of False, the result will be the same (0 xor X = X) While I'm not sure how useful it would be, here's a version of the first function that returns one of the operands (ala AND and OR), except in the case where there is an even number of True elements, where it returns False: def xor(*operands): r, rprime = False, False for x in operands: xprime = bool(x) if rprime: if xprime: r, rprime = False, False else: r, rprime = x, xprime return r xor(0, 0) 0 xor(0, 1) 1 xor(1, 0) 1 xor(1, 1) False xor(0, 1, 2) False xor(0, 1, 2, 3) 3 xor(None, []) [] -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On 7/16/2009 2:06 AM Jean-Michel Pichavant said... Ok then, why or does not return True, if the first element is considered True ? Why returning the element itself. Any reason for that ? Because it's confusing, maybe people used to that logic find it obvious, but I really don't. For example, I sometimes use it to set defaults: daysInAdvance = int(inputVar) or 25 Emile -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Emile van Sebille wrote: On 7/16/2009 2:06 AM Jean-Michel Pichavant said... Ok then, why or does not return True, if the first element is considered True ? Why returning the element itself. Any reason for that ? Because it's confusing, maybe people used to that logic find it obvious, but I really don't. For example, I sometimes use it to set defaults: daysInAdvance = int(inputVar) or 25 Emile Sure this looks like an elegant way to set default values and I will use this form , but I'm not sure this justifies by itself the trickery. Python has extended the algebra definition of or and and top any type, but it is so unintuitive (I'm no LISP programmer). I think than using the short-circuiting mechanism of bool operators along with the python tricks is just error prone and may result in bug difficult to spot, unless you are very aware of all python boolean mechanisms. JM -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On 2009-07-16, Emile van Sebille em...@fenx.com wrote: On 7/16/2009 2:06 AM Jean-Michel Pichavant said... Ok then, why or does not return True, if the first element is considered True ? Why returning the element itself. Any reason for that ? Because it's confusing, maybe people used to that logic find it obvious, but I really don't. For example, I sometimes use it to set defaults: daysInAdvance = int(inputVar) or 25 I don't get it. That doesn't work right when inputVar == 0. -- Grant Edwards grante Yow! What I want to find at out is -- do parrots know visi.commuch about Astro-Turf? -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On 7/16/2009 7:04 AM Unknown said... On 2009-07-16, Emile van Sebille em...@fenx.com wrote: daysInAdvance = int(inputVar) or 25 I don't get it. That doesn't work right when inputVar == 0. Aah, but you didn't get to define right. :) For that particular example 0 is not a valid response. Emile -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Emile van Sebille wrote: On 7/16/2009 7:04 AM Unknown said... On 2009-07-16, Emile van Sebille em...@fenx.com wrote: daysInAdvance = int(inputVar) or 25 I don't get it. That doesn't work right when inputVar == 0. Aah, but you didn't get to define right. :) For that particular example 0 is not a valid response. Emile When I was talking about such error prone form of boolean operations, I didn't expect to be right so quickly :p Steven explained the truth notion with the Something/Nothing. 0 is Something, 0 would be Nothing. I'm not sure it makes sens anyway. I mean, I could easily argue that the number 0 is something. In the end I wonder if I shouldn't just acknowledge the python mechanism without trying to find any intuitive/natural/obvious logic in it, knowing that sometimes the Truth lies far away from the Evidence. JM -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Quoting Jean-Michel Pichavant jeanmic...@sequans.com: Emile van Sebille wrote: On 7/16/2009 7:04 AM Unknown said... On 2009-07-16, Emile van Sebille em...@fenx.com wrote: daysInAdvance = int(inputVar) or 25 I don't get it. That doesn't work right when inputVar == 0. Aah, but you didn't get to define right. :) For that particular example 0 is not a valid response. When I was talking about such error prone form of boolean operations, I didn't expect to be right so quickly :p What do you mean by being right so quickly, and error prone in this context? I would also ask Unknown why he believes that int(intputVar) or 25 doesn't work right when inputVar == 0. The only false value that int() may return is zero, so the or 25 clause is there only for that case. I can't see then how you think that is an error. I'm not sure it makes sens anyway. I mean, I could easily argue that the number 0 is something. In the end I wonder if I shouldn't just acknowledge the python mechanism Then look it another way. The Empty/Nothing is just standard practice, there is nothing in python that forces you to be false if you are empty, or true otherwise. Instead, answer this: why do you need a /boolean/ value? Is there any case where you need to be certain that the object's type is 'bool'? If you think the answer is yes, you may want to get more familiar with the duck typing concept. (That doesn't mean that there are no legitimate cases where duck typing is inappropriate, but that there are many cases where people, specially if they come from statically typed languages, may believe that it is inappropriate when it isn't). In the python world, one should care more about how an object /behaves/ than from what clase it came. If something quacks like a duck, then assume that it is a duck, at least for the quacking part. Most python objects carry a truth value. Sometimes it feels natural (None is false, boolean True and False are true and false, empty containers are expected to be false, 0 and '' are false). Sometimes, it is everything but natural, but that's a problem with the object's implementation (datetime.time comes to mind). So, you shouldn't care if you have a bool instance - it should be enough that it behaves like a bool (though, if you need a bool, you can always construct one). The True or False expression could return Giraffes, as long as Giraffes behave like the bool True in boolean context. If you care about the class of the result, you can ask for its truth value, and if you don't care about it, you can just ignore it, and use it as you would use a bool. And then, if you can return any object as long as it behaves properly, what would be better to return? A new bool? Why not new Giraffe, if they will have the same behaviour? Guido chose to return the a value that will say more about the result of the operation than just a boolean. It acts as a boolean - if you don't need anything else, treat it as such -, but it will be, whenever is possible, one of the objects in the sequence, in case you need more info. without trying to find any intuitive/natural/obvious logic in it, knowing that sometimes the Truth lies far away from the Evidence. Don't do that. Many of python's decisions are very well thought. You may disagree with them, as I do with some, but they are rarely taken lightly. And this is one that I find very agreeable and in line with the rest of python's philosophy. -- Luis Zarrabeitia Facultad de Matemática y Computación, UH http://profesores.matcom.uh.cu/~kyrie -- Participe en Universidad 2010, del 8 al 12 de febrero de 2010 La Habana, Cuba http://www.universidad2010.cu -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Thu, 16 Jul 2009 11:06:54 +0200, Jean-Michel Pichavant wrote: So if I resume: - not 'foo' = False - 'foo' or 'foo' = 'foo' I may be missing something, but honestly, Guido must have smoked some heavy stuff to write such logic, has he ? Several languages (e.g. Lisp, Bourne shell) behave the same way, i.e. or returns the first element which is considered true while and returns the last element provided that all preceding elements are considered true. [snip] Ok then, why or does not return True, if the first element is considered True ? If the first element is true, returning the first element is returning true. Why returning the element itself. Any reason for that ? Why not? Where is the benefit in collapsing all true values to True? You can convert values to True/False with bool(), but the conversion cannot be reversed. It only makes a difference if you are interested in the representation rather than the value. Do you normally test for equality with is or ==? -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Wed, 15 Jul 2009 18:14:10 +0200, Hrvoje Niksic wrote: If the question was Why is there no 'or' operator ?, would because A or B = not(not A and not B) be a proper answer ? Note that in Python A or B is in fact not equivalent to not(not A and not B). Ah, but it *is* equivalent; it isn't identical, but that's not the point. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On 7/16/2009 1:29 PM Nobody said... On Wed, 15 Jul 2009 18:14:10 +0200, Hrvoje Niksic wrote: If the question was Why is there no 'or' operator ?, would because A or B = not(not A and not B) be a proper answer ? Note that in Python A or B is in fact not equivalent to not(not A and not B). Ah, but it *is* equivalent; it isn't identical, but that's not the point. I'm not sure I'd call it equivalent. A or B returns either unaltered, and not(not A and not B) always returns a boolean. The equivalent would be not(not( A or B )). Emile -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Thu, 16 Jul 2009 14:59:22 -0700, Emile van Sebille wrote: If the question was Why is there no 'or' operator ?, would because A or B = not(not A and not B) be a proper answer ? Note that in Python A or B is in fact not equivalent to not(not A and not B). Ah, but it *is* equivalent; it isn't identical, but that's not the point. I'm not sure I'd call it equivalent. That depends upon what definition of equivalent you are using. Apart from all manner of vague, informal definitions, there is a formal definition: A relation = is an equivalence relation if the following hold for all x, y, and z: x = x x = y = y = x x = y and y = z = x = z An equivalence relation partitions its domain into equivalence classes, such that an element is = (equivalent) to every element in the same class, and not = to every element in every other class. This is a lesser notion of equality to identity, which also requires that x = y = f(x) = f(y) for all f. Python's notion of boolean values treats x and y as equivalent if bool(x) == bool(y). On this basis, the result of A or B is *equivalent* to the result of not(not A and not B). If you use either result as a boolean value (e.g. the test of an if or while statement, as an operand to and or or, etc), the effect is the same. The results aren't *identical*, as there exist ways to distinguish the two. As for the utility of this behaviour, returning an actual value rather than True/False allows the operators to be used as gates. The term gate in digital logic follows from the axioms: 0 and x = 0 1 and x = x 0 or x = x 1 or x = 1 [0 = False, 1 = True] If you consider the left operand as the control and the right operand as the data, the control determines whether or not the data can pass through the gate to the output (i.e. whether the gate is open or closed). In Python: and: False and 7 = False False and 0 = False True and 7 = 7 True and 0 = 0 or: False or 7 = 7 False or 0 = 0 True or 7 = True True or 0 = True -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Dr. Phillip M. Feldman pfeld...@verizon.net wrote: Here's a related issue: I would like to see an option for type checking on operands of logical operators, so that attempting to apply a logical operator to non-Boolean entities generates a warning message. With operand type checking, 'xor' and != would be different. How would you define Boolean entities? Do you mean the True and False values? Such a change would break virtually every Python program ever written. In any case, this idea is dead in the water. It would break a whole bunch of existing code from before the conditional operator: xxx = testme and truevalue or falsevalue -- Tim Roberts, t...@probo.com Providenza Boekelheide, Inc. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 15, 5:07 am, Dr. Phillip M. Feldman pfeld...@verizon.net wrote: I appreciate the effort that people have made, but I'm not impressed with any of the answers. For one thing, xor should be able to accept an arbitrary number of input arguments (not just two), and should return True if and only if the number of input arguments that evaluate to True is odd. Well, that's not exactly what you originally asked for. But it's still a one-liner: def xor(*args): return bool(sum(map(bool, args)) % 2) or perhaps def xor(*args): return bool(len(filter(None, args)) 1) Here's my code: def xor(*args): xor accepts an arbitrary number of input arguments, returning True if and only if bool() evaluates to True for an odd number of the input arguments. result= False for arg in args: if bool(arg): result= not result It's more idiomatic to say if arg: ... rather than if bool (arg): return result Mark -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Scott David Daniels wrote: Ethan Furman wrote: and returns the last object that is true A little suspect this. _and_ returns the first object that is not true, or the last object. or returns the first object that is true Similarly: _or_ returns the first object that is true, or the last object. Thanks for the correction. ~Ethan~ --Scott David Daniels scott.dani...@acm.org -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 15, 5:44 pm, Mark Dickinson dicki...@gmail.com wrote: On Jul 15, 5:07 am, Dr. Phillip M. Feldman pfeld...@verizon.net wrote: [snip] for arg in args: if bool(arg): result= not result It's more idiomatic to say if arg: ... rather than if bool (arg): Ah yes, but not once conditional tests, (just like logical operators), type-check to ensure they have been supplied with Boolean entities. ;) -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 14, 4:48 pm, Ethan Furman et...@stoneleaf.us wrote: A whole family of supers. :) You should pick up Lisp. The whole concept of a binary operator doesn't exist over there. All the things binary operators can do, Lisp does with 0, 1, 2, or more arguments. [1] (+) 0 [2] (+ 1) 1 [3] (+ 1 2) 3 [4] (+ 1 2 3) 6 Once you get used to that, binary operators don't seem so useful anymore. The equivalent in Python is dropping the operators and replacing them with built-in functions that take 0, 1, 2, or more arguments. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Tue, 14 Jul 2009 11:25:08 -0700, Dr. Phillip M. Feldman wrote: Current Boolean operators are 'and', 'or', and 'not'. It would be nice to have an 'xor' operator as well. I've often wished there was too, for the sake of completeness and aesthetics, I'd love to be able to write: a xor b instead of defining a function xor(a, b). Unfortunately, outside of boolean algebra and simulating electrical circuits, I can't think of any use-cases for an xor operator. Do you have any? -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 15, 12:08 am, Christian Heimes li...@cheimes.de wrote: Chris Rebert wrote: Using the xor bitwise operator is also an option: bool(x) ^ bool(y) I prefer something like: bool(a) + bool(b) == 1 It works even for multiple tests (super xor): if bool(a) + bool(b) + bool(c) + bool(d) != 1: raise ValueError(Exactly one of a, b, c and d must be true) Christian if bool(a) + bool(b) + bool(c) + bool(d) != 1: is not equivalent to xor. 1 xor 1 xor 1 = 1 xor (1 xor 1) = 1 xor 0 = 1 (or = (1 xor 1) xor 1 = 0 xor 1 = 1 if you assicate to the left) -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Wed, 2009-07-15 at 02:02 -0700, Jonathan Gardner wrote: On Jul 14, 4:48 pm, Ethan Furman et...@stoneleaf.us wrote: A whole family of supers. :) All the things binary operators can do, Lisp does with 0, 1, 2, or more arguments. +1 n-ary operators are great, but binary operators are just syntactic sugar for functions (which are obviously a generalisation of an n-ary operator in python). The fact that lisp-like languages don't expose this sugar is good for making you think about what you're actually doing, but just like mathematicians use binary (and unary) operators in equations rather than working in the lambda calculus, having that sugar is useful to python. [1] (+) 0 [2] (+ 1) 1 [3] (+ 1 2) 3 [4] (+ 1 2 3) 6 c.f. : sum([]) 0 sum([1]) 1 sum([1,2]) 3 sum([1,2,3]) 6 Once you get used to that, binary operators don't seem so useful anymore. The equivalent in Python is dropping the operators and replacing them with built-in functions that take 0, 1, 2, or more arguments. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Steven D'Aprano wrote: On Tue, 14 Jul 2009 11:25:08 -0700, Dr. Phillip M. Feldman wrote: Current Boolean operators are 'and', 'or', and 'not'. It would be nice to have an 'xor' operator as well. I've often wished there was too, for the sake of completeness and aesthetics, I'd love to be able to write: a xor b instead of defining a function xor(a, b). Unfortunately, outside of boolean algebra and simulating electrical circuits, I can't think of any use-cases for an xor operator. Do you have any? I was pondering on this yesterday, and the only case I've come across in my code -- and it's reasonably common -- is checking that one and only one of two params has been passed. I have code which wants, say, an id or a name but doesn't want both. It's hardly difficult to write the check even now, but an built-in xor would make it fractionally cleaner. TJG -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
pdpi wrote: On Jul 15, 12:08 am, Christian Heimes li...@cheimes.de wrote: Chris Rebert wrote: Using the xor bitwise operator is also an option: bool(x) ^ bool(y) I prefer something like: bool(a) + bool(b) == 1 It works even for multiple tests (super xor): if bool(a) + bool(b) + bool(c) + bool(d) != 1: raise ValueError(Exactly one of a, b, c and d must be true) Christian if bool(a) + bool(b) + bool(c) + bool(d) != 1: is not equivalent to xor. 1 xor 1 xor 1 = 1 xor (1 xor 1) = 1 xor 0 = 1 (or = (1 xor 1) xor 1 = 0 xor 1 = 1 if you assicate to the left) I'm well aware of the fact that I've described something differently. 'xor' can be explained as 'check if exactly one element of two elements is true'. My algorithms describes a super xor, aka 'check if exactly one element of n elements is true'. Christian -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 15, 12:37 pm, Christian Heimes li...@cheimes.de wrote: pdpi wrote: On Jul 15, 12:08 am, Christian Heimes li...@cheimes.de wrote: Chris Rebert wrote: Using the xor bitwise operator is also an option: bool(x) ^ bool(y) I prefer something like: bool(a) + bool(b) == 1 It works even for multiple tests (super xor): if bool(a) + bool(b) + bool(c) + bool(d) != 1: raise ValueError(Exactly one of a, b, c and d must be true) Christian if bool(a) + bool(b) + bool(c) + bool(d) != 1: is not equivalent to xor. 1 xor 1 xor 1 = 1 xor (1 xor 1) = 1 xor 0 = 1 (or = (1 xor 1) xor 1 = 0 xor 1 = 1 if you assicate to the left) I'm well aware of the fact that I've described something differently. 'xor' can be explained as 'check if exactly one element of two elements is true'. My algorithms describes a super xor, aka 'check if exactly one element of n elements is true'. Christian Well, I just wouldn't call it a super xor then, when unicity test works much better -- especially as an alternative to an actual xor without any specification to the actual intended functionality except the exception text. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
MRAB pyt...@mrabarnett.plus.com wrote in message news:mailman.3158.1247667680.8015.python-l...@python.org... Steven D'Aprano wrote: On Tue, 14 Jul 2009 11:25:08 -0700, Dr. Phillip M. Feldman wrote: Current Boolean operators are 'and', 'or', and 'not'. It would be nice to have an 'xor' operator as well. I've often wished there was too, for the sake of completeness and aesthetics, I'd love to be able to write: a xor b instead of defining a function xor(a, b). Unfortunately, outside of boolean algebra and simulating electrical circuits, I can't think of any use-cases for an xor operator. Do you have any? The problem is that 'and' and 'or' are not limited to Boolean values: 'and' returns the first false value or the last true value. 'or' returns the first true value or the last false value. What values should 'xor' return? IMHO, if only one of the values is true then it should return that value, otherwise it should return False. 1 xor 0 = 1 0 xor 2 = 2 1 xor 2 = False 0 xor 0 = False This is because it's a Boolean operator, so it should fall back to Boolean values when necessary, like 'not': not 0 = True not 1 = False Also: x and y and z = (x and y) and z x or y or z = (x or y) or z therefore: x xor y xor z = (x xor y) xor z Gosh, let's all discuss commutation and distribution. And surely in quantum merchanics there is something about non-commuting operatiomns letting in Planck's constant. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Christian Heimes wrote: Chris Rebert wrote: Using the xor bitwise operator is also an option: bool(x) ^ bool(y) I prefer something like: bool(a) + bool(b) == 1 It works even for multiple tests (super xor): if bool(a) + bool(b) + bool(c) + bool(d) != 1: raise ValueError(Exactly one of a, b, c and d must be true) Christian While everyone's trying to tell the OP how to workaround the missing xor operator, nobody answered the question why is there no xor operator ?. If the question was Why is there no 'or' operator ?, would because A or B = not(not A and not B) be a proper answer ? JM -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On 2009-07-15 10:15, Jean-Michel Pichavant wrote: Christian Heimes wrote: Chris Rebert wrote: Using the xor bitwise operator is also an option: bool(x) ^ bool(y) I prefer something like: bool(a) + bool(b) == 1 It works even for multiple tests (super xor): if bool(a) + bool(b) + bool(c) + bool(d) != 1: raise ValueError(Exactly one of a, b, c and d must be true) Christian While everyone's trying to tell the OP how to workaround the missing xor operator, nobody answered the question why is there no xor operator ?. If the question was Why is there no 'or' operator ?, would because A or B = not(not A and not B) be a proper answer ? That's not the only answer the OP has been given. The most compelling answer is that an xor keyword cannot be implemented with similar semantics to and and or, in particular short-circuiting and returning one of the actual inputs instead of a coerced bool. -- Robert Kern I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth. -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Jean-Michel Pichavant jeanmic...@sequans.com writes: While everyone's trying to tell the OP how to workaround the missing xor operator, nobody answered the question why is there no [boolean] xor operator ?. Probably because there isn't one in C. The bitwise XOR operator, on the other hand, exists in both C and Python. If the question was Why is there no 'or' operator ?, would because A or B = not(not A and not B) be a proper answer ? Note that in Python A or B is in fact not equivalent to not(not A and not B). -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Hrvoje Niksic hnik...@xemacs.org writes: While everyone's trying to tell the OP how to workaround the missing xor operator, nobody answered the question why is there no [boolean] xor operator ?. Probably because there isn't one in C. The bitwise XOR operator, on the other hand, exists in both C and Python. A non-bitwise XOR really sounds almost useless. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Wed, 15 Jul 2009 13:37:22 +0200, Christian Heimes li...@cheimes.de wrote: pdpi wrote: On Jul 15, 12:08 am, Christian Heimes li...@cheimes.de wrote: Chris Rebert wrote: Using the xor bitwise operator is also an option: bool(x) ^ bool(y) I prefer something like: bool(a) + bool(b) == 1 It works even for multiple tests (super xor): if bool(a) + bool(b) + bool(c) + bool(d) != 1: raise ValueError(Exactly one of a, b, c and d must be true) Christian if bool(a) + bool(b) + bool(c) + bool(d) != 1: is not equivalent to xor. 1 xor 1 xor 1 = 1 xor (1 xor 1) = 1 xor 0 = 1 (or = (1 xor 1) xor 1 = 0 xor 1 = 1 if you assicate to the left) I'm well aware of the fact that I've described something differently. 'xor' can be explained as 'check if exactly one element of two elements is true'. My algorithms describes a super xor, aka 'check if exactly one element of n elements is true'. But that's not a super xor (commonly known as XOR). Rather than describing xor as: check if exactly one element of two elements is true describe it as: check if an odd number of two elements is true then you'll get the correct definition of super xor: check if an odd number of n elements is true I.e., XOR determines parity, and: XOR some binary vector v = 0 if v has an even number of 1s and 1 if v has an odd number of 1s wayne Christian -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Hrvoje Niksic wrote: [snip] Note that in Python A or B is in fact not equivalent to not(not A and not B). l = [(True, True), (True, False), (False, True), (False, False)] for p in l: ... p[0] or p[1] ... True True True False for p in l: ... not(not p[0] and not p[1]) ... True True True False Did I make twice the same obvious error ? JM -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On 7/15/2009 10:43 AM Jean-Michel Pichavant said... Hrvoje Niksic wrote: [snip] Note that in Python A or B is in fact not equivalent to not(not A and not B). l = [(True, True), (True, False), (False, True), (False, False)] for p in l: ... p[0] or p[1] ... True True True False for p in l: ... not(not p[0] and not p[1]) ... True True True False Did I make twice the same obvious error ? No -- but in the not(not... example it doesn't short-circuit. Emile -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 15, 2009, at 1:43 PM, Jean-Michel Pichavant wrote:
Hrvoje Niksic wrote:
[snip]
Note that in Python A or B is in fact not equivalent to not(not A and
not B).
l = [(True, True), (True, False), (False, True), (False, False)]
for p in l:
... p[0] or p[1]
[snip]
Did I make twice the same obvious error ?
Try again with:
l = [('foo','bar'), ('foo', ''), ('', 'bar'), ('', '')]
-Miles
--
http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 15, 2009, at 1:55 PM, Emile van Sebille wrote: On 7/15/2009 10:43 AM Jean-Michel Pichavant said... Hrvoje Niksic wrote: [snip] Note that in Python A or B is in fact not equivalent to not(not A and not B). Did I make twice the same obvious error ? No -- but in the not(not... example it doesn't short-circuit. No; like 'A or B', 'not (not A and not B)' does in fact short-circuit if A is True. (The 'and' condition does not have to evaluate the right operand when 'not A' is False.) -Miles -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Tim Golden wrote: I was pondering on this yesterday, and the only case I've come across in my code -- and it's reasonably common -- is checking that one and only one of two params has been passed. I have code which wants, say, an id or a name but doesn't want both. It's hardly difficult to write the check even now, but an built-in xor would make it fractionally cleaner. I think I would just have one parameter id_name or identifier so no check is needed. This is a common idiom in Python where names are not typed. Example: param 'source' is a string (with a file name to be opened) or an already opened file. If you want two local vars inside the function, that should be kept private to the function. tjr -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
I did initially ask for an infix xor operator, but eventually gave up on this. I like the first of your two one-line solutions below; this is clean and easy to understand. Thanks! I'd still like to be able to write an expression like '(a and b) xor (c and d) xor (e and f)', but it looks as though that can't be done. snip Well, that's not exactly what you originally asked for. But it's still a one-liner: def xor(*args): return bool(sum(map(bool, args)) % 2) or perhaps def xor(*args): return bool(len(filter(None, args)) 1) -- View this message in context: http://www.nabble.com/missing-%27xor%27-Boolean-operator-tp24485116p24503248.html Sent from the Python - python-list mailing list archive at Nabble.com. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Tue, 14 Jul 2009 11:47:41 -0700 (PDT), Mark Dickinson dicki...@gmail.com wrote: On Jul 14, 7:25 pm, Dr. Phillip M. Feldman pfeld...@verizon.net wrote: Current Boolean operators are 'and', 'or', and 'not'. It would be nice to have an 'xor' operator as well. Hmm. I don't think 'nice' is sufficient. You'd need to make the case that it's sufficiently useful to justify adding a new keyword 'xor' to the language; I suspect that would be an uphill struggle. :) I'll just note that: (1) It's easy to emulate xor: 'x xor y' - bool(x) != bool(y) (2) 'and' and 'or' are special in that they have useful short- circuiting behaviour; xor doesn't have this property (that is, you always need to evaluate *both* operands to determine the result). I'd also guess that 'xor' would be much less used than 'and' or 'or', but maybe that's just a reflection of the sort of code that I tend to write. You're right about that!. It's used everywhere in: - coding and encryption theory (and practice) (e.g., http://www.mathcs.emory.edu/~whalen/Hash/Hash_Articles/IEEE/XOR-based%20hash%20functions.pdf) - emulation and simulation of hardware (since all but the most trivial logic circuits are likely to include XOR-gates) - hence, for design of new architectures or simulators or virtual machines and simplification of existing ones--(e.g., http://www.date-conference.com/archive/conference/proceedings/PAPERS/1999/DATE99/PDFFILES/05A_6.PDF) and http://bochs.sourceforge.net/Virtualization_Without_Hardware_Final.pdf which includes: The answer relies on the observation that subtracting an integer value from 0x gives the same result as XOR-ing that same value to 0x. And, perhaps the most useful use of all, for Bouton's solution of the game of Nim--both for the proof that his strategy solves the game and for an easy implementation of a Nim-playing program--and the only operator needed is XOR (e.g., http://www.wordiq.com/definition/Nim). wayne -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On 2009-07-15 13:29, Wayne Brehaut wrote: On Tue, 14 Jul 2009 11:47:41 -0700 (PDT), Mark Dickinson dicki...@gmail.com wrote: On Jul 14, 7:25 pm, Dr. Phillip M. Feldmanpfeld...@verizon.net wrote: Current Boolean operators are 'and', 'or', and 'not'. It would be nice to have an 'xor' operator as well. Hmm. I don't think 'nice' is sufficient. You'd need to make the case that it's sufficiently useful to justify adding a new keyword 'xor' to the language; I suspect that would be an uphill struggle. :) I'll just note that: (1) It's easy to emulate xor: 'x xor y'- bool(x) != bool(y) (2) 'and' and 'or' are special in that they have useful short- circuiting behaviour; xor doesn't have this property (that is, you always need to evaluate *both* operands to determine the result). I'd also guess that 'xor' would be much less used than 'and' or 'or', but maybe that's just a reflection of the sort of code that I tend to write. You're right about that!. It's used everywhere in: - coding and encryption theory (and practice) (e.g., http://www.mathcs.emory.edu/~whalen/Hash/Hash_Articles/IEEE/XOR-based%20hash%20functions.pdf) - emulation and simulation of hardware (since all but the most trivial logic circuits are likely to include XOR-gates) - hence, for design of new architectures or simulators or virtual machines and simplification of existing ones--(e.g., http://www.date-conference.com/archive/conference/proceedings/PAPERS/1999/DATE99/PDFFILES/05A_6.PDF) and http://bochs.sourceforge.net/Virtualization_Without_Hardware_Final.pdf which includes: The answer relies on the observation that subtracting an integer value from 0x gives the same result as XOR-ing that same value to 0x. And, perhaps the most useful use of all, for Bouton's solution of the game of Nim--both for the proof that his strategy solves the game and for an easy implementation of a Nim-playing program--and the only operator needed is XOR (e.g., http://www.wordiq.com/definition/Nim). All of those can use the bitwise XOR operator, not the boolean XOR. Python already has the ^ operator for those purposes. -- Robert Kern I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth. -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 15, 7:29 pm, Wayne Brehaut wbreh...@mcsnet.ca wrote:
On Tue, 14 Jul 2009 11:47:41 -0700 (PDT), Mark Dickinson dicki...@gmail.com
wrote:
I'd also guess that 'xor' would be much less used than 'and' or 'or',
but maybe that's just a reflection of the sort of code that I tend to
write.
You're right about that!. It's used everywhere in:
[snip examples and references]
Those examples are (almost) all about the *bitwise* xor operator,
which exists in Python ('^') and, as you point out, has no shortage of
good uses. The discussion was about a *logical* xor, to parallel the
existing 'and' and 'or' operators.
--
Mark
--
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Re: missing 'xor' Boolean operator
On Jul 14, 2:25 pm, Dr. Phillip M. Feldman pfeld...@verizon.net wrote: Current Boolean operators are 'and', 'or', and 'not'. It would be nice to have an 'xor' operator as well. My $0.02 on this discussion: There would be nothing gained by having non-bitwise XOR operator. You can't short-circuit XOR, because you must evaluate all operands to produce a result. Consequently, returning the true item also doesn't make much sense. XOR is closer to the the logical NOT operator than it is to AND or OR. Here's my own take on a function that can handle any number of arguments (it should probably raise an exception for 0 or 1 arguments, but I'm lazy): def xor(*operands): if operands: operands = list(operands) a = bool(operands.pop(0)) while operands: b = bool(operands.pop(0)) if a: if b: a = False elif b: a = True return a return False -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Miles Kaufmann wrote:
On Jul 15, 2009, at 1:43 PM, Jean-Michel Pichavant wrote:
Hrvoje Niksic wrote:
[snip]
Note that in Python A or B is in fact not equivalent to not(not A and
not B).
l = [(True, True), (True, False), (False, True), (False, False)]
for p in l:
... p[0] or p[1]
[snip]
Did I make twice the same obvious error ?
Try again with:
l = [('foo','bar'), ('foo', ''), ('', 'bar'), ('', '')]
-Miles
Didn't know that.
So if I resume:
- not 'foo' = False
- 'foo' or 'foo' = 'foo'
I may be missing something, but honestly, Guido must have smoked some
heavy stuff to write such logic, has he ?
Let's play again
False or 'foo' = 'foo'
False and 'foo' = False
So funny. I would have expected boolean operators to return a boolean
value. I should have read the f*** manual (this is an anticipated RTFM
counter-attack). Anyway Miles, thanks for the update.
JM
--
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Re: missing 'xor' Boolean operator
On Wed, 15 Jul 2009 11:51:44 -0700 (PDT), Mark Dickinson
dicki...@gmail.com wrote:
On Jul 15, 7:29 pm, Wayne Brehaut wbreh...@mcsnet.ca wrote:
On Tue, 14 Jul 2009 11:47:41 -0700 (PDT), Mark Dickinson
dicki...@gmail.com wrote:
I'd also guess that 'xor' would be much less used than 'and' or 'or',
but maybe that's just a reflection of the sort of code that I tend to
write.
You're right about that!. It's used everywhere in:
[snip examples and references]
Those examples are (almost) all about the *bitwise* xor operator,
which exists in Python ('^') and, as you point out, has no shortage of
good uses. The discussion was about a *logical* xor, to parallel the
existing 'and' and 'or' operators.
I thought you were saying your program domains didn't include a lot of
requirements for xor in general, rather than just no uses for Boolean
xor--and I'm used to thinking of xor on binary vectors as Boolean
anyway so would still have been confused.
The most common non-binary-bit-wise xor is the general symmetric
difference of sets, most likely to be useful in list or dictionary
processing or database-like contexts. Please see my suggested use-case
for Steven below.
wayne
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Re: missing 'xor' Boolean operator
Jean-Michel Pichavant jeanmic...@sequans.com writes: Hrvoje Niksic wrote: [snip] Note that in Python A or B is in fact not equivalent to not(not A and not B). l = [(True, True), (True, False), (False, True), (False, False)] for p in l: ... p[0] or p[1] [...] Try with a different data set, for example: 10 or 20 10 not(not 10 and not 20) True -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Tue, 14 Jul 2009 11:47:41 -0700 (PDT), Mark Dickinson dicki...@gmail.com wrote: On Jul 14, 7:25 pm, Dr. Phillip M. Feldman pfeld...@verizon.net wrote: Current Boolean operators are 'and', 'or', and 'not'. It would be nice to have an 'xor' operator as well. Hmm. I don't think 'nice' is sufficient. You'd need to make the case that it's sufficiently useful to justify adding a new keyword 'xor' to the language; I suspect that would be an uphill struggle. :) === 8 === And for the objects for which it *is* sufficiently useful (sets) the xor operator ^ is available: cheese = set(['cheddar', 'limburger', 'stilton']) stinky = set(['skunk', 'limburger', 'stilton', 'polecat', 'doggy-doo', 'civet']) nasty = set(['doggy-doo', 'polecat', 'limburger', 'Perl']) cheese stinky # stinky cheese set(['limburger', 'stilton']) cheese ^ stinky # either cheese or stinky but not both set(['doggy-doo', 'civet', 'polecat', 'skunk', 'cheddar']) cheese ^ stinky ^ nasty # in an odd number of these sets (1 or 3) set(['civet', 'cheddar', 'Perl', 'limburger', 'skunk']) wayne -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On 15 Jul 2009 09:11:44 GMT, Steven D'Aprano ste...@remove.this.cybersource.com.au wrote: On Tue, 14 Jul 2009 11:25:08 -0700, Dr. Phillip M. Feldman wrote: Current Boolean operators are 'and', 'or', and 'not'. It would be nice to have an 'xor' operator as well. I've often wished there was too, for the sake of completeness and aesthetics, I'd love to be able to write: a xor b instead of defining a function xor(a, b). Unfortunately, outside of boolean algebra and simulating electrical circuits, I can't think of any use-cases for an xor operator. Do you have any? Since xor in set theory is the symmetric difference, perhaps we'd like to know all items in exactly one of two word lists or dictionaries, or anything else we could easily set-ize: cheese = set(['cheddar', 'limburger', 'stilton']) stinky = set(['skunk', 'limburger', 'stilton', 'polecat', 'doggy-doo', 'civet']) nasty = set(['doggy-doo', 'polecat', 'limburger', 'Perl']) cheese stinky # stinky cheese set(['limburger', 'stilton']) cheese ^ stinky # either cheese or stinky but not both set(['doggy-doo', 'civet', 'polecat', 'skunk', 'cheddar']) cheese ^ stinky ^ nasty # in an odd number of these sets (1 or 3) set(['civet', 'cheddar', 'Perl', 'limburger', 'skunk']) Who hasn't needed that occasionally? wayne -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Wed, Jul 15, 2009 at 3:57 PM, Wayne Brehautwbreh...@mcsnet.ca wrote: On 15 Jul 2009 09:11:44 GMT, Steven D'Aprano ste...@remove.this.cybersource.com.au wrote: On Tue, 14 Jul 2009 11:25:08 -0700, Dr. Phillip M. Feldman wrote: Current Boolean operators are 'and', 'or', and 'not'. It would be nice to have an 'xor' operator as well. I've often wished there was too, for the sake of completeness and aesthetics, I'd love to be able to write: a xor b instead of defining a function xor(a, b). Unfortunately, outside of boolean algebra and simulating electrical circuits, I can't think of any use-cases for an xor operator. Do you have any? Since xor in set theory is the symmetric difference, perhaps we'd like to know all items in exactly one of two word lists or dictionaries, or anything else we could easily set-ize: cheese = set(['cheddar', 'limburger', 'stilton']) stinky = set(['skunk', 'limburger', 'stilton', 'polecat', 'doggy-doo', 'civet']) nasty = set(['doggy-doo', 'polecat', 'limburger', 'Perl']) cheese stinky # stinky cheese set(['limburger', 'stilton']) cheese ^ stinky # either cheese or stinky but not both set(['doggy-doo', 'civet', 'polecat', 'skunk', 'cheddar']) cheese ^ stinky ^ nasty # in an odd number of these sets (1 or 3) set(['civet', 'cheddar', 'Perl', 'limburger', 'skunk']) Who hasn't needed that occasionally? This discussion is about adding a *logical* operator for use in boolean expressions. We obviously already have ^ for non-boolean use. Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Wed, 15 Jul 2009 21:05:16 +0200, Jean-Michel Pichavant wrote: So if I resume: - not 'foo' = False - 'foo' or 'foo' = 'foo' I may be missing something, but honestly, Guido must have smoked some heavy stuff to write such logic, has he ? Several languages (e.g. Lisp, Bourne shell) behave the same way, i.e. or returns the first element which is considered true while and returns the last element provided that all preceding elements are considered true. Let's play again False or 'foo' = 'foo' False and 'foo' = False So funny. I would have expected boolean operators to return a boolean value. In Python, almost everything is a boolean value. Compare with Lisp, where everything is a boolean value: nil (the empty list) is false and everything else (including integer zero) is true. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Anthony Tolle anthony.to...@gmail.com writes: def xor(*operands): if operands: operands = list(operands) a = bool(operands.pop(0)) while operands: b = bool(operands.pop(0)) if a: if b: a = False elif b: a = True return a return False Among other things, that uses quadratic time! Why do you want to keep popping items from that list instead of iterating through it anyway? Anyway, I think you wrote something close to this: def xor(*operands): r = False for x in operands: r = (r != bool(x)) return r or in map-reduce style: from operator import ne def xor(*operands): return reduce(ne, map(bool, operands), False) -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Wed, 15 Jul 2009 21:05:16 +0200, Jean-Michel Pichavant wrote: Didn't know that. So if I resume: - not 'foo' = False - 'foo' or 'foo' = 'foo' I may be missing something, but honestly, Guido must have smoked some heavy stuff to write such logic, has he ? No, it's perfectly reasonable, and not at all the product of mind- altering drugs. Other languages do the same thing. for x in alist or blist or clist: print x will select the first non-empty list and iterate over that. Every object in Python has a truth value. By convention, we say that objects are Something or Nothing. 0 None [] '' are examples of Nothing, or false values. 1 2.5 [x, y, z] 'foo' are examples of Something, or true values. Note the distinction between lower-case false and true (adjectives), and title-case False and True (nouns). `if x: ... else:` branches according to whether x is Something or Nothing, not whether it is True or False. The operators `and` and `or` also return Something or Nothing, short- circuiting as appropriate. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Tue, Jul 14, 2009 at 11:47 AM, Mark Dickinsondicki...@gmail.com wrote: On Jul 14, 7:25 pm, Dr. Phillip M. Feldman pfeld...@verizon.net wrote: Current Boolean operators are 'and', 'or', and 'not'. It would be nice to have an 'xor' operator as well. Hmm. I don't think 'nice' is sufficient. You'd need to make the case that it's sufficiently useful to justify adding a new keyword 'xor' to the language; I suspect that would be an uphill struggle. :) I'll just note that: (1) It's easy to emulate xor: 'x xor y' - bool(x) != bool(y) Using the xor bitwise operator is also an option: bool(x) ^ bool(y) Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Robert Kern wrote: On 2009-07-14 14:56, Dr. Phillip M. Feldman wrote: != does do what I want, except that it doesn't indicate to someone reading the code that the operands are being treated as logicals. (Readability is supposed to be one of the major selling points of Python). But, this is probably good enough. In the words of those greater than myself, Not every one-liner needs to be in the standard library. def xor(a, b): return bool(a) != bool(b) Let's see... and returns the last object that is true or returns the first object that is true so should xor return the only object that is true, else False/None? def xor(a, b) if a and b: return None elif a: return a elif b: return b else: return None ~Ethan~ -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Ethan Furman wrote: Robert Kern wrote: On 2009-07-14 14:56, Dr. Phillip M. Feldman wrote: != does do what I want, except that it doesn't indicate to someone reading the code that the operands are being treated as logicals. (Readability is supposed to be one of the major selling points of Python). But, this is probably good enough. In the words of those greater than myself, Not every one-liner needs to be in the standard library. def xor(a, b): return bool(a) != bool(b) Let's see... and returns the last object that is true or returns the first object that is true so should xor return the only object that is true, else False/None? def xor(a, b) if a and b: return None elif a: return a elif b: return b else: return None How about: def xor(a, b): return not b and a or not a and b -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
MRAB wrote: Ethan Furman wrote: Robert Kern wrote: On 2009-07-14 14:56, Dr. Phillip M. Feldman wrote: != does do what I want, except that it doesn't indicate to someone reading the code that the operands are being treated as logicals. (Readability is supposed to be one of the major selling points of Python). But, this is probably good enough. In the words of those greater than myself, Not every one-liner needs to be in the standard library. def xor(a, b): return bool(a) != bool(b) Let's see... and returns the last object that is true or returns the first object that is true so should xor return the only object that is true, else False/None? def xor(a, b) if a and b: return None elif a: return a elif b: return b else: return None How about: def xor(a, b): return not b and a or not a and b In [12]: not 1 and 0 or 1 and not 0 Out[12]: True In [13]: not 0 and 0 or 0 and not 0 Out[13]: 0 In [14]: not 1 and 1 or 1 and not 1 Out[14]: False In [15]: not [] and [] or [] and not [] Out[15]: [] Doesn't produce consistent objects, sometimes bool, sometimes something else. 'Course, it all depends on what you need. ~Ethan~ -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Chris Rebert wrote: Using the xor bitwise operator is also an option: bool(x) ^ bool(y) I prefer something like: bool(a) + bool(b) == 1 It works even for multiple tests (super xor): if bool(a) + bool(b) + bool(c) + bool(d) != 1: raise ValueError(Exactly one of a, b, c and d must be true) Christian -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Ethan Furman wrote: and returns the last object that is true A little suspect this. _and_ returns the first object that is not true, or the last object. or returns the first object that is true Similarly: _or_ returns the first object that is true, or the last object. so should xor return the only object that is true, else False/None? Xor has the problem that in two cases it can return neither of its args. Not has behavior similar in those cases, and we see it returns False or True. The Pythonic solution is therefore to use False. def xor(a, b) if a and b: return None elif a: return a elif b: return b else: return None def xor(a, b): if bool(a) == bool(b): return False else: return a or b Side-effect counting in applications of bool(x) is ignored here. If minimizing side-effects is needed: def xor(a, b): if a: if not b: return a elif b: return b return False --Scott David Daniels scott.dani...@acm.org -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
Christian Heimes wrote:
Chris Rebert wrote:
Using the xor bitwise operator is also an option:
bool(x) ^ bool(y)
I prefer something like:
bool(a) + bool(b) == 1
It works even for multiple tests (super xor):
if bool(a) + bool(b) + bool(c) + bool(d) != 1:
raise ValueError(Exactly one of a, b, c and d must be true)
Christian
super_xor! I like it!
In [23]: def super_xor(args, failure=False):
: found_one = False
: for item in args:
: if item:
: if found_one:
: return failure
: else:
: found_one = item
: return found_one or failure
In [25]: super_xor((0, 1, 0, []))
Out[25]: 1
In [26]: super_xor((0, 1, 0, [],('this',)))
Out[26]: False
In [27]: super_xor((0, {}, 0, [],()))
Out[27]: False
In [16]: def super_or(args, failure=False):
: for item in args:
: if item:
: return item
: else:
: return failure
:
In [17]: super_or((None, [], 0, 3, {}))
Out[17]: 3
In [18]: super_or((None, [], 0, (), {}))
Out[18]: False
In [19]: def super_and(args, failure=False):
: for item in args:
: if not item:
: return failure
: else:
: return item
:
In [20]: super_and((1, 2, 3))
Out[20]: 3
In [21]: super_and((1, 2, 3, 4))
Out[21]: 4
In [22]: super_and((1, 0, 3, 4))
Out[22]: False
A whole family of supers. :)
~Ethan~
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Re: missing 'xor' Boolean operator
I appreciate the effort that people have made, but I'm not impressed with any of the answers. For one thing, xor should be able to accept an arbitrary number of input arguments (not just two), and should return True if and only if the number of input arguments that evaluate to True is odd (see www.mathworld.com article on xor). Here's my code: def xor(*args): xor accepts an arbitrary number of input arguments, returning True if and only if bool() evaluates to True for an odd number of the input arguments. result= False for arg in args: if bool(arg): result= not result return result MRAB-2 wrote: Ethan Furman wrote: Robert Kern wrote: On 2009-07-14 14:56, Dr. Phillip M. Feldman wrote: != does do what I want, except that it doesn't indicate to someone reading the code that the operands are being treated as logicals. (Readability is supposed to be one of the major selling points of Python). But, this is probably good enough. In the words of those greater than myself, Not every one-liner needs to be in the standard library. def xor(a, b): return bool(a) != bool(b) Let's see... and returns the last object that is true or returns the first object that is true so should xor return the only object that is true, else False/None? def xor(a, b) if a and b: return None elif a: return a elif b: return b else: return None How about: def xor(a, b): return not b and a or not a and b -- http://mail.python.org/mailman/listinfo/python-list -- View this message in context: http://www.nabble.com/missing-%27xor%27-Boolean-operator-tp24485116p24491580.html Sent from the Python - python-list mailing list archive at Nabble.com. -- http://mail.python.org/mailman/listinfo/python-list
Re: missing 'xor' Boolean operator
On Jul 15, 2009, at 12:07 AM, Dr. Phillip M. Feldman wrote: I appreciate the effort that people have made, but I'm not impressed with any of the answers. For one thing, xor should be able to accept an arbitrary number of input arguments (not just two) You originally proposed this in the context of the existing short- circuit boolean operators. Those operators (being infix) take only two operands. and should return True if and only if the number of input arguments that evaluate to True is odd The existing and/or operators always return the value of one of the operands--not necessarily True or False--which is another important property, but one that can't be translated fully to xor. Given the lack of context in your original post, hopefully you'll forgive me being unimpressed by your not being impressed. :) Here's my code: def xor(*args): xor accepts an arbitrary number of input arguments, returning True if and only if bool() evaluates to True for an odd number of the input arguments. result= False for arg in args: if bool(arg): result= not result return result If all you want is a True or False result, I'd write it like this: import operator def xor(*args): return reduce(operator.xor, map(bool, args)) # or imap In order to make it act more like the other logical operators, I'd use MRAB's 2-argument xor as the reducer--though I can't really see the utility. def xor2(a, b): return (not b and a) or (not a and b) def xor(*args): return reduce(xor2, args) You may also find this question interesting: http://stackoverflow.com/questions/432842/ -Miles -- http://mail.python.org/mailman/listinfo/python-list
