2010/12/17 Stefan Behnel :

Hi,

it seems that Py3 doesn't support setting a "nonlocal" value as part of the "nonlocal" command

   Python 3.2a4+ (py3k:86480, Nov 16 2010, 16:43:22)    [GCC 4.4.3] on linux2    Type "help", "copyright", "credits" or "license" for more information.    >>> def x():    ...   y = 5    ...   def f():    ...     nonlocal y = 6      File "<stdin>", line 4        nonlocal y = 6                   ^    SyntaxError: invalid syntax

even though the PEP clearly describes this feature.

http://www.python.org/dev/peps/pep-3104/#proposed-solution

Either the PEP or the implementation should be updated. Personally, I think the PEP should be changed as I think that the syntax complicates the grammar more than it's worth. Also, the moratorium applies here, given that Py3.1 does not implement this.

Comments?

There's a issue and a patch for this somewhere. I personally don't care; it's not too painful to write two lines. -- Regards, Benjamin

On Fri, Dec 17, 2010 at 11:52 AM, Laurens Van Houtven wrote:

+1 for throwing it out of the PEP. Assignment is a thing, nonlocal/global is a thing, don't mix them up :) (That in addition to the grammar cleanliness argument Stephan already made)

Another +1 for the same reasons. Also, since global does not allow assignment, neither should nonlocal.

On Fri, Dec 17, 2010 at 12:33 PM, Benjamin Peterson wrote: ..

...

Another +1 for the same reasons.  Also, since global does not allow assignment, neither should nonlocal.

Note that the PEP stated that global would also be extended.

I missed that, so for future reference, the PEP says: """ A shorthand form is also permitted, in which nonlocal is prepended to an assignment or augmented assignment: nonlocal x = 3 The above has exactly the same meaning as nonlocal x; x = 3. (Guido supports a similar form of the global statement.) """ and refers to Guido's post at http://mail.python.org/pipermail/python-3000/2006-November/004166.html In any case, the relevant issue is http://bugs.python.org/issue4199 and it should probably be marked as "after moratorium". Meanwhile an implementation status note can be added to the PEP to avoid this issue being brought up again.

On Sat, Dec 18, 2010 at 1:12 PM, Georg Brandl wrote:

Am 17.12.2010 17:52, schrieb Laurens Van Houtven:

...

+1 for throwing it out of the PEP. Assignment is a thing, nonlocal/global is a thing, don't mix them up :) (That in addition to the grammar cleanliness argument Stephan already made)

The trouble is what to make of

nonlocal x = 3, y

Is it two nonlocal declarations or one with a tuple assignment?

Georg

I'm not sure I understand. Isn't that another reason to throw it out? If you don't allow such assignments, there can't be any ambiguity, right? (Or am I missing something?) lvh

Am 22.12.2010 23:11, schrieb Laurens Van Houtven:

On Sat, Dec 18, 2010 at 1:12 PM, Georg Brandl wrote:

...

Am 17.12.2010 17:52, schrieb Laurens Van Houtven:

...

+1 for throwing it out of the PEP. Assignment is a thing, nonlocal/global is a thing, don't mix them up :) (That in addition to the grammar cleanliness argument Stephan already made)

The trouble is what to make of

nonlocal x = 3, y

Is it two nonlocal declarations or one with a tuple assignment?

Georg

I'm not sure I understand. Isn't that another reason to throw it out? If you don't allow such assignments, there can't be any ambiguity, right? (Or am I missing something?)

Yes and no -- there may not be an ambiguity to the parser, but still to the human. Except if you disallow the syntax in any case, requiring people to write nonlocal x = (3, y) which is then again inconsistent with ordinary assignment statements. Georg

Am 23.12.2010 22:03, schrieb Laurens Van Houtven:

On Thu, Dec 23, 2010 at 9:51 PM, Georg Brandl wrote:

...

Yes and no -- there may not be an ambiguity to the parser, but still to the human. Except if you disallow the syntax in any case, requiring people to write

nonlocal x = (3, y)

which is then again inconsistent with ordinary assignment statements.

Georg

Right -- but (and hence the confusion) I was arguing for not mixing global/nonlocal with assignment at all, and instead having nonlocal and global only take one or more names. That would (obviously) remove any such ambiguity ;-)

Oh yes, I see -- not sure why I worded it like I did. I just wanted to reiterate the most obvious problematic point to people who hadn't followed the earlier discussions about it. Sorry. Georg

Hrvoje Niksic, 24.12.2010 09:45:

On 12/23/2010 10:03 PM, Laurens Van Houtven wrote:

...

On Thu, Dec 23, 2010 at 9:51 PM, Georg Brandl wrote:

...

Yes and no -- there may not be an ambiguity to the parser, but still to the human. Except if you disallow the syntax in any case, requiring people to write

nonlocal x = (3, y)

which is then again inconsistent with ordinary assignment statements.

Right -- but (and hence the confusion) I was arguing for not mixing global/nonlocal with assignment at all, and instead having nonlocal and global only take one or more names. That would (obviously) remove any such ambiguity ;-)

I would like to offer the opposing viewpoint: nonlocal x = value is a useful shortcut because nonlocal is used in closure callbacks where brevity matters.

I doubt that it really matters so much that one line more kills readability. It's still a relatively rare use case after all.

The reason nonlocal is introduced is to change the variable, so it makes sense that the two can be done in the same line of code.

But still, this is just a special case. If the variable is changed more than once, you'd end up with one assignment with nonlocal and one without. This just adds to the growing list of code inconsistencies presented in this thread.

As for global x = value being disallowed, I have been annoyed at times with that, so that sounds like a good argument to change both.

Requiring the parentheses for tuple creation sounds like a good compromise for resolving the ambiguity, consistent with similar limitations of the generator expression syntax.

Why introduce such a pitfall for coders here? "Require" doesn't mean the parser can enforce it. If it's not there, it just means something else, so it's up to the coder to get it right. The exact same kind of situation that was fixed for the except syntax in Python 3. Stefan

On Mon, Dec 27, 2010 at 9:43 AM, Raymond Hettinger wrote:

FWIW, I'm entirely opposed to doing an assignment in a nonlocal definition. [...]

-1 for assignment in nonlocal and global statements from me, too. Mark

Am 27.12.2010 10:43, schrieb Raymond Hettinger:

FWIW, I'm entirely opposed to doing an assignment in a nonlocal definition.

* It is easily mis-parsed by human (as shown by Georg's examples). * It looks very much like an initialization of a local variable in many languages, but it is not -- the variable has already been initialized in another scope.

Hah, and we'd probably get requests for a mandatory declaration of locals, ala local a = 5 for reasons of "consistency" or "explicit is better than implicit". Georg