warn on assignment to None, True, False

[jhylton]

BPO	549213
Nosy	@gvanrossum, @loewis
Files	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 = None
closed_at = <Date 2004-08-18.11:37:48.000>
created_at = <Date 2002-04-26.19:14:27.000>
labels = ['interpreter-core']
title = 'warn on assignment to None, True, False'
updated_at = <Date 2004-08-18.11:37:48.000>
user = 'https://bugs.python.org/jhylton'

bugs.python.org fields:

activity = <Date 2004-08-18.11:37:48.000>
actor = 'loewis'
assignee = 'jhylton'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2002-04-26.19:14:27.000>
creator = 'jhylton'
dependencies = []
files = ['4197']
hgrepos = []
issue_num = 549213
keywords = ['patch']
message_count = 10.0
messages = ['39723', '39724', '39725', '39726', '39727', '39728', '39729', '39730', '39731', '39732']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'loewis', 'jhylton']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue549213'
versions = []

[jhylton]

This patch issues a warning when None, True, or False
is assigned to. I don't know what the text of the
warning should say, but that's a minor matter.

[gvanrossum]

Logged In: YES
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Nice, but doesn't catch enough cases. E.g. using None as an
argument name, assigning to an attribute named None, "import
foo.None", "from foo.None import bar", "from None import
bar", "from None.foo import bar", using None as a keyword
argument in a call: "f(1, 2, None=3)".

[jhylton]

Logged In: YES
user_id=31392

Right. I'll have to fiddle the patch to make the check into
a function and we can see where else it should be applied.

[jhylton]

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Here's an improved pass, although the calls are getting a
bit messy. Not sure how worried I am, since I'd like to
replace this code anyway. (The worry is that I don't get
around to replacing it before 2.3.)

Also, fix com_arglist() to do only the work that is necessary.

[gvanrossum]

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Better. Some strange behavior remains:

from foo import None    # gives three warnings!
import None   # gives two warnings!

We need a test suite that systematically tests all cases.

[loewis]

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user_id=21627

Is this going to be part of Python 2.3?

[gvanrossum]

Logged In: YES
user_id=6380

Yes, I'd like it to be. I think the nits (duplicate warnings
in some cases) can be banged out during alpha testing.
Jeremy, can you check this in? It needs a small note in
Misc/NEWS.

[gvanrossum]

Logged In: YES
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Checked in, compile.c 2.269.

[gvanrossum]

Logged In: YES
user_id=6380

Alas, I spoke too soon. This patch broke some unit tests,
e.g. test_grammar.py; its seems it doesn't correctly
traverse into tuple-shaped arguments. It also causes some
warnings because there's code out there that assigns to True
and False in case it's being run under older Python versions.