with statement looks up __exit__ incorrectly

[benrg]

BPO	11428
Nosy	@birkenfeld, @benjaminp, @merwok

^{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/benjaminp'
closed_at = <Date 2011-03-07.07:35:02.078>
created_at = <Date 2011-03-07.07:19:16.528>
labels = ['interpreter-core', 'type-bug', 'invalid']
title = 'with statement looks up __exit__ incorrectly'
updated_at = <Date 2011-03-11.21:27:25.767>
user = 'https://bugs.python.org/benrg'

bugs.python.org fields:

activity = <Date 2011-03-11.21:27:25.767>
actor = 'eric.araujo'
assignee = 'benjamin.peterson'
closed = True
closed_date = <Date 2011-03-07.07:35:02.078>
closer = 'georg.brandl'
components = ['Interpreter Core']
creation = <Date 2011-03-07.07:19:16.528>
creator = 'benrg'
dependencies = []
files = []
hgrepos = []
issue_num = 11428
keywords = []
message_count = 5.0
messages = ['130231', '130235', '130241', '130257', '130615']
nosy_count = 4.0
nosy_names = ['georg.brandl', 'benjamin.peterson', 'eric.araujo', 'benrg']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue11428'
versions = ['Python 3.2']

[benrg]

class MakeContextHandler:
  def __init__(self, enter, exit):
    self.__enter__ = enter
    self.__exit__ = exit

with MakeContextHandler(lambda: None, lambda *e: None): pass

In 3.1.3 this worked; in 3.2 it raises AttributeError('__exit__'), which appears to be a bug.

[birkenfeld]

The bug is actually in 3.1 and fixed in 3.2: special methods (those with __underscore__ names) are supposed to be looked up on the class, not the instance.

[benrg]

But when I translate my example according to PEP-343, it works (i.e., doesn't raise an exception) in 3.2, and PEP-343 says "[t]he details of the above translation are intended to prescribe the exact semantics." So I think that at least one of PEP-343, the evaluation of mgr.__exit__, or the evaluation of with mgr: pass must be broken, though I'm no longer sure which.

[benjaminp]

Well, the pep is wrong, too.

[merwok]

After a quick perusal of the PEP, there are no wrong definitions of context managers (IOW they define methods in a class, not attribute on an object), but the pseudo-code explaining how the statement works use things like “mgr.__enter__” instead of “type(mgr).__enter__(mgr)”, which might be misleading.