Re: [Tutor] if n == 0 vs if not n
On Tue, Oct 6, 2009 at 2:40 AM, Vern Ceder vce...@canterburyschool.orgwrote: Dave Angel wrote: Now in this case where it is only used as boolean checks which would be the most pythonic way if writing these checks? The shorter version may be preferable, but it doesn't generally give the same results. Without knowing the possible data, these substitutions are not safe. For example, replacing if not n == 0with if n will give different results for values of , [] and so on. It WILL work if you know that n is an int or float, however. DaveA True, I took the OP's statement that they were to be used only as boolean checks to mean that there was no type mixing going on. Personally, I would say that checking a list or string for equality (or lack thereof) with 0 is even less preferable. ;) Otherwise, one would at least prefer if n != 0 to if not n == 0, I would think. Actually, I just realized that not has higher precedence than == so this is really checking if (not n) is equal to 0, not if (n == 0) is (not) True. So essentially not n is evaluated and is turned into a bool, which is then compared to 0, which is the same as False in a boolean context, but n != 0 is comparing n to 0, where n may not be the same type (eg. a string). I'm not sure if there's a situation where this difference matters, but I feel like there might be. Anyone have some counter-examples to n != 0 being the same as not n == 0? ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
On Mon, Oct 5, 2009 at 9:13 PM, Luke Paireepinart rabidpoob...@gmail.com wrote: Actually, I just realized that not has higher precedence than == so this is really checking if (not n) is equal to 0, not if (n == 0) is (not) True. No, not is lower precedence than ==. See http://docs.python.org/reference/expressions.html#summary Anyone have some counter-examples to n != 0 being the same as not n == 0? Well, you can create a pathological class where they are different: In [17]: class funky(object): : def __eq__(self, other): : return True : def __ne__(self, other): : return True In [18]: f = funky() In [19]: f != 0 Out[19]: True In [20]: f == 0 Out[20]: True In [21]: not f == 0 Out[21]: False For a less contrived example, if n is a numpy array, n==0 is also a numpy array but (not n != 0) gives an error: In [1]: from numpy import * In [2]: a = array( [ 0, 10, 20, 30, 40 ] ) In [3]: a== 0 Out[3]: array([ True, False, False, False, False], dtype=bool) In [5]: not a!= 0 --- ValueErrorTraceback (most recent call last) C:\Project\MangoLib\ipython console in module() ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all() Kent ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
Thanks all for the informative discussion. To re-confirm it was mostly for boolean checks like if b == True. wow, as the OP, you must have been surprised to see how far we have taken your (seemingly) simple question. we went from boolean checks to interning! commenting on my previous reply, i was addressing only the boolean results when shortening the comparisons. as i mentioned earlier, every Python object has the concept of a boolean value. zeros and empty containers have False values while all others are True. however, what i did *not* mention is that these (abbreviated) comparisons do not work should you care to distinguish between multiple Python objects/values that have the same boolean value. in other words, if not b will catch False, None, 0, etc. if your application is using 3 values like None (for unset value), False (bad/errror code), and True (correct behavior), then None and False will both cause the if clause to be executed. in other words, if you care about the actual objects, then you need to use either == or is, rather than just checking their boolean outcomes. now on to the difference between == vs. is and interning. == is the object *value* comparison operator while is is the object *identity* comparison operator. although is 0 does work, it's easier to read and less confusing than == 0. also as others have already mentioned, it's not a good idea to rely on the undocumented underlying implementation, and for it to stay consistent. for example, back in the earlier 2.x releases, the interned integer range was (-1, 101). when i worked on the 2nd edition of Core Python, i still had that range in the original manuscript. i then discovered that it changed to (-5, 256)... no one warned me ahead of time, and i was not paying enough attention to the python-dev list! again, the bottom line is to *not* rely on the implementation because it is always subject to change, granted not for numbers like -1, 0, 1, but why confuse your fellow Python coder? is is better suited for constants that are not numbers and that is always only one instance of: None, True, False. best regards, -- wesley - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Python Web Development with Django, Addison Wesley, (c) 2009 http://withdjango.com wesley.j.chun :: wescpy-at-gmail.com python training and technical consulting cyberweb.consulting : silicon valley, ca http://cyberwebconsulting.com ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
On Thu, 2009-10-08 at 12:58 -0700, wesley chun wrote: wow, as the OP, you must have been surprised to see how far we have taken your (seemingly) simple question. Pleasently suprised :-) And I am gratefull to see the heavy weights join in. however, what i did *not* mention is that these (abbreviated) comparisons do not work should you care to distinguish between multiple Python objects/values that have the same boolean value. in other words, if not b will catch False, None, 0, etc. if your application is using 3 values like None (for unset value), False (bad/errror code), and True (correct behavior), then None and False will both cause the if clause to be executed. in other words, if you care about the actual objects, then you need to use either == or is, rather than just checking their boolean outcomes. Good one, I have to remember this as it will very likely bite me someday. I really appriciate all the feedback and good advice! Thanks Sander ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
On Mon, Oct 5, 2009 at 3:37 PM, Sander Sweers sander.swe...@gmail.comwrote: Thanks Wesly/Vern for the replies. On Mon, 2009-10-05 at 21:56 +0200, Luke Paireepinart wrote: if not n == 0 if b == True can be written as if b. However, if not n == 0 can be written as if n != 0 but NOT as if n. The reason why is that 0 is not equivalent to False even though it evaluates to False. So if not n: would be true for n = 0 and for n = and for n = None but if n != 0: would be true for n = and n = None but not n = 0. Ah, have not thought about this one. In this case it checks the return code of a subprocess command which if not zero means build failure. I am leaving these as they are because in my opinion if not returncode == 0 shows clearer what is going on than if returncode. If it's checking the returncode against a value, Vern makes a good point: if returncode != 0 makes a whole lot more sense than if not returncode == 0 Though when dealing with an integer return code, doesn't it make more sense to use the is operator? if returncode is 0: #do something if returncode is not 0: #do something -Wayne ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
Wayne wrote: On Mon, Oct 5, 2009 at 3:37 PM, Sander Sweers sander.swe...@gmail.comwrote: Thanks Wesly/Vern for the replies. On Mon, 2009-10-05 at 21:56 +0200, Luke Paireepinart wrote: if not n == 0 if b == True can be written as if b. However, if not n == 0 can be written as if n != 0 but NOT as if n. The reason why is that 0 is not equivalent to False even though it evaluates to False. So if not n: would be true for n = 0 and for n = and for n = None but if n != 0: would be true for n = and n = None but not n = 0. Ah, have not thought about this one. In this case it checks the return code of a subprocess command which if not zero means build failure. I am leaving these as they are because in my opinion if not returncode == 0 shows clearer what is going on than if returncode. If it's checking the returncode against a value, Vern makes a good point: if returncode != 0 makes a whole lot more sense than if not returncode == 0 Though when dealing with an integer return code, doesn't it make more sense to use the is operator? if returncode is 0: #do something if returncode is not 0: #do something -Wayne No, because you're not assured that all integers that are equal are the same object. Python optimizes that for small integers, but there's no documented range that you can count on it. x = 374 y = 374 print id(x), id(y) 12721272 12721296 x==y True x is y False DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
On Tue, Oct 6, 2009 at 9:58 AM, Dave Angel da...@ieee.org wrote: snip No, because you're not assured that all integers that are equal are the same object. Python optimizes that for small integers, but there's no documented range that you can count on it. But for this specific case - checking a return code against zero, should it still be considered unreliable? The only case that you are looking for correctness is 0 == 0, any other case should evaluate as false, so I guess the question is does python always optimize for zero? Any other optimization is irrelevant, AFAIK. -Wayne ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
On Tue, Oct 6, 2009 at 5:08 PM, Wayne sri...@gmail.com wrote: On Tue, Oct 6, 2009 at 9:58 AM, Dave Angel da...@ieee.org wrote: snip No, because you're not assured that all integers that are equal are the same object. Python optimizes that for small integers, but there's no documented range that you can count on it. But for this specific case - checking a return code against zero, should it still be considered unreliable? The only case that you are looking for correctness is 0 == 0, any other case should evaluate as false, so I guess the question is does python always optimize for zero? Any other optimization is irrelevant, AFAIK. Never rely on optimizations like this being done, or on them not being done. The only save way to code is having your code work in both cases. -- André Engels, andreeng...@gmail.com ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
Thanks all for the informative discussion. To re-confirm it was mostly for boolean checks like if b == True. Greets Sander ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
On Tue, Oct 6, 2009 at 11:08 AM, Wayne sri...@gmail.com wrote: On Tue, Oct 6, 2009 at 9:58 AM, Dave Angel da...@ieee.org wrote: snip No, because you're not assured that all integers that are equal are the same object. Python optimizes that for small integers, but there's no documented range that you can count on it. But for this specific case - checking a return code against zero, should it still be considered unreliable? Yes. It is undocumented, implementation-specific behaviour and you should not depend on it without good reason. In this case, there is no reason at all to prefer if x is 0 to if x == 0. The only case that you are looking for correctness is 0 == 0, any other case should evaluate as false, so I guess the question is does python always optimize for zero? Any other optimization is irrelevant, AFAIK. Define always. In all past and future versions of every implementation of Python? Unlikely and unknowable. In all current versions of CPython? Yes, AFAIK. Kent ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
On Tue, Oct 6, 2009 at 9:04 AM, Wayne sri...@gmail.com wrote: If it's checking the returncode against a value, Vern makes a good point: if returncode != 0 makes a whole lot more sense than if not returncode == 0 Though when dealing with an integer return code, doesn't it make more sense to use the is operator? if returncode is 0: #do something if returncode is not 0: #do something No! Equality and identity are different. CPython does cache small integers but that is an implementation detail that you should not rely on. For large values you cannot presume that equal numbers are also identical. For example: In [1]: a=10 In [2]: b=10 In [3]: a==b Out[3]: True In [4]: a is b Out[4]: False Kent ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
Hi Sander, PEP 8, the Style Guide for Python Code http://www.python.org/dev/peps/pep-0008/ is pretty clear that the shorter version is preferable: if s: if n: if b: if not b: and so on... Cheers, Vern Sander Sweers wrote: Hi Tutors, I am going through someone's python script and I am seeing a lot of the following boolean checks. if not s == if not n == 0 if b == True if not b == True etc.. All of these can be written without the == notation like if n, if s etc. Now in this case where it is only used as boolean checks which would be the most pythonic way if writing these checks? Thanks Sander ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor -- This time for sure! -Bullwinkle J. Moose - Vern Ceder, Director of Technology Canterbury School, 3210 Smith Road, Ft Wayne, IN 46804 vce...@canterburyschool.org; 260-436-0746; FAX: 260-436-5137 ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
Thanks Wesly/Vern for the replies. On Mon, 2009-10-05 at 21:56 +0200, Luke Paireepinart wrote: if not n == 0 if b == True can be written as if b. However, if not n == 0 can be written as if n != 0 but NOT as if n. The reason why is that 0 is not equivalent to False even though it evaluates to False. So if not n: would be true for n = 0 and for n = and for n = None but if n != 0: would be true for n = and n = None but not n = 0. Ah, have not thought about this one. In this case it checks the return code of a subprocess command which if not zero means build failure. I am leaving these as they are because in my opinion if not returncode == 0 shows clearer what is going on than if returncode. Whoever wrote your code probably thinks he knows the types of the variables beforehand so he's just making assumptions as to whether a variable is an int / string / etc. So you can probably safely assume that they're boolean checks, but I take no responsibility if you break something :) When I make the changes and send in a patch it will be reviewed. So any siliness I come up with will be shot down quickly ;-) Thanks Sander ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
I am going through someone's python script and I am seeing a lot of the following boolean checks. if not s == if not n == 0 if b == True if not b == True etc.. All of these can be written without the == notation like if n, if s etc.Now in this case where it is only used as boolean checks which would be the most pythonic way if writing these checks? it would be the same as what you have already described. checking against Boolean literals follows the same logic, i.e., if b, if not b, etc. of course, the reasoning behind what you said and my suggestion is that all Python objects evaluate to some sort of Boolean value. the == 0 and '== ' (and their corresponding nots) aren't necessary because both 0 and have a Boolean False value, as does False. the general rule is that any numeric zero (0, 0.0, 0.0+0.0J, etc.) or empty container (i.e., str, list, tuple, dict, set, etc.), are all False. all other values are True. hope this helps! -- wesley - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Core Python Programming, Prentice Hall, (c)2007,2001 Python Fundamentals, Prentice Hall, (c)2009 http://corepython.com wesley.j.chun :: wescpy-at-gmail.com python training and technical consulting cyberweb.consulting : silicon valley, ca http://cyberwebconsulting.com ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
Vern Ceder wrote: div class=moz-text-flowed style=font-family: -moz-fixedHi Sander, PEP 8, the Style Guide for Python Code http://www.python.org/dev/peps/pep-0008/ is pretty clear that the shorter version is preferable: if s: if n: if b: if not b: and so on... Cheers, Vern Sander Sweers wrote: Hi Tutors, I am going through someone's python script and I am seeing a lot of the following boolean checks. if not s == if not n == 0 if b == True if not b == True etc.. All of these can be written without the == notation like if n, if s etc. Now in this case where it is only used as boolean checks which would be the most pythonic way if writing these checks? Thanks Sander ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor The shorter version may be preferable, but it doesn't generally give the same results. Without knowing the possible data, these substitutions are not safe. For example, replacing if not n == 0with if n will give different results for values of , [] and so on. It WILL work if you know that n is an int or float, however. DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
Dave Angel wrote: Now in this case where it is only used as boolean checks which would be the most pythonic way if writing these checks? The shorter version may be preferable, but it doesn't generally give the same results. Without knowing the possible data, these substitutions are not safe. For example, replacing if not n == 0with if n will give different results for values of , [] and so on. It WILL work if you know that n is an int or float, however. DaveA True, I took the OP's statement that they were to be used only as boolean checks to mean that there was no type mixing going on. Personally, I would say that checking a list or string for equality (or lack thereof) with 0 is even less preferable. ;) Otherwise, one would at least prefer if n != 0 to if not n == 0, I would think. Cheers, Vern -- This time for sure! -Bullwinkle J. Moose - Vern Ceder, Director of Technology Canterbury School, 3210 Smith Road, Ft Wayne, IN 46804 vce...@canterburyschool.org; 260-436-0746; FAX: 260-436-5137 ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] if n == 0 vs if not n
On Mon, Oct 5, 2009 at 9:28 PM, Sander Sweers sander.swe...@gmail.comwrote: Hi Tutors, I am going through someone's python script and I am seeing a lot of the following boolean checks. if not s == if not n == 0 if b == True if not b == True etc.. All of these can be written without the == notation like if n, if s No, they cannot. Some of them can be, others cannot. if b == True can be written as if b. However, if not n == 0 can be written as if n != 0 but NOT as if n. The reason why is that 0 is not equivalent to False even though it evaluates to False. So if not n: would be true for n = 0 and for n = and for n = None but if n != 0: would be true for n = and n = None but not n = 0. The same is true for if not s == Now in this case where it is only used as boolean checks which would be the most pythonic way if writing these checks? If you're sure they're boolean checks, if n: or if not n: is usually how I see it written. Whoever wrote your code probably thinks he knows the types of the variables beforehand so he's just making assumptions as to whether a variable is an int / string / etc. So you can probably safely assume that they're boolean checks, but I take no responsibility if you break something :) ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
