Make "global after use" a SyntaxError

[ilevkivskyi]

BPO	27999
Nosy	@gvanrossum, @1st1, @ilevkivskyi
Files	patch-v1.diff patch-v2.diff

^{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 = 'https://github.com/gvanrossum'
closed_at = <Date 2016-09-09.16:53:54.889>
created_at = <Date 2016-09-07.14:08:38.915>
labels = ['interpreter-core', 'type-bug', 'docs']
title = 'Make "global after use" a SyntaxError'
updated_at = <Date 2016-09-09.16:53:54.888>
user = 'https://github.com/ilevkivskyi'

bugs.python.org fields:

activity = <Date 2016-09-09.16:53:54.888>
actor = 'gvanrossum'
assignee = 'gvanrossum'
closed = True
closed_date = <Date 2016-09-09.16:53:54.889>
closer = 'gvanrossum'
components = ['Documentation', 'Interpreter Core']
creation = <Date 2016-09-07.14:08:38.915>
creator = 'levkivskyi'
dependencies = []
files = ['44501', '44503']
hgrepos = []
issue_num = 27999
keywords = ['patch']
message_count = 10.0
messages = ['274813', '274836', '274888', '274894', '274895', '275314', '275318', '275332', '275344', '275346']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'docs@python', 'python-dev', 'yselivanov', 'levkivskyi']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27999'
versions = ['Python 3.6']

[ilevkivskyi]

The documentation at https://docs.python.org/3/reference/simple_stmts.html says that:

"Names listed in a global statement must not be used in the same code block textually preceding that global statement"

But then later:

"CPython implementation detail: The current implementation does not enforce the two restrictions,
but programs should not abuse this freedom, as future implementations may enforce them..."

Code like this

def f():
    x = 1
    global x

gives SyntaxWarning for several releases, maybe it is time to make it a SyntaxError?

[gvanrossum]

I like fixing this. I suppose it'll be fixed for nonlocal too?

[ilevkivskyi]

Yes, nonlocal will be fixed too. This will be a relatively small patch, but I am a bit afraid there could be merge conflicts (the code affected could overlap with changes for PEP-526).

Easiest way would be to simply make this a single patch with PEP-526 implementation (this will actually even slightly simplify the implementation).

Alternatively we could fix this right after the implementation patch is applied. What do you prefer?

[gvanrossum]

I strongly prefer to have it as a separate patch (either before or
after) to avoid the appearance of piggy-backing this in with a much
larger change.

[ilevkivskyi]

OK, then I think after will be safer. Let us came back to this right after PEP-526.

[ilevkivskyi]

I am attaching the patch.

Yury, I also added you here, just in case you have time for this.

Btw, while working on this I have found that the second restriction:

"""
Names listed in a global statement must not be ... in a for loop control target, class definition, function definition, or import statement.
"""

is not enforced at all, it does not give even a SyntaxWarning.
Is it something worth fixing?

[ilevkivskyi]

Added few tests to the patch

[gvanrossum]

I'll commit this.

[python-dev]

New changeset 804b71d43c85 by Guido van Rossum in branch 'default':
Issue bpo-27999: Make "global after use" a SyntaxError, and ditto for nonlocal.
https://hg.python.org/cpython/rev/804b71d43c85