mock's autospec's behavior on method-bound builtin functions is broken

[habnabit]

BPO	29403
Nosy	@voidspace, @berkerpeksag, @habnabit, @csabella
PRs	bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions #3 [3.6] bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions (GH-3) #2807 [3.5] bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions (GH-3) #2809
Files	mock.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 = <Date 2017-07-21.23:27:59.508>
created_at = <Date 2017-01-31.23:01:55.161>
labels = ['3.7', 'type-bug', 'library']
title = "mock's autospec's behavior on method-bound builtin functions is broken"
updated_at = <Date 2017-07-21.23:27:59.506>
user = 'https://github.com/habnabit'

bugs.python.org fields:

activity = <Date 2017-07-21.23:27:59.506>
actor = 'berker.peksag'
assignee = 'none'
closed = True
closed_date = <Date 2017-07-21.23:27:59.508>
closer = 'berker.peksag'
components = ['Library (Lib)']
creation = <Date 2017-01-31.23:01:55.161>
creator = 'Aaron Gallagher'
dependencies = []
files = ['46468']
hgrepos = []
issue_num = 29403
keywords = ['patch']
message_count = 7.0
messages = ['286565', '286566', '295249', '298696', '298827', '298828', '298829']
nosy_count = 4.0
nosy_names = ['michael.foord', 'berker.peksag', 'Aaron Gallagher', 'cheryl.sabella']
pr_nums = ['3', '2807', '2809']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29403'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7']

[habnabit]

Cython will, in the right circumstances, offer a MethodType instance where im_func is a builtin function. Any instance of MethodType is automatically assumed to be a python-defined function (more specifically, a function that has an inspectable signature), but _set_signature was still conservative in its assumptions. As a result _set_signature would return early with None instead of a mock since the im_func had no inspectable signature. This causes problems deeper inside mock, as _set_signature is assumed to always return a mock, and nothing checked its return value.

In similar corner cases, autospec will simply not check the spec of the function, so _set_signature is amended to now return early with the original, not-wrapped mock object.

There is another None early return in _set_signature, but it seems.. harder to trigger? It seems more appropriate to raise an exception there if this is being fixed.

[habnabit]

Patch adapted from testing-cabal/mock#389

[csabella]

This PR appears to have been approved, perhaps pending input from michael.foord?

[berkerpeksag]

New changeset 856cbcc by Berker Peksag (Aaron Gallagher) in branch 'master':
bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions (GH-3)
856cbcc

[berkerpeksag]

New changeset 64b9a15 by Berker Peksag in branch '3.6':
[3.6] bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions (GH-3)
64b9a15

[berkerpeksag]

New changeset e78dc0a by Berker Peksag in branch '3.5':
[3.5] bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions (GH-3)
e78dc0a

[berkerpeksag]

Thank you for the PR, Aaron!