add shorthand global and nonlocal statements #48449

benjaminp · 2008-10-24T22:32:14Z

BPO	4199
Nosy	@akuchling, @birkenfeld, @rhettinger, @gpshead, @amauryfa, @benjaminp, @bitdancer, @ericsnowcurrently
Files	global_nonlocal_shorthand.patch global_nonlocal_shorthand2.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2013-06-30.14:12:36.980>
created_at = <Date 2008-10-24.22:32:14.485>
labels = ['interpreter-core', 'type-feature']
title = 'add shorthand global and nonlocal statements'
updated_at = <Date 2013-06-30.14:12:36.945>
user = 'https://github.com/benjaminp'

bugs.python.org fields:

activity = <Date 2013-06-30.14:12:36.945>
actor = 'gregory.p.smith'
assignee = 'none'
closed = True
closed_date = <Date 2013-06-30.14:12:36.980>
closer = 'gregory.p.smith'
components = ['Interpreter Core']
creation = <Date 2008-10-24.22:32:14.485>
creator = 'benjamin.peterson'
dependencies = []
files = ['11884', '12254']
hgrepos = []
issue_num = 4199
keywords = ['patch', 'needs review']
message_count = 20.0
messages = ['75193', '77089', '77134', '77159', '88874', '99705', '99958', '99965', '99966', '99977', '99978', '100015', '100043', '100188', '110694', '110698', '155723', '155865', '192049', '192070']
nosy_count = 10.0
nosy_names = ['jhylton', 'akuchling', 'georg.brandl', 'rhettinger', 'gregory.p.smith', 'amaury.forgeotdarc', 'benjamin.peterson', 'r.david.murray', 'BreamoreBoy', 'eric.snow']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'patch review'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue4199'
versions = ['Python 3.4']

benjaminp · 2008-10-24T22:32:10Z

PEP-3104 says that the nonlocal and global statements should allow a
shorthand. ("global x; x = 3" == "global x = 3") This patch implements that.

benjaminp · 2008-12-06T01:22:54Z

Please review.

amauryfa · 2008-12-06T15:30:49Z

the add_ast_fields() function seems to be added by this patch, but it
is already in svn.
Are 1-tuple supported?

    t = [1]
    global a, = t

benjaminp · 2008-12-06T18:28:35Z

I think I may have been merging add_ast_fields when I wrote the patch.

Here's a new patch that handles "global x, = (5,)". To do it, I added a
new flag* to the Global and Nonlocal AST objects that indicates whether
the value needs to be unpacked or not.

The flag is an int. The bool AST type was removed in py3k.

birkenfeld · 2009-06-04T10:08:19Z

Postponed to 3.2.

akuchling · 2010-02-22T02:58:57Z

Bumping priority so this doesn't get forgotten before 3.2; it seems important because it fixes noncompliance with a PEP.

birkenfeld · 2010-02-23T21:37:24Z

"High" is not high enough :)

jhylton · 2010-02-23T22:51:38Z

Is deferred blocker a higher priority?

Jeremy

On Tue, Feb 23, 2010 at 4:37 PM, Georg Brandl <report@bugs.python.org> wrote:

Georg Brandl <georg@python.org> added the comment:

"High" is not high enough :)

----------
priority: high -> deferred blocker

Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue4199\>

birkenfeld · 2010-02-23T22:52:55Z

I interpret the "Priority" list view contents to be sorted by priority, so "deferred blocker" is the second-highest.

jhylton · 2010-02-23T23:41:08Z

On Sat, Dec 6, 2008 at 1:28 PM, Benjamin Peterson
<report@bugs.python.org> wrote:

Benjamin Peterson <musiccomposition@gmail.com> added the comment:

I think I may have been merging add_ast_fields when I wrote the patch.

Here's a new patch that handles "global x, = (5,)". To do it, I added a
new flag* to the Global and Nonlocal AST objects that indicates whether
the value needs to be unpacked or not.

You shouldn't need to do this. The unpack is implied if the number of
identifiers is greater than 1.

Jeremy

The flag is an int. The bool AST type was removed in py3k.

Added file: http://bugs.python.org/file12254/global_nonlocal_shorthand2.patch

Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue4199\>

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jhylton · 2010-02-23T23:50:07Z

I also notice that the Grammar in the PEP is more complicated:
nonlocal_stmt ::=
"nonlocal" identifier ("," identifier)*
["=" (target_list "=")+ expression_list]
| "nonlocal" identifier augop expression_list

The Grammar in the patch is:
+global_stmt: 'global' NAME (',' NAME)* [','] ['=' testlist]
+nonlocal_stmt: 'nonlocal' NAME (',' NAME)* [','] ['=' testlist]

It appears that the PEP is trying to support:

nonlocal x = y = z = 1
nonlocal a, b = c, d = 1

If we're going to support the PEP as written, I think we need to
modify Global() and Nonlocal() to look exactly like Assign(), but add
an extra check to verify that all of the expressions in the targets
are Name, List, or Tuple. You'd probably want to check this at the
time you are generating the AST, so that you're not stuck with some
extra state in the compiler traversal about whether you are generating
code for a Global() or an Assign().

This approach makes the compiler code very simple. We use exactly the
same code for Global(), Nonlocal(), and Assign(). It does have the
downside that you need to enforce this unwritten constraint of the AST
in ast.c and in the ast module.

It also means that the AST will change in a non-backwards compatible
way. I don't see how to do that given that we're also changing the
language spec. (Do you want to include the spec change in your
patch?)

Jeremy

On Tue, Feb 23, 2010 at 6:41 PM, Jeremy Hylton <jeremy@alum.mit.edu> wrote:

On Sat, Dec 6, 2008 at 1:28 PM, Benjamin Peterson
<report@bugs.python.org> wrote:
>
> Benjamin Peterson <musiccomposition@gmail.com> added the comment:
>
> I think I may have been merging add_ast_fields when I wrote the patch.
>
> Here's a new patch that handles "global x, = (5,)". To do it, I added a
> new flag* to the Global and Nonlocal AST objects that indicates whether
> the value needs to be unpacked or not.

You shouldn't need to do this. The unpack is implied if the number of
identifiers is greater than 1.

Jeremy

> * The flag is an int. The bool AST type was removed in py3k.
>
> Added file: http://bugs.python.org/file12254/global_nonlocal_shorthand2.patch
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue4199\>
> _______________________________________
> _______________________________________________
> Python-bugs-list mailing list
> Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/jeremy%40alum.mit.edu
>
>

birkenfeld · 2010-02-24T09:20:19Z

I also notice that the Grammar in the PEP is more complicated:
nonlocal_stmt ::=
"nonlocal" identifier ("," identifier)*
["=" (target_list "=")+ expression_list]
| "nonlocal" identifier augop expression_list

The Grammar in the patch is:
+global_stmt: 'global' NAME (',' NAME)* [','] ['=' testlist]
+nonlocal_stmt: 'nonlocal' NAME (',' NAME)* [','] ['=' testlist]

It appears that the PEP is trying to support:

nonlocal x = y = z = 1
nonlocal a, b = c, d = 1

It also tries to support augmented assignment; however I'm not sure what the semantics of that should be.

Further, there is an ambiguity if too much freedom is allowed: what about

global x = 1, y

Is it declaring a global "x" and assigning a tuple, or declaring a global "x" and a global "y"?

If we're going to support the PEP as written, I think we need to
modify Global() and Nonlocal() to look exactly like Assign(), but add
an extra check to verify that all of the expressions in the targets
are Name, List, or Tuple. You'd probably want to check this at the
time you are generating the AST, so that you're not stuck with some
extra state in the compiler traversal about whether you are generating
code for a Global() or an Assign().

I would not support List or Tuple as targets. Same basic problem as
above, and I don't see a use case.

I see two possibilities for the actual syntax:

global *either* supports multiple identifiers, *or* one identifier
and an assignment.
global always supports multiple identifiers, each with an optional
assignment; tuples need parentheses.

In both cases, I would keep it simple and not allow multiple targets or
augmented assignment.

jhylton · 2010-02-24T16:24:53Z

I guess there's some question about whether the syntax in the PEP was
considered carefully when it was approved. If so, I'm not sure that
we want to re-open the discussion. On the other hand, it's been a
long time since the PEP was approved and we do have a moratorium on
language changes, so it doesn't hurt to slow things down.

I'd argue that the intent of the PEP is pretty clear. You can take
any assignment statement where the LHS doesn't have subscripts, put a
nonlocal or global in front of it, and it means that all those
assignments are to global/nonlocal variables.

Jeremy

On Wed, Feb 24, 2010 at 4:20 AM, Georg Brandl <report@bugs.python.org> wrote:

Georg Brandl <georg@python.org> added the comment:

> I also notice that the Grammar in the PEP is more complicated:
> nonlocal_stmt ::=
> "nonlocal" identifier ("," identifier)*
> ["=" (target_list "=")+ expression_list]
> | "nonlocal" identifier augop expression_list
>
> The Grammar in the patch is:
> +global_stmt: 'global' NAME (',' NAME)* [','] ['=' testlist]
> +nonlocal_stmt: 'nonlocal' NAME (',' NAME)* [','] ['=' testlist]
>
> It appears that the PEP is trying to support:
>
> nonlocal x = y = z = 1
> nonlocal a, b = c, d = 1

It also tries to support augmented assignment; however I'm not sure what the semantics of that should be.

Further, there is an ambiguity if too much freedom is allowed: what about

global x = 1, y

Is it declaring a global "x" and assigning a tuple, or declaring a global "x" and a global "y"?

> If we're going to support the PEP as written, I think we need to
> modify Global() and Nonlocal() to look exactly like Assign(), but add
> an extra check to verify that all of the expressions in the targets
> are Name, List, or Tuple. You'd probably want to check this at the
> time you are generating the AST, so that you're not stuck with some
> extra state in the compiler traversal about whether you are generating
> code for a Global() or an Assign().

I would not support List or Tuple as targets. Same basic problem as
above, and I don't see a use case.

I see two possibilities for the actual syntax:

global *either* supports multiple identifiers, *or* one identifier
and an assignment.

global always supports multiple identifiers, each with an optional
assignment; tuples need parentheses.

In both cases, I would keep it simple and not allow multiple targets or
augmented assignment.

----------

Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue4199\>

birkenfeld · 2010-02-27T16:33:26Z

I do think a brief discussion after the moratorium is over would be good.

BreamoreBoy · 2010-07-18T21:56:12Z

I'm not sure as to the status of this. Could it go back to (say) 3.3, is it still a candidate for 3.2(.x), or what?

benjaminp · 2010-07-18T22:48:48Z

2010/7/18 Mark Lawrence <report@bugs.python.org>:

Mark Lawrence <breamoreboy@yahoo.co.uk> added the comment:

I'm not sure as to the status of this. Could it go back to (say) 3.3, is it still a candidate for 3.2(.x), or what?

It can't do anything until 3.3.

bitdancer · 2012-03-14T06:11:49Z

We could also just decide we don't need it :)

If we do (I haven't read the PEP) does a statement with an assignment make the variable global in that scope, or does it only affect the global variable for the duration of the assignment, and otherwise the variable remains local in the scope? (I don't know which to guess, and the ambiguity is disturbing. I like the current clarity even if it is more typing.)

rhettinger · 2012-03-15T06:35:26Z

+1 on the feature as described in the PEP.

akuchling · 2013-06-29T21:49:40Z

Bumping version to 3.4. I'll send a note to python-dev about this issue.

gpshead · 2013-06-30T14:12:37Z

Closing and rejecting based on said discussion. http://mail.python.org/pipermail/python-dev/2013-June/127143.html

benjaminp added the interpreter-core (Objects, Python, Grammar, and Parser dirs) label Oct 24, 2008

benjaminp added the type-feature A feature request or enhancement label Dec 6, 2008

birkenfeld added the deferred-blocker label Feb 23, 2010

benjaminp removed the deferred-blocker label Feb 27, 2010

gpshead closed this as completed Jun 30, 2013

ezio-melotti transferred this issue from another repository Apr 10, 2022

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add shorthand global and nonlocal statements #48449

add shorthand global and nonlocal statements #48449

benjaminp commented Oct 24, 2008

benjaminp commented Oct 24, 2008

benjaminp commented Dec 6, 2008

amauryfa commented Dec 6, 2008

benjaminp commented Dec 6, 2008

birkenfeld commented Jun 4, 2009

akuchling commented Feb 22, 2010

birkenfeld commented Feb 23, 2010

jhylton mannequin commented Feb 23, 2010

birkenfeld commented Feb 23, 2010

jhylton mannequin commented Feb 23, 2010

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birkenfeld commented Feb 24, 2010

jhylton mannequin commented Feb 24, 2010

birkenfeld commented Feb 27, 2010

BreamoreBoy mannequin commented Jul 18, 2010

benjaminp commented Jul 18, 2010

bitdancer commented Mar 14, 2012

rhettinger commented Mar 15, 2012

akuchling commented Jun 29, 2013

gpshead commented Jun 30, 2013

add shorthand global and nonlocal statements #48449

add shorthand global and nonlocal statements #48449

Comments

benjaminp commented Oct 24, 2008

benjaminp commented Oct 24, 2008

benjaminp commented Dec 6, 2008

amauryfa commented Dec 6, 2008

benjaminp commented Dec 6, 2008

birkenfeld commented Jun 4, 2009

akuchling commented Feb 22, 2010

birkenfeld commented Feb 23, 2010

jhylton mannequin commented Feb 23, 2010

birkenfeld commented Feb 23, 2010

jhylton mannequin commented Feb 23, 2010

jhylton mannequin commented Feb 23, 2010

birkenfeld commented Feb 24, 2010

jhylton mannequin commented Feb 24, 2010

birkenfeld commented Feb 27, 2010

BreamoreBoy mannequin commented Jul 18, 2010

benjaminp commented Jul 18, 2010

bitdancer commented Mar 14, 2012

rhettinger commented Mar 15, 2012

akuchling commented Jun 29, 2013

gpshead commented Jun 30, 2013