Document "exec(stmt, global_dict, local_dict)" form in Python 2?

[mdickinson]

BPO	16339
Nosy	@mdickinson
Files	issue16339.patch issue16339_v2.patch issue16339_v3.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 = 'https://github.com/mdickinson'
closed_at = <Date 2012-11-25.13:28:33.952>
created_at = <Date 2012-10-27.17:31:07.387>
labels = ['easy', 'docs']
title = 'Document "exec(stmt, global_dict, local_dict)" form in Python 2?'
updated_at = <Date 2012-11-25.13:28:33.952>
user = 'https://github.com/mdickinson'

bugs.python.org fields:

activity = <Date 2012-11-25.13:28:33.952>
actor = 'mark.dickinson'
assignee = 'mark.dickinson'
closed = True
closed_date = <Date 2012-11-25.13:28:33.952>
closer = 'mark.dickinson'
components = ['Documentation']
creation = <Date 2012-10-27.17:31:07.387>
creator = 'mark.dickinson'
dependencies = []
files = ['28106', '28108', '28109']
hgrepos = []
issue_num = 16339
keywords = ['patch', 'easy']
message_count = 6.0
messages = ['173947', '174906', '176339', '176342', '176343', '176349']
nosy_count = 3.0
nosy_names = ['mark.dickinson', 'docs@python', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'patch review'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue16339'
versions = ['Python 2.7']

[mdickinson]

In Python 2, the 'exec' statement supports 'exec'-ing a (statement, globals, locals) tuple:

>>> exec("print 2", {}, {})
2

This doesn't seem to be documented at http://docs.python.org/reference/simple_stmts.html#the-exec-statement.

If I understand correctly, the 'exec tuple' form was originally there for backwards compatibility with ancient versions of Python; however, it now also conveniently provides the semblance of forwards compatibility with Python 3. It appears to be supported by current versions of Jython (though judging by bpo-403345 that wasn't always the case) and PyPy.

Is this omission intentional? Is this form of exec an official, supported part of the Python 2 language?

[mdickinson]

Thread on #python-dev: http://mail.python.org/pipermail/python-dev/2012-November/122543.html

If this is documented, direct tests for this form of exec should also be added.

[mdickinson]

Here's a patch for the documentation and tests.

[mdickinson]

Updated patch: drops mention of backwards compatibility, since that's not so useful (thanks, Ezio!). Adds note about resemblance to Python 3 code.

[mdickinson]

Rewording the sentence about Python 3.

[python-dev]

New changeset fd9c9579050d by Mark Dickinson in branch '2.7':
Issue bpo-16339: Document and test exec(stmt, globals, locals) form in Python 2.7.
http://hg.python.org/cpython/rev/fd9c9579050d