[Python-Dev] Lambda [was Re: PEP 8 modernisation]
Hi Alexander, On 02/08/13 00:48, Alexander Shorin wrote: Hi Ronald, I understand this, but I'm a bit confused about fate of lambdas with such guideline since I see no more reasons to use them with p.9 statement: long lines, code duplicate, no mock and well tests etc. - all these problems could be solved with assigning lambda to some name, but now they are looks useless (or useful only for very trivial cases) Lambda is still useful for the reason lambda has always been useful: it is an expression, not a statement, so you can embed it directly where needed. # Preferred: sorted(data, key=lambda value: value['spam'].casefold()) # Allowed: def f(value): return value['spam'].casefold() sorted(data, key=f) # Prohibited: f = lambda value: value['spam'].casefold() sorted(data, key=f) # SyntaxError: sorted(data, key=def f(value): value['spam'].casefold()) -- Steven ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Lambda [was Re: PEP 8 modernisation]
Hi Steven, On Thu, Aug 1, 2013 at 7:06 PM, Steven D'Aprano st...@pearwood.info wrote: Hi Alexander, On 02/08/13 00:48, Alexander Shorin wrote: Hi Ronald, I understand this, but I'm a bit confused about fate of lambdas with such guideline since I see no more reasons to use them with p.9 statement: long lines, code duplicate, no mock and well tests etc. - all these problems could be solved with assigning lambda to some name, but now they are looks useless (or useful only for very trivial cases) Lambda is still useful for the reason lambda has always been useful: it is an expression, not a statement, so you can embed it directly where needed. # Preferred: sorted(data, key=lambda value: value['spam'].casefold()) # Allowed: def f(value): return value['spam'].casefold() sorted(data, key=f) # Prohibited: f = lambda value: value['spam'].casefold() sorted(data, key=f) # SyntaxError: sorted(data, key=def f(value): value['spam'].casefold()) The case: items = [[0, 'foo'], [3, 'baz'], [2, 'foo'], [1, 'bar']] Need to group by second item. Quite common task: from itertools import groupby for key, items in groupby(items, key=lambda i: i[1]): print(key, ':', list(items)) foo : [[0, 'foo']] baz : [[3, 'baz']] foo : [[2, 'foo']] bar : [[1, 'bar']] oops, failed, we need to sort things first by this item and it looks we have to duplicate grouping function: fun = lambda i: i[1] for key, items in groupby(sorted(items, key=fun), key=fun): print(key, ':', list(items)) Ok, PEP suggests to use defs, so we adds 3 more lines (before and after def + return) to code: def fun(i): return i[1] for key, items in groupby(sorted(items, key=fun), key=fun): print(key, ':', list(items)) so that's the question: what is the rationale of this if lambdas successfully solves the problem with minimal amount of typing, code and thinking? I thought there should be only one way to do something, but this PEP-8 statement conflicts with PEP-20 one: There should be one-- and preferably only one --obvious way to do it. It's really not oblivious why lambdas couldn't be assignment to some name, especially in the light of fact that if they are been passed to some function as argument, they will be assignee to some name. -- ,,,^..^,,, ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Lambda [was Re: PEP 8 modernisation]
On Thu, Aug 1, 2013 at 5:58 PM, Alexander Shorin kxe...@gmail.com wrote: fun = lambda i: i[1] for key, items in groupby(sorted(items, key=fun), key=fun): print(key, ':', list(items)) I'd do a direct translation to def here: def fun(i): return i[1] for key, items in groupby(sorted(items, key=fun), key=fun): print(key, ':', list(items)) ChrisA ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Lambda [was Re: PEP 8 modernisation]
On Thu, Aug 1, 2013 at 12:58 PM, Alexander Shorin kxe...@gmail.com wrote: Hi Steven, On Thu, Aug 1, 2013 at 7:06 PM, Steven D'Aprano st...@pearwood.info wrote: Hi Alexander, On 02/08/13 00:48, Alexander Shorin wrote: Hi Ronald, I understand this, but I'm a bit confused about fate of lambdas with such guideline since I see no more reasons to use them with p.9 statement: long lines, code duplicate, no mock and well tests etc. - all these problems could be solved with assigning lambda to some name, but now they are looks useless (or useful only for very trivial cases) Lambda is still useful for the reason lambda has always been useful: it is an expression, not a statement, so you can embed it directly where needed. # Preferred: sorted(data, key=lambda value: value['spam'].casefold()) # Allowed: def f(value): return value['spam'].casefold() sorted(data, key=f) # Prohibited: f = lambda value: value['spam'].casefold() sorted(data, key=f) # SyntaxError: sorted(data, key=def f(value): value['spam'].casefold()) The case: items = [[0, 'foo'], [3, 'baz'], [2, 'foo'], [1, 'bar']] Need to group by second item. Quite common task: from itertools import groupby for key, items in groupby(items, key=lambda i: i[1]): print(key, ':', list(items)) foo : [[0, 'foo']] baz : [[3, 'baz']] foo : [[2, 'foo']] bar : [[1, 'bar']] oops, failed, we need to sort things first by this item and it looks we have to duplicate grouping function: fun = lambda i: i[1] for key, items in groupby(sorted(items, key=fun), key=fun): print(key, ':', list(items)) Ok, PEP suggests to use defs, so we adds 3 more lines (before and after def + return) to code: def fun(i): return i[1] for key, items in groupby(sorted(items, key=fun), key=fun): print(key, ':', list(items)) so that's the question: what is the rationale of this if lambdas successfully solves the problem with minimal amount of typing, code and thinking? I thought there should be only one way to do something, but this PEP-8 statement conflicts with PEP-20 one: There should be one-- and preferably only one --obvious way to do it. It's really not oblivious why lambdas couldn't be assignment to some name, especially in the light of fact that if they are been passed to some function as argument, they will be assignee to some name. Just because you can doesn't mean you should. This guideline is all about being explicit over implicit, not about saving typing. If you want to bind a function to a name then you should use a def to specify that fact; you also lose some things otherwise (e.g. __name__ is not set). Lambdas should be thought of one-off functions you write inline because it expresses the intent of the code just as well. Assigning a lambda to a variable is in no way more beneficial compared to using def and thus this guideline suggesting you use def to make it at least as clear, if not more and to gain benefits such as __name__ being set (which helps with debugging, etc.). ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Lambda [was Re: PEP 8 modernisation]
Chris Angelico writes: On Thu, Aug 1, 2013 at 5:58 PM, Alexander Shorin kxe...@gmail.com wrote: fun = lambda i: i[1] for key, items in groupby(sorted(items, key=fun), key=fun): print(key, ':', list(items)) I'd do a direct translation to def here: def fun(i): return i[1] for key, items in groupby(sorted(items, key=fun), key=fun): print(key, ':', list(items)) As long as it's about readability, why not make it readable? def second(pair): return pair[1] for key, items in groupby(sorted(items, key=second), key=second): print(key, ':', list(items)) I realize it's somewhat unfair (for several reasons) to compare that to Alexander's fun = lambda i: i[1], but I can't help feeling that in another sense it is fair. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Lambda [was Re: PEP 8 modernisation]
* Stephen J. Turnbull wrote: Chris Angelico writes: On Thu, Aug 1, 2013 at 5:58 PM, Alexander Shorin kxe...@gmail.com wrote: fun = lambda i: i[1] for key, items in groupby(sorted(items, key=fun), key=fun): print(key, ':', list(items)) I'd do a direct translation to def here: def fun(i): return i[1] for key, items in groupby(sorted(items, key=fun), key=fun): print(key, ':', list(items)) As long as it's about readability, why not make it readable? def second(pair): return pair[1] for key, items in groupby(sorted(items, key=second), key=second): print(key, ':', list(items)) I realize it's somewhat unfair (for several reasons) to compare that to Alexander's fun = lambda i: i[1], but I can't help feeling that in another sense it is fair. Seems to run OT somewhat, but second is probably a bad name here. If the key changes, you have to rename it in several places (or worse, you DON'T rename it, and then the readability is gone). Usually I'm using a name with key in it - describing what it's for, not how it's done. The minimal distance to its usage is supporting that, too. nd -- Das Verhalten von Gates hatte mir bewiesen, dass ich auf ihn und seine beiden Gefährten nicht zu zählen brauchte -- Karl May, Winnetou III Im Westen was neues: http://pub.perlig.de/books.html#apache2 ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] Lambda [was Re: PEP 8 modernisation]
Steven D'Aprano wrote: Lambda is still useful for the reason lambda has always been useful: it is an expression, not a statement, so you can embed it directly where needed. are there some possibilities to change def to an expression? do I need to wait 'till python9k? yes, this brings to the possibility to write something like foo = def bar(): pass but at least should let the lambda to die in peace... -- ZeD ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
