Re: C-like assignment expression?
On May 21, 4:38 am, [EMAIL PROTECTED] wrote: Hello, I have an if-elif chain in which I'd like to match a string against several regular expressions. Also I'd like to use the match groups within the respective elif... block. The C-like idiom that I would like to use is this: if (match = my_re1.match(line): # use match elsif (match = my_re2.match(line)): # use match elsif (match = my_re3.match(line)) # use match ...buy this is illegal in python. The other way is to open up an else: block in each level, do the assignment and then the test. This unneccessarily leads to deeper and deeper nesting levels which I find ugly. Just as ugly as first testing against the RE in the elif: clause and then, if it matches, to re-evaluate the RE to access the match groups. Thanks, robert Try this. -- Paul class TestValue(object): Class to support assignment and test in single operation def __init__(self,v=None): self.value = v Add support for quasi-assignment syntax using '' in place of '='. def __lshift__(self,other): self.value = other return bool(self.value) import re tv = TestValue() integer = re.compile(r[-+]?\d+) real = re.compile(r[-+]?\d*\.\d+) word = re.compile(r\w+) for inputValue in (123 abc 3.1.split()): if (tv real.match(inputValue)): print Real, float(tv.value.group()) elif (tv integer.match(inputValue)): print Integer, int(tv.value.group()) elif (tv word.match(inputValue)): print Word, tv.value.group() Prints: Integer 123 Word abc Real 3.1 -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
This version is a bit better, since it follows the convention that '' should return self. class TestValue(object): Class to support assignment and test in single operation def __init__(self,v=None): self.value = v Add support for quasi-assignment syntax using '' in place of '='. def __lshift__(self,other): self.value = other return self def __bool__(self): return bool(self.value) __nonzero__ = __bool__ -- Paul -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
On 2008-05-21, Diez B. Roggisch [EMAIL PROTECTED] wrote: [EMAIL PROTECTED] wrote: On May 21, 1:47 pm, Hrvoje Niksic [EMAIL PROTECTED] wrote: Although that solution is pretty, it is not the canonical solution because it doesn't cover the important case of if bodies needing to access common variables in the enclosing scope. (This will be easier in Python 3 with 'nonlocal', though.) The snippet posted by Diez is IMHO closer to a canonical solution to this FAQ. Hello everybody, thanks for the various answers. I'm actually pretty puzzled because I expected to see some obvious solution that I just hadn't found before. In general I find Python more elegant and syntactically richer than C (that's where I come from), so I didn't expect the solutions to be a lot more verbose and/or ugly (no offense) than the original idea which would have worked if Python's assignment statement would double as expression, as in C. Well, it's a design-decision - and I'm pretty ok with it being a bit verbose here - as it prevents a *great* deal of programming errors that would otherwise happen from accidentally writing a = b where a == b was meant. But that is an error that occurs because of the specific symbols chosen to represent an assignment or equality test. At the time python was first designed, other symbols for assignment were already used for in other languages, like := and - So if preventing errors was the main motivation why not allow an assignment to be an expression but use a symbol for the assignment that would prevent those kind of errors? I find it hard to believe that a design choice like whether or not to have the assignment behave as an expression or not, was decided on the ground of a particulare lexical representation of the assignment symbol. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
[EMAIL PROTECTED] wrote: Hello, I have an if-elif chain in which I'd like to match a string against several regular expressions. Also I'd like to use the match groups within the respective elif... block. The C-like idiom that I would like to use is this: if (match = my_re1.match(line): # use match elsif (match = my_re2.match(line)): # use match elsif (match = my_re3.match(line)) # use match ...buy this is illegal in python. The other way is to open up an else: block in each level, do the assignment and then the test. This unneccessarily leads to deeper and deeper nesting levels which I find ugly. Just as ugly as first testing against the RE in the elif: clause and then, if it matches, to re-evaluate the RE to access the match groups. This might help: --- s = foo class Tester(object): def __call__(self, pattern): self.m = re.match(pattern, s) return self.m is not None def __getattr__(self, name): return getattr(self.m, name) test = Tester() if test(bar): print wrong elif test(foo): print right - Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
[EMAIL PROTECTED] wrote: I have an if-elif chain in which I'd like to match a string against several regular expressions. Also I'd like to use the match groups within the respective elif... block. The C-like idiom that I would like to use is this: if (match = my_re1.match(line): # use match elsif (match = my_re2.match(line)): # use match elsif (match = my_re3.match(line)) # use match ...buy this is illegal in python. The other way is to open up an else: block in each level, do the assignment and then the test. This unneccessarily leads to deeper and deeper nesting levels which I find ugly. How about this (untested) code: for re in (re1, re2, re3): match = re.match(line) if match: # use it This requires that use it means the same for each regular expression though... Uli -- Sator Laser GmbH Geschäftsführer: Thorsten Föcking, Amtsgericht Hamburg HR B62 932 -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
[EMAIL PROTECTED] a écrit : Hello, I have an if-elif chain in which I'd like to match a string against several regular expressions. Also I'd like to use the match groups within the respective elif... block. The C-like idiom that I would like to use is this: if (match = my_re1.match(line): # use match elsif (match = my_re2.match(line)): # use match elsif (match = my_re3.match(line)) # use match ot Isn't it the third or fourth time this very same question pops up here ? Starts to look like a FAQ. /ot The canonical solution is to iterate over a list of expression,function pairs, ie: def use_match1(match): # code here def use_match2(match): # code here def use_match3(match): # code here for exp, func in [ (my_re1, use_match1), (my_re2, use_match2), (my_re3, use_match3) ]: match = exp.match(line) if match: func(match) break The alternate solution is Diez's Test object. HTH -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
Bruno Desthuilliers [EMAIL PROTECTED] writes: The canonical solution is to iterate over a list of expression,function pairs, ie: Although that solution is pretty, it is not the canonical solution because it doesn't cover the important case of if bodies needing to access common variables in the enclosing scope. (This will be easier in Python 3 with 'nonlocal', though.) The snippet posted by Diez is IMHO closer to a canonical solution to this FAQ. -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
On May 21, 1:47 pm, Hrvoje Niksic [EMAIL PROTECTED] wrote: Although that solution is pretty, it is not the canonical solution because it doesn't cover the important case of if bodies needing to access common variables in the enclosing scope. (This will be easier in Python 3 with 'nonlocal', though.) The snippet posted by Diez is IMHO closer to a canonical solution to this FAQ. Hello everybody, thanks for the various answers. I'm actually pretty puzzled because I expected to see some obvious solution that I just hadn't found before. In general I find Python more elegant and syntactically richer than C (that's where I come from), so I didn't expect the solutions to be a lot more verbose and/or ugly (no offense) than the original idea which would have worked if Python's assignment statement would double as expression, as in C. Thanks again, robert PS: Since I'm testing only three REs, and I only need the match results from one of them, I just re-evaluate that one. -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
On May 21, 3:12 pm, [EMAIL PROTECTED] [EMAIL PROTECTED] wrote: On May 21, 1:47 pm, Hrvoje Niksic [EMAIL PROTECTED] wrote: Although that solution is pretty, it is not the canonical solution because it doesn't cover the important case of if bodies needing to access common variables in the enclosing scope. (This will be easier in Python 3 with 'nonlocal', though.) The snippet posted by Diez is IMHO closer to a canonical solution to this FAQ. Hello everybody, thanks for the various answers. I'm actually pretty puzzled because I expected to see some obvious solution that I just hadn't found before. In general I find Python more elegant and syntactically richer than C (that's where I come from), so I didn't expect the solutions to be a lot more verbose and/or ugly (no offense) than the original idea which would have worked if Python's assignment statement would double as expression, as in C. Thanks again, robert PS: Since I'm testing only three REs, and I only need the match results from one of them, I just re-evaluate that one. Is it really a lot to change to have it if my_re1.match(line): match = my_re1.match(line) elseif my_re2.match(line): match = my_re2.match(line) elseif my_re3.match(line): match = my_re3.match(line) ? That reads clearly to me... -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
[EMAIL PROTECTED] wrote: On May 21, 1:47 pm, Hrvoje Niksic [EMAIL PROTECTED] wrote: Although that solution is pretty, it is not the canonical solution because it doesn't cover the important case of if bodies needing to access common variables in the enclosing scope. (This will be easier in Python 3 with 'nonlocal', though.) The snippet posted by Diez is IMHO closer to a canonical solution to this FAQ. Hello everybody, thanks for the various answers. I'm actually pretty puzzled because I expected to see some obvious solution that I just hadn't found before. In general I find Python more elegant and syntactically richer than C (that's where I come from), so I didn't expect the solutions to be a lot more verbose and/or ugly (no offense) than the original idea which would have worked if Python's assignment statement would double as expression, as in C. Well, it's a design-decision - and I'm pretty ok with it being a bit verbose here - as it prevents a *great* deal of programming errors that would otherwise happen from accidentally writing a = b where a == b was meant. One could argue that regular expressions - which seem to be THE case where it bugs people - should offer a standard way that essentially works as my solution - by keeping state around, making series of tests easier. Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
[EMAIL PROTECTED] wrote: On May 21, 3:12 pm, [EMAIL PROTECTED] [EMAIL PROTECTED] wrote: On May 21, 1:47 pm, Hrvoje Niksic [EMAIL PROTECTED] wrote: Although that solution is pretty, it is not the canonical solution because it doesn't cover the important case of if bodies needing to access common variables in the enclosing scope. (This will be easier in Python 3 with 'nonlocal', though.) The snippet posted by Diez is IMHO closer to a canonical solution to this FAQ. Hello everybody, thanks for the various answers. I'm actually pretty puzzled because I expected to see some obvious solution that I just hadn't found before. In general I find Python more elegant and syntactically richer than C (that's where I come from), so I didn't expect the solutions to be a lot more verbose and/or ugly (no offense) than the original idea which would have worked if Python's assignment statement would double as expression, as in C. Thanks again, robert PS: Since I'm testing only three REs, and I only need the match results from one of them, I just re-evaluate that one. Is it really a lot to change to have it if my_re1.match(line): match = my_re1.match(line) elseif my_re2.match(line): match = my_re2.match(line) elseif my_re3.match(line): match = my_re3.match(line) ? That reads clearly to me... And wastes time. regular expressions can become expensive to match - doing it twice might be hurtful. Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
And wastes time. regular expressions can become expensive to match - doing it twice might be hurtful. Diez match = (my_re1.match(line) or my_re2.match(line)) or my_re3.match(line) ? -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
[EMAIL PROTECTED] wrote: And wastes time. regular expressions can become expensive to match - doing it twice might be hurtful. Diez match = (my_re1.match(line) or my_re2.match(line)) or my_re3.match(line) How do you know *which* of the three has matched then? Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
On May 21, 4:09 pm, Diez B. Roggisch [EMAIL PROTECTED] wrote: [EMAIL PROTECTED] wrote: And wastes time. regular expressions can become expensive to match - doing it twice might be hurtful. Diez match = (my_re1.match(line) or my_re2.match(line)) or my_re3.match(line) How do you know *which* of the three has matched then? Diez Depends if the OP wants to know that... -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
one of the few things i miss from C is being able to use assignment in expressions. that's the only thing, really. also there's no switch/case, you have to use a dictionary of functions instead, although i rarely need that, usually i just use elif. [EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED] On May 21, 1:47 pm, Hrvoje Niksic [EMAIL PROTECTED] wrote: Although that solution is pretty, it is not the canonical solution because it doesn't cover the important case of if bodies needing to access common variables in the enclosing scope. (This will be easier in Python 3 with 'nonlocal', though.) The snippet posted by Diez is IMHO closer to a canonical solution to this FAQ. Hello everybody, thanks for the various answers. I'm actually pretty puzzled because I expected to see some obvious solution that I just hadn't found before. In general I find Python more elegant and syntactically richer than C (that's where I come from), so I didn't expect the solutions to be a lot more verbose and/or ugly (no offense) than the original idea which would have worked if Python's assignment statement would double as expression, as in C. Thanks again, robert PS: Since I'm testing only three REs, and I only need the match results from one of them, I just re-evaluate that one. -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
[EMAIL PROTECTED] wrote: On May 21, 4:09 pm, Diez B. Roggisch [EMAIL PROTECTED] wrote: [EMAIL PROTECTED] wrote: And wastes time. regular expressions can become expensive to match - doing it twice might be hurtful. Diez match = (my_re1.match(line) or my_re2.match(line)) or my_re3.match(line) How do you know *which* of the three has matched then? Diez Depends if the OP wants to know that... Well, in *general* one wants that. So as a general-purpose solution this is certainly *not* the way to go. Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
On May 21, 4:57 pm, inhahe [EMAIL PROTECTED] wrote: one of the few things i miss from C is being able to use assignment in expressions. that's the only thing, really. also there's no switch/case, you have to use a dictionary of functions instead, although i rarely need that, usually i just use elif. One thing I hate from C is the assignment in expressions...Forcing myself to write 0 == Something rather than Something == 0 just to make sure I was mistakenly assigning values in statements is annoying, it ruins the ease of reading. I kind of agree with the select:case, but I think a key issue is how to implement it. Elif is reasonable for now. Diez, true I guess, but then we haven't seen what these expressions are, and why there has to be three. -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
On May 21, 9:09 am, Diez B. Roggisch [EMAIL PROTECTED] wrote:
[EMAIL PROTECTED] wrote:
And wastes time. regular expressions can become expensive to match -
doing it twice might be hurtful.
Diez
match = (my_re1.match(line) or my_re2.match(line)) or
my_re3.match(line)
How do you know *which* of the three has matched then?
Diez
Since the OP is processing alternative regexp's, he may be drifting
close to parsing territory. Here is a pyparsing example showing how
to match one of 'n' alternatives, and then apply functional logic
depending on which expression was matched. This example actually
shows 4 ways to address this question (in roughly increasing order of
OO-ness):
- explicit if/elif/... test on the name of the specific match to see
which alternative matched
- more Pythonic dispatch using a dict of names and corresponding
expression processing functions
- parse-time processing using pyparsing parse actions
- parse actions invoke class constructors, to return expression-
specific objects
-- Paul
from pyparsing import oneOf, Combine, Optional, Word, \
alphas, nums, alphanums, QuotedString
# define basic expressions
sign = oneOf(+ -)
integer = Combine(Optional(sign) + Word(nums))
real = Combine(Optional(sign) + Word(nums) + . +
Optional(Word(nums)))
name = Word(alphas,alphanums) | QuotedString(')
# define alternates with results names
item = real(real) | integer(integer) | name(name)
print \nUse results names to determine which pattern matched
for it in item.searchString(abc 123 -3.14 'phase of the moon'):
if it.getName() == real:
print Real:, (float(it[0]))
elif it.getName() == integer:
print Int:, (int(it[0]))
else:
print String:, it[0]
print \nUse dict to dispatch to type-specific functions \
- more Pythonically canonical
def processRealItem(it):
print Real:, (float(it[0]))
def processIntegerItem(it):
print Int:, (int(it[0]))
def processNameItem(it):
print String:, it[0]
for it in item.searchString(abc 123 -3.14 'phase of the moon'):
{real: processRealItem,
integer : processIntegerItem,
name: processNameItem }[it.getName()](it)
print \nMove expression-specific logic into parse-time parse actions
def convertInt(t):
return int(t[0])
def convertReal(t):
return float(t[0])
integer.setParseAction(convertInt)
real.setParseAction(convertReal)
item = real(real) | integer(integer) | name(name)
for it in item.searchString(abc 123 -3.14 'phase of the moon'):
print %s: %s %s % (it.getName(), it[0], type(it[0]))
print \nUse class constructors as parse-time parse actions \
- results names no longer needed
class IntObject(object):
def __init__(self,t):
self.type, self.value = Int, int(t[0])
class RealObject(object):
def __init__(self,t):
self.type, self.value = Real, float(t[0])
class StringObject(object):
def __init__(self,t):
self.type, self.value = String, t[0]
integer.setParseAction(IntObject)
real.setParseAction(RealObject)
name.setParseAction(StringObject)
item = real | integer | name
for it in item.searchString(abc 123 -3.14 'phase of the moon'):
print %s: %s (%s) % (it[0].type, it[0].value,
it[0].__class__.__name__)
Prints:
Use results names to determine which pattern matched
String: abc
Int: 123
Real: -3.14
String: phase of the moon
Use dict to dispatch to type-specific functions - more Pythonically
canonical
String: abc
Int: 123
Real: -3.14
String: phase of the moon
Move expression-specific logic into parse-time parse actions
name: abc type 'str'
integer: 123 type 'int'
real: -3.14 type 'float'
name: phase of the moon type 'str'
Use class constructors as parse-time parse actions - results names no
longer needed
String: abc (StringObject)
Int: 123 (IntObject)
Real: -3.14 (RealObject)
String: phase of the moon (StringObject)
--
http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
On 21 Mai, 11:38, [EMAIL PROTECTED] wrote: Hello, I have an if-elif chain in which I'd like to match a string against several regular expressions. Also I'd like to use the match groups within the respective elif... block. The C-like idiom that I would like to use is this: if (match = my_re1.match(line): # use match elsif (match = my_re2.match(line)): # use match elsif (match = my_re3.match(line)) # use match ...buy this is illegal in python. The other way is to open up an else: block in each level, do the assignment and then the test. This unneccessarily leads to deeper and deeper nesting levels which I find ugly. Just as ugly as first testing against the RE in the elif: clause and then, if it matches, to re-evaluate the RE to access the match groups. Thanks, robert You are perfectly correct. Pythons design is lacking here IMO. But what is your question? -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
[EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED] On May 21, 4:57 pm, inhahe [EMAIL PROTECTED] wrote: one of the few things i miss from C is being able to use assignment in expressions. that's the only thing, really. also there's no switch/case, you have to use a dictionary of functions instead, although i rarely need that, usually i just use elif. One thing I hate from C is the assignment in expressions...Forcing myself to write 0 == Something rather than Something == 0 interesting trick, i've never thought of that/seen it although if Python implemented it I think it should default to giving warnings when you use = in an expression, that way you don't have to worry. just to make sure I was mistakenly assigning values in statements is annoying, it ruins the ease of reading. I kind of agree with the select:case, but I think a key issue is how to implement it. Elif is reasonable for now. Diez, true I guess, but then we haven't seen what these expressions are, and why there has to be three. -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
One thing I hate from C is the assignment in expressions...Forcing myself to write 0 == Something rather than Something == 0 interesting trick, i've never thought of that/seen it although if Python implemented it I think it should default to giving warnings when you use = in an expression, that way you don't have to worry. That introduces complications though, do you want to see a pagefull of warnings every time you import a module that uses the ='s? You could specify in your python file that you want to suppress that warning, but then you'd never know when you used = by accident when you meant to use ==. anyway i was thinking you could have a second assignment operator to use just in expressions, and only allow that. it could be := since some languages tend to use that. i wouldn't like it as a general assignment operator but assignment in expressions is a special case. also - or -. C uses - for functions but I think math/calculators use that for assignment. -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
On May 21, 5:50 pm, inhahe [EMAIL PROTECTED] wrote: One thing I hate from C is the assignment in expressions...Forcing myself to write 0 == Something rather than Something == 0 interesting trick, i've never thought of that/seen it although if Python implemented it I think it should default to giving warnings when you use = in an expression, that way you don't have to worry. That introduces complications though, do you want to see a pagefull of warnings every time you import a module that uses the ='s? You could specify in your python file that you want to suppress that warning, but then you'd never know when you used = by accident when you meant to use ==. anyway i was thinking you could have a second assignment operator to use just in expressions, and only allow that. it could be := since some languages tend to use that. i wouldn't like it as a general assignment operator but assignment in expressions is a special case. also - or -. C uses - for functions but I think math/calculators use that for assignment. My preference would be ?=. if match ?= my_re1.match(line): # use match elif match ?= my_re2.match(line): # use match elif match ?= my_re3.match(line): # use match -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
On May 21, 11:38 am, [EMAIL PROTECTED] wrote: if (match = my_re1.match(line): # use match elsif (match = my_re2.match(line)): # use match elsif (match = my_re3.match(line)) # use match ...buy this is illegal in python. Assignment expressions is disallowed in Python to protect against a very common bug in C/C++ programs, the (accidental) confusion of if (match = my_re1.match(line)) with if (match == my_re1.match(line)) or vice versa. -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
On May 21, 10:38 am, [EMAIL PROTECTED] wrote: Hello, I have an if-elif chain in which I'd like to match a string against several regular expressions. Also I'd like to use the match groups within the respective elif... block. The C-like idiom that I would like to use is this: if (match = my_re1.match(line): # use match elsif (match = my_re2.match(line)): # use match elsif (match = my_re3.match(line)) # use match ...buy this is illegal in python. The other way is to open up an else: block in each level, do the assignment and then the test. This unneccessarily leads to deeper and deeper nesting levels which I find ugly. Just as ugly as first testing against the RE in the elif: clause and then, if it matches, to re-evaluate the RE to access the match groups. Thanks, robert You could use named groups to search for all three patterns at once like this: original: prog1 = re.compile(r'pat1') prog2 = re.compile(r'pat2') prog3 = re.compile(r'pat3') ... Becomes: prog = re.compile(r'(?Pp1pat1)|(?Pp2pat2)|(?Pp3pat3)') match = prog.match(line) for p in 'p1 p2 p3'.split(): if match.groupdict()[p]: do_something_for_prog(p) - Paddy. -- http://mail.python.org/mailman/listinfo/python-list
Re: C-like assignment expression?
On 21 Mai, 19:56, sturlamolden [EMAIL PROTECTED] wrote: On May 21, 11:38 am, [EMAIL PROTECTED] wrote: if (match = my_re1.match(line): # use match elsif (match = my_re2.match(line)): # use match elsif (match = my_re3.match(line)) # use match ...buy this is illegal in python. Assignment expressions is disallowed in Python to protect against a very common bug in C/C++ programs, the (accidental) confusion of if (match = my_re1.match(line)) with if (match == my_re1.match(line)) or vice versa. This is just a syntactical issue. But what is the *value* of an assigment? In Python it is always None: assigments are statements, not expressions. However Guido and team have found a *pragmatic* solution for this at another place: with open(myFile) as f: BLOCK Compare this with a possible syntactical form of an if-statement: if EXPR as NAME: BLOCK This isn't ugly syntax-wise. It's just a bit harder to understand the semantics of an if-statement. It might read like this: Evaluate EXPR and compute bool(EXPR). If this value is True assign EXPR to NAME and execute BLOCK. Otherwise refuse both assigment and BLOCK execution. Maybe assignment can be performed unconditionally as in the C case. I'm not sure about this. -- http://mail.python.org/mailman/listinfo/python-list
