AttributeError should report the same details when raised by lookup_special() as when raised in the REPL

[dholth]

BPO	12022
Nosy	@gvanrossum, @benjaminp, @merwok, @durban, @dholth, @serhiy-storchaka, @iritkatriel
PRs	bpo-12022: Change error type for bad objects in "with" and "async with" #26809

^{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/serhiy-storchaka'
closed_at = <Date 2021-06-29.08:29:06.821>
created_at = <Date 2011-05-06.20:07:21.538>
labels = ['interpreter-core', 'type-bug', '3.11']
title = 'AttributeError should report the same details when raised by lookup_special() as when raised in the REPL'
updated_at = <Date 2021-06-29.08:29:06.820>
user = 'https://github.com/dholth'

bugs.python.org fields:

activity = <Date 2021-06-29.08:29:06.820>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2021-06-29.08:29:06.821>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2011-05-06.20:07:21.538>
creator = 'dholth'
dependencies = []
files = []
hgrepos = []
issue_num = 12022
keywords = ['patch']
message_count = 9.0
messages = ['135365', '135388', '135484', '135491', '136640', '354977', '396148', '396165', '396707']
nosy_count = 7.0
nosy_names = ['gvanrossum', 'benjamin.peterson', 'eric.araujo', 'daniel.urban', 'dholth', 'serhiy.storchaka', 'iritkatriel']
pr_nums = ['26809']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue12022'
versions = ['Python 3.11']

[dholth]

"How much do we care about special method lookup?" Python recently bypasses __getattr__ entirely when looking up context managers. http://mail.python.org/pipermail/python-dev/2009-May/089535.html

Could this be the reason that ZODB's transaction module, which attempts to be a context manager by declaring

manager = ThreadTransactionManager()
__enter__ = manager.get
__exit__ = manager.__exit__

Does not work in Python 2.7.1 on Ubuntu 11.04 or RHEL5? Frustratingly, the exception is no more specific than an AttributeError, even though hasattr(transaction, '__exit__')?

I would prefer to never get AttributeError: transaction.__exit__ when hasattr(transaction, '__exit__') as I find that to be very confusing. Maybe the interpreter could raise SpecialAttributeError('transaction.__exit__ is not sufficiently special') instead.

http://svn.zope.org/repos/main/transaction/trunk/transaction/__init__.py

[benjaminp]

Yes, that's why. I suggest you appeal to python-ideas about the new exception.

[dholth]

Python should explain AttributeError in the same way when it's raised by the interpreter. The with: statement below should raise the second AttributeError, not the first.

import transaction
with transaction: pass
>>> AttributeError: __exit__

import sys
sys.__exit__
>>> AttributeError: 'module' object has no attribute '__exit__'

[dholth]

Thank you Benjamin for following up on this issue

[merwok]

hasattr(transaction, '__exit__')

http://docs.python.org/dev/reference/datamodel#special-method-names explains that magic methods are looked up on the class, not on the instances. There’s a lot of code out there that erroneously checks for __len__ or __call__ on instances, and this is the second time to my knowledge that a project abused a module-level __enter__ function.

[serhiy-storchaka]

Would not be better to change an AttributeError to TypeError? Seems this is the only place in the core when missing special method causes an AttributeError and not TypeError.

[iritkatriel]

I've reproduced this on 3.11 (though the AttributeError is now on __enter__ and not __exit__ as was the case in msg135484):

>>> import transaction
>>> with transaction: pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: __enter__

>>> import sys
>>> sys.__exit__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'sys' has no attribute '__exit__'
>>>