Incorrect scope of list comprehension variables
in [Python]
|
Prev: psycopg2 / psycopg2.ProgrammingError: syntax error at or near "E'mytable'"
Next: Question on templates and python logging
From: Stephen Hansen on 4 Apr 2010 03:17 On 2010-04-03 23:30:32 -0700, Steve Howell said: > On Apr 3, 9:58�pm, Tim Roberts <t...(a)probo.com> wrote: >> Alain Ketterlin <al...(a)dpt-info.u-strasbg.fr> wrote: >> >>> I've just spent a few hours debugging code similar to this: >> >>> d = dict() >>> for r in [1,2,3]: >>> � �d[r] = [r for r in [4,5,6]] >>> print d >> >> Yes, this has been fixed in later revisions, but I'm curious to know what >> led you to believe that a list comprehension created a new scope. �I don't >> that was ever promised. > > Common sense about how programming languages should work? As > confirmed by later revisions? Where exactly does this common sense come from? A list comprehension is basically syntactic sugar over a for loop, and... Python 3.1.2 (r312:79360M, Mar 24 2010, 01:33:18) [GCC 4.0.1 (Apple Inc. build 5493)] on darwin Type "copyright", "credits" or "license()" for more information. >>> for x in range(10): pass >>> x 9 -- --S .... p.s: change the ".invalid" to ".com" in email address to reply privately.
From: Ethan Furman on 4 Apr 2010 08:50 Steve Howell wrote: > On Apr 3, 9:58 pm, Tim Roberts <t...(a)probo.com> wrote: > >>Alain Ketterlin <al...(a)dpt-info.u-strasbg.fr> wrote: >> >> >>>I've just spent a few hours debugging code similar to this: >> >>>d = dict() >>>for r in [1,2,3]: >>> d[r] = [r for r in [4,5,6]] >>>print d >> >>Yes, this has been fixed in later revisions, but I'm curious to know what >>led you to believe that a list comprehension created a new scope. I don't >>that was ever promised. > > > Common sense about how programming languages should work? As > confirmed by later revisions? Common sense? About *somebody else's* idea of how a programming language should work? Please. Experiment and read the manual. ~Ethan~
From: Alf P. Steinbach on 4 Apr 2010 09:06 * Ethan Furman: > Steve Howell wrote: >> On Apr 3, 9:58 pm, Tim Roberts <t...(a)probo.com> wrote: >> >>> Alain Ketterlin <al...(a)dpt-info.u-strasbg.fr> wrote: >>> >>> >>>> I've just spent a few hours debugging code similar to this: >>> >>>> d = dict() >>>> for r in [1,2,3]: >>>> d[r] = [r for r in [4,5,6]] >>>> print d >>> >>> Yes, this has been fixed in later revisions, but I'm curious to know >>> what >>> led you to believe that a list comprehension created a new scope. I >>> don't >>> that was ever promised. >> >> >> Common sense about how programming languages should work? As >> confirmed by later revisions? > > Common sense? About *somebody else's* idea of how a programming > language should work? Common sense is about practical solutions. Since there is no practical gain from a list comprehension affecting the bindings of outside variables, and there correspondingly is a practical pay-off from list comprehensions not affecting the bindings of outside variables, common sense is to expect the latter. It's in the nature of common sense that those who possess this ability often tend to make the same tentative assumptions when presented with the same problem. It doesn't mean that they're consulting each other, like your "somebody else's": it just means that they're applying similar common sense reasoning. So, there's no great conspiracy. > Please. Experiment and read the manual. Common sense is applied first, as a heuristic. You really wouldn't want to drill down into the architect's drawings in order to get office 215 in a building. First you apply common sense. Cheers & hth., - Alf
From: D'Arcy J.M. Cain on 4 Apr 2010 09:43 On Sun, 04 Apr 2010 15:06:46 +0200 "Alf P. Steinbach" <alfps(a)start.no> wrote: > Common sense is applied first, as a heuristic. You really wouldn't want to drill > down into the architect's drawings in order to get office 215 in a building. > First you apply common sense. Oh goodie, bad analogies. Can I play too? Getting to office 215 is not analogous to writing a program. It is analogous to using the program. Writing the program is like building the office tower. You need to know about the tools and materials that you are working with. You don't use "common sense" to decide what materials to use. You study the literature and the specs. -- D'Arcy J.M. Cain <darcy(a)druid.net> | Democracy is three wolves http://www.druid.net/darcy/ | and a sheep voting on +1 416 425 1212 (DoD#0082) (eNTP) | what's for dinner.
From: Paul Rubin on 4 Apr 2010 17:50
Alain Ketterlin <alain(a)dpt-info.u-strasbg.fr> writes: > d[r] = [r for r in [4,5,6]] > THe problem is that the "r" in d[r] somehow captures the value of the > "r" in the list comprehension, and somehow kills the loop interator. Yes, this is a well known design error in Python 2.x. The 3.x series fixes this error but introduces other errors of its own. It is evil enough that I almost always use this syntax instead: d[r] = list(r for r in [4,5,6]) that works in 3.x and the later releases of 2.x. In early 2.x (maybe up to 2.2) it throws an error at compile time rather than at run time. |