Validate callbacks in 'contextlib.ExitStack.callback()'

[YurySelivanov]

BPO	15185
Nosy	@ncoghlan, @serhiy-storchaka, @1st1
Files	contextlib.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 = None
closed_at = None
created_at = <Date 2012-06-25.22:31:40.198>
labels = ['type-feature', 'library']
title = "Validate callbacks in 'contextlib.ExitStack.callback()'"
updated_at = <Date 2020-04-25.16:18:48.598>
user = 'https://bugs.python.org/YurySelivanov'

bugs.python.org fields:

activity = <Date 2020-04-25.16:18:48.598>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2012-06-25.22:31:40.198>
creator = 'Yury.Selivanov'
dependencies = []
files = ['26159']
hgrepos = []
issue_num = 15185
keywords = ['patch']
message_count = 4.0
messages = ['164021', '164024', '164025', '367276']
nosy_count = 4.0
nosy_names = ['ncoghlan', 'Yury.Selivanov', 'serhiy.storchaka', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue15185'
versions = ['Python 3.5']

[YurySelivanov]

With this patch 'contextlib.ExitStack.callback(callback, *args, **kwds)' will validate that the passed callback actually accept the passed '*args' and '**kwds'.

P.S. If this patch comes through - I'll create two more issues for 'atexit.register' and 'unittest.TestCase.addCleanup'. (I'll also look through the stdlib to find if there are some other places where this is needed.)

[1st1]

(my previous message edit: no support for 'atexit' - it's implemented in C)

[ncoghlan]

I'm not comfortable adding a dependency on inspect to the contextlib module, and I think it would be undesirable to have too many cases where the early validation is silently skipped.

While I like the idea of adding early signature validation to these APIs in principle, given the above concerns and the fact this probably crosses the line into "feature" territory, I think we want to wait until 3.4 and the introduction of a mechanism that adds introspection support to callables implemented in C.

That will likely involve moving the signature support somewhere lower in the module stack (perhaps to a private "_signature" module, with inspect remaining the official API), and will reduce the number of cases where pre-validation is skipped.

[serhiy-storchaka]

I think it is rather a work for linter's and static type checking. Runtime check adds an overhead, and it is expected that in correct program it will always pass. You need the check only while write the program, it may help in debugging. But once it has been debugged and tested, the check becomes a waste of CPU time.

Since this issue was not touched for 8 years and there is a large progress in static code analysis, I suggest to close it.