Re: generator expression works in shell, NameError in script
On Jun 18, 11:56 pm, nn wrote: > On Jun 18, 8:38 am, guthrie wrote: > > > > > On Jun 17, 6:38 pm, Steven Samuel Cole > > wrote: > > > > Still don't really understand why my initial code didn't work, though... > > > Your code certainly looks reasonable, and looks to me like it "should" > > work. The comment of partial namespace is interesting, but > > unconvincing (to me) - but I am not a Python expert! It would > > certainly seem that within that code block it is in the local > > namespace, not removed from that scope to be in another. > > > Seems that it should either be a bug, or else is a design error in the > > language! > > > Just as in the above noted "WTF" - non-intuitive language constructs > > that surprise users are poor design. > > This is certainly an odd one. This code works fine under 2.6 but fails > in Python 3.1. > > >>> class x: > > ... lst=[2] > ... gen=[lst.index(e) for e in lst] > ... > Traceback (most recent call last): > File "", line 1, in > File "", line 3, in x > File "", line 3, in > NameError: global name 'lst' is not defined > Arghh. I see thousands of future wtf!? posts to c.l.p. triggered by this new feature. Might as well save some time and add it to the FAQ already. -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
In article <13e776b0-6ca8-479e-ba82-eb5b7eb8f...@a5g2000pre.googlegroups.com>, ssc wrote: >On Jun 18, 10:49=A0am, Jon Clements wrote: >> >> Why are you doing this? I'm assuming a code to title look up is >> required (don't forget military, royal and honorable titles >> etc... :) ) > >I'm in New Zealand. Hardly any need for military titles, rarely any >for royal and damn sure none for honorable :-D (scnr) +1 QOTW -- Aahz (a...@pythoncraft.com) <*> http://www.pythoncraft.com/ "as long as we like the same operating system, things are cool." --piranha -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On Jun 18, 10:49 am, Jon Clements wrote: > Why are you doing this? I'm assuming a code to title look up is > required (don't forget military, royal and honorable titles > etc... :) ) I'm in New Zealand. Hardly any need for military titles, rarely any for royal and damn sure none for honorable :-D (scnr) -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
guthrie wrote: -- I don't get this - the only local loop variable is "e", not lst. Yes, but the whole list comprehension gets put into a nested function, including the part that evaluates lst. (It's not strictly *necessary* to do that, but that's the way it happens to be implemented at the moment.) It would generally be the case that a nested function would have access to its enclosing scope. Usually, but class namespaces are a special case -- they're not considered to be enclosing scopes, even though textually they're written that way. it certainly appears un-intuitive It is, but it's hard to see what could be done to improve the situation without introducing worse problems. -- Greg -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On Jun 19, 2009, at 8:45 AM, Bruno Desthuilliers wrote: >>> class Foo(object): ... bar = ['a', 'b', 'c'] ... baaz = list((b, b) for b in bar) but it indeed looks like using bar.index *in a generator expression* fails (at least in 2.5.2) : >>> class Foo(object): ... bar = ['a', 'b', 'c'] ... baaz = list((bar.index(b), b) for b in bar) ... Traceback (most recent call last): File "", line 1, in File "", line 3, in Foo File "", line 3, in NameError: global name 'bar' is not defined The reason that the first one works but the second fails is clearer if you translate each generator expression to the approximately equivalent generator function: class Foo(object): bar = ['a', 'b', 'c'] def _gen(_0): for b in _0: yield (b, b) baaz = list(_gen(iter(bar)) # PEP 227: "the name bindings that occur in the class block # are not visible to enclosed functions" class Foo(object): bar = ['a', 'b', 'c'] def _gen(_0): for b in _0: yield (bar.index(b), b) baaz = list(_gen(iter(bar)) -Miles -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On Jun 18, 11:28 pm, greg wrote: > nn wrote: > > This is certainly an odd one. This code works fine under 2.6 but fails > > in Python 3.1. > > class x: > > > ... lst=[2] > > ... gen=[lst.index(e) for e in lst] > > In 3.x it was decided that the loop variables in a list > comprehension should be local, and not leak into the > surrounding scope. This was implemented by making the > list comprehension into a nested function. > > Unfortunately this leads to the same unintuitive > behaviour as a genexp when used in a class scope. > -- I don't get this - the only local loop variable is "e", not lst. And lst is used twice in the generator expression, which is being complained about? It would generally be the case that a nested function would have access to its enclosing scope. In any case, even if/when the current behavior is explained or rationalized, it certainly appears un-intuitive, and that is another level of "error"! :-) -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
ryles wrote: However, in 3.0 list comprehensions are actually treated as list(). That's not quite true -- it would be rather inefficient if it was. The code generated for the LC is much the same as it was in 2.x. But the whole LC is put into a nested function so that the loop variables are local to it. -- Greg -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
> Does the generator expression have its own little namespace or so ? Yes, generators create a new scope: http://docs.python.org/reference/expressions.html#grammar-token-generator_expression -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On Jun 19, 1:45 pm, Bruno Desthuilliers wrote: > [...] > but it indeed looks like using bar.index *in a generator expression* > fails (at least in 2.5.2) : > > >>> class Foo(object): > ... bar = ['a', 'b', 'c'] > ... baaz = list((bar.index(b), b) for b in bar) > ... > Traceback (most recent call last): > File "", line 1, in > File "", line 3, in Foo > File "", line 3, in > NameError: global name 'bar' is not defined > > Looks like a bug to me :-/ I think it's working as intended: name resolution for nested scopes skips intermediate class scopes. The 'Discussion' section of PEP 227 (Statically Nested Scopes) is illuminating; here's a snippet: """Names in class scope are not accessible. Names are resolved in the innermost enclosing function scope. If a class definition occurs in a chain of nested scopes, the resolution process skips class definitions. This rule prevents odd interactions between class attributes and local variable access.""" Mark -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
Emile van Sebille a écrit : On 6/17/2009 3:54 PM ssc said... Wow! Didn't expect that kind of instant support. Thank you very much, I'll give both zip and enumerate a try. The code I've shown is actually copied pretty straight from a Django form class, but I didn't want to mention that as not to dilute the conversation. Don't think it matters, anyway. This is the relevant excerpt: from django.forms import Form class SignupForm(Form): titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms',] title_choices = [(0, '')] + list((titles.index(t)+1, t) for t in titles) Now that I look at it again, it seems odd to me to not have the code e.g. in __init__(...), but just 'class-global'. And as class is an executable statement, I imagine titles exists in a namespace that hasn't yet been completely defined. Still, that does not seem a reason for titles not to be not defined, as I do define it just in the line above. Well, again, titles will exists in the SignupForm namespace once it exists, which it doesn't yet when you get to title_choices and want to access it. The namespace itself is as defined at any point of the class statement's body as the local namespace of a function at any given point of the function's body. Else decorators (or their alternate pre '@' syntactic-sugar version) wouldn't work. FWIW, the following code works JustFine(tm): br...@bruno:~$ python Python 2.5.2 (r252:60911, Oct 5 2008, 19:24:49) [GCC 4.3.2] on linux2 >>> class Foo(object): ... bar = ['a', 'b', 'c'] ... baaz = list(enumerate(bar)) as well as this one: >>> class Foo(object): ... bar = ['a', 'b', 'c'] ... baaz = [(bar.index(t)+1, t) for t in bar] and this one: >>> class Foo(object): ... bar = ['a', 'b', 'c'] ... baaz = list((b, b) for b in bar) but it indeed looks like using bar.index *in a generator expression* fails (at least in 2.5.2) : >>> class Foo(object): ... bar = ['a', 'b', 'c'] ... baaz = list((bar.index(b), b) for b in bar) ... Traceback (most recent call last): File "", line 1, in File "", line 3, in Foo File "", line 3, in NameError: global name 'bar' is not defined Looks like a bug to me :-/ -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
nn wrote: This is certainly an odd one. This code works fine under 2.6 but fails in Python 3.1. class x: ... lst=[2] ... gen=[lst.index(e) for e in lst] In 3.x it was decided that the loop variables in a list comprehension should be local, and not leak into the surrounding scope. This was implemented by making the list comprehension into a nested function. Unfortunately this leads to the same unintuitive behaviour as a genexp when used in a class scope. -- Greg -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
ssc wrote: class SignupForm(Form): titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms',] title_choices = [(0, '')] + list((titles.index(t)+1, t) for t in titles) Does the generator expression have its own little namespace or so ? Yes. The generator expression is a function, with its own local namespace. Since the class scope is not visible from inside functions declared within it, the behaviour you're seeing results. -- Greg -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On Jun 18, 9:56 am, nn wrote: > On Jun 18, 8:38 am, guthrie wrote: > > > > > On Jun 17, 6:38 pm, Steven Samuel Cole > > wrote: > > > > Still don't really understand why my initial code didn't work, though... > > > Your code certainly looks reasonable, and looks to me like it "should" > > work. The comment of partial namespace is interesting, but > > unconvincing (to me) - but I am not a Python expert! It would > > certainly seem that within that code block it is in the local > > namespace, not removed from that scope to be in another. > > > Seems that it should either be a bug, or else is a design error in the > > language! > > > Just as in the above noted "WTF" - non-intuitive language constructs > > that surprise users are poor design. > > This is certainly an odd one. This code works fine under 2.6 but fails > in Python 3.1. > > >>> class x: > > ... lst=[2] > ... gen=[lst.index(e) for e in lst] > ... > Traceback (most recent call last): > File "", line 1, in > File "", line 3, in x > File "", line 3, in > NameError: global name 'lst' is not defined > > > > I believe it works in 2.x because unlike generator expressions, list comprehensions do not create a new scope. However, in 3.0 list comprehensions are actually treated as list(). http://docs.python.org/reference/expressions.html http://www.python.org/dev/peps/pep-0289 -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On Jun 18, 8:38 am, guthrie wrote: > On Jun 17, 6:38 pm, Steven Samuel Cole > wrote: > > > Still don't really understand why my initial code didn't work, though... > > Your code certainly looks reasonable, and looks to me like it "should" > work. The comment of partial namespace is interesting, but > unconvincing (to me) - but I am not a Python expert! It would > certainly seem that within that code block it is in the local > namespace, not removed from that scope to be in another. > > Seems that it should either be a bug, or else is a design error in the > language! > > Just as in the above noted "WTF" - non-intuitive language constructs > that surprise users are poor design. This is certainly an odd one. This code works fine under 2.6 but fails in Python 3.1. >>> class x: ... lst=[2] ... gen=[lst.index(e) for e in lst] ... Traceback (most recent call last): File "", line 1, in File "", line 3, in x File "", line 3, in NameError: global name 'lst' is not defined >>> -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On Jun 17, 6:38 pm, Steven Samuel Cole wrote: > Still don't really understand why my initial code didn't work, though... Your code certainly looks reasonable, and looks to me like it "should" work. The comment of partial namespace is interesting, but unconvincing (to me) - but I am not a Python expert! It would certainly seem that within that code block it is in the local namespace, not removed from that scope to be in another. Seems that it should either be a bug, or else is a design error in the language! Just as in the above noted "WTF" - non-intuitive language constructs that surprise users are poor design. -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
Both zip and enumerate do the trick. Thanks for the pointers, that's 2 more built-ins under my belt :-) Still don't really understand why my initial code didn't work, though... Thanks everyone! :-) -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On 6/17/2009 3:54 PM ssc said... Wow! Didn't expect that kind of instant support. Thank you very much, I'll give both zip and enumerate a try. The code I've shown is actually copied pretty straight from a Django form class, but I didn't want to mention that as not to dilute the conversation. Don't think it matters, anyway. This is the relevant excerpt: from django.forms import Form class SignupForm(Form): titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms',] title_choices = [(0, '')] + list((titles.index(t)+1, t) for t in titles) Now that I look at it again, it seems odd to me to not have the code e.g. in __init__(...), but just 'class-global'. And as class is an executable statement, I imagine titles exists in a namespace that hasn't yet been completely defined. Still, that does not seem a reason for titles not to be not defined, as I do define it just in the line above. Well, again, titles will exists in the SignupForm namespace once it exists, which it doesn't yet when you get to title_choices and want to access it. Define titles above the class and you should be OK. Does the generator expression have its own little namespace or so ? Sometimes -- in some python versions generator expressions leak... Emile -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On Wed, Jun 17, 2009 at 3:54 PM, ssc wrote: > Wow! Didn't expect that kind of instant support. Thank you very much, > I'll give both zip and enumerate a try. > > The code I've shown is actually copied pretty straight from a Django > form class, but I didn't want to mention that as not to dilute the > conversation. Don't think it matters, anyway. This is the relevant > excerpt: > > from django.forms import Form > > class SignupForm(Form): > > titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms',] > title_choices = [(0, '')] + list((titles.index(t)+1, t) for t in > titles) > > Now that I look at it again, it seems odd to me to not have the code > e.g. in __init__(...), but just 'class-global'. > Still, that does not seem a reason for titles not to be not defined, > as I do define it just in the line above. > > Does the generator expression have its own little namespace or so ? No, which leads to the common "WTF?" reaction upon seeing stuff like: >>> funcs = ((lambda: x) for x in range(7)) >>> list_of = list(funcs) >>> list_of[0]() 6 Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
Wow! Didn't expect that kind of instant support. Thank you very much, I'll give both zip and enumerate a try. The code I've shown is actually copied pretty straight from a Django form class, but I didn't want to mention that as not to dilute the conversation. Don't think it matters, anyway. This is the relevant excerpt: from django.forms import Form class SignupForm(Form): titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms',] title_choices = [(0, '')] + list((titles.index(t)+1, t) for t in titles) Now that I look at it again, it seems odd to me to not have the code e.g. in __init__(...), but just 'class-global'. Still, that does not seem a reason for titles not to be not defined, as I do define it just in the line above. Does the generator expression have its own little namespace or so ? -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On Jun 17, 11:19 pm, ssc wrote:
> Hello,
>
> I am trying to generate this list of tuples:
> [(0, ''), (1, 'Dr'), (2, 'Miss'), (3, 'Mr'), (4, 'Mrs'), (5, 'Ms')]
>
> My code works fine in the Python shell:
>
> >>> titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms',]
> >>> title_choices = [(0, '')] + list((titles.index(t)+1, t) for t in titles)
> >>> title_choices
>
> [(0, ''), (1, 'Dr'), (2, 'Miss'), (3, 'Mr'), (4, 'Mrs'), (5, 'Ms')]
>
> The same code run in a script fails with
> NameError: global name 'titles' is not defined
>
> Does anybody know why ? How can I fix the error ?
>
> Thank you very much :-)
>
> Steve
Why are you doing this? I'm assuming a code to title look up is
required (don't forget military, royal and honorable titles
etc... :) )
Why not just:
>>> titles = ['', 'Dr', 'Miss', 'Mr', 'Mrs', 'Ms',]
>>> lookup = dict(enumerate(titles))
>>> print lookup
{0: '', 1: 'Dr', 2: 'Miss', 3: 'Mr', 4: 'Mrs', 5: 'Ms'}
or if you don't want a dict, then just:
>>> print list(enumerate(titles))
[(0, ''), (1, 'Dr'), (2, 'Miss'), (3, 'Mr'), (4, 'Mrs'), (5, 'Ms')]
--
http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On 6/17/2009 3:41 PM Jason Tackaberry said... How about: enumerate([''] + titles) or perhaps, depending on usage... list(enumerate(titles)) Emile -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On Wed, 2009-06-17 at 15:38 -0700, Chris Rebert wrote: > See what Emile said, but here's a nicer way to code it, IMHO: > > titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms'] > title_choices = zip(range(len(titles)+1), ['']+titles) > > zip() to the rescue! How about: enumerate([''] + titles) -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On Wed, Jun 17, 2009 at 3:19 PM, ssc wrote: > Hello, > > I am trying to generate this list of tuples: > [(0, ''), (1, 'Dr'), (2, 'Miss'), (3, 'Mr'), (4, 'Mrs'), (5, 'Ms')] > > My code works fine in the Python shell: > titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms',] title_choices = [(0, '')] + list((titles.index(t)+1, t) for t in titles) title_choices > [(0, ''), (1, 'Dr'), (2, 'Miss'), (3, 'Mr'), (4, 'Mrs'), (5, 'Ms')] > > The same code run in a script fails with > NameError: global name 'titles' is not defined > > Does anybody know why ? How can I fix the error ? See what Emile said, but here's a nicer way to code it, IMHO: titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms'] title_choices = zip(range(len(titles)+1), ['']+titles) zip() to the rescue! Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
Re: generator expression works in shell, NameError in script
On 6/17/2009 3:19 PM ssc said... Hello, I am trying to generate this list of tuples: [(0, ''), (1, 'Dr'), (2, 'Miss'), (3, 'Mr'), (4, 'Mrs'), (5, 'Ms')] My code works fine in the Python shell: titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms',] title_choices = [(0, '')] + list((titles.index(t)+1, t) for t in titles) title_choices [(0, ''), (1, 'Dr'), (2, 'Miss'), (3, 'Mr'), (4, 'Mrs'), (5, 'Ms')] The same code run in a script fails with NameError: global name 'titles' is not defined You get this because titles doesn't exist in the builtin, local or global namespaces. Post the code that fails. It's hard to debug working code. :) Emile Does anybody know why ? How can I fix the error ? Thank you very much :-) Steve -- http://mail.python.org/mailman/listinfo/python-list
generator expression works in shell, NameError in script
Hello, I am trying to generate this list of tuples: [(0, ''), (1, 'Dr'), (2, 'Miss'), (3, 'Mr'), (4, 'Mrs'), (5, 'Ms')] My code works fine in the Python shell: >>> titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms',] >>> title_choices = [(0, '')] + list((titles.index(t)+1, t) for t in titles) >>> title_choices [(0, ''), (1, 'Dr'), (2, 'Miss'), (3, 'Mr'), (4, 'Mrs'), (5, 'Ms')] The same code run in a script fails with NameError: global name 'titles' is not defined Does anybody know why ? How can I fix the error ? Thank you very much :-) Steve -- http://mail.python.org/mailman/listinfo/python-list
