Patcher stop method should be idempotent

[sfreilich]

BPO	36366
Nosy	@brettcannon, @cjw296, @voidspace, @matrixise, @mariocj89, @miss-islington, @tirkarthi, @sfreilich
PRs	bpo-36366: Return None on stopping unstarted patch object #12472

^{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 2019-03-28.21:10:27.538>
created_at = <Date 2019-03-19.15:57:41.523>
labels = ['type-bug', '3.8']
title = 'Patcher stop method should be idempotent'
updated_at = <Date 2019-03-28.21:10:27.535>
user = 'https://github.com/sfreilich'

bugs.python.org fields:

activity = <Date 2019-03-28.21:10:27.535>
actor = 'brett.cannon'
assignee = 'none'
closed = True
closed_date = <Date 2019-03-28.21:10:27.538>
closer = 'brett.cannon'
components = []
creation = <Date 2019-03-19.15:57:41.523>
creator = 'sfreilich'
dependencies = []
files = []
hgrepos = []
issue_num = 36366
keywords = ['patch']
message_count = 7.0
messages = ['338371', '338372', '338379', '338395', '338436', '339075', '339076']
nosy_count = 8.0
nosy_names = ['brett.cannon', 'cjw296', 'michael.foord', 'matrixise', 'mariocj89', 'miss-islington', 'xtreak', 'sfreilich']
pr_nums = ['12472']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue36366'
versions = ['Python 3.8']

[sfreilich]

Currently, it's an error to call the stop() method of a patcher object created by mock.patch repeatedly:

>>> patch = mock.patch.object(Foo, 'BAR', 'x')
>>> patch.start()
'x'
>>> patch.stop()
>>> patch.stop()
RuntimeError: stop called on unstarted patcher

This causes unnecessary problems when test classes using mock.patch.stopall for cleanup are mixed with those cleaning up patches individually:

class TestA(unittest.TestCase):

  def setUp():
    patcher = mock.patch.object(...)
    self.mock_a = patcher.start()
    self.addCleanup(patcher.stop)

...

class TestB(TestA):

  def setUp():
    super().setUp()
    self.addCleanup(mock.patch.stopall)
    self.mock_b = mock.patch.object(...).start()

...

This fails because mock.patch.stopall stops the patch set up in TestA.setUp(), then that raises an exception when it's stopped again.

But why does patcher.stop() enforce that precondition? Wouldn't it be sufficient for it to just enforce the postcondition, that after stop() is called the patch is stopped()? That would make it easier to write test code which makes use of mock.patch.stopall, which allows the proper cleanup of patches to be configured much more concisely.

[matrixise]

@samuel,

1. What's the version of Python?
2. Could you provide a script with an example?

Thank you

[tirkarthi]

When mock.patch is creates a patch object and patch.start calls __enter__ that sets is_local. On stop __exit__ is called where a check is done is to make sure is_local attribute is present and then cleanup is done along with deleting calling del self.is_local so calling stop second time causes the attribute check to fail. There is no specific reason I could find with git history.

It seems that calling patch.stop without patch.start makes cleanup to happen on unpatched objects and raises errors which I hope is avoided by always setting is_local on start and removing it on stop like a flag. That being said I am not sure why a early return couldn't be made when is_local is absent instead of proceeding with cleanup logic or raising a runtime error. I see no tests failing on early return except a test where RuntimeError is intentionally tested by calling stop on unstarted patch. I have added mock module devs for some context.

A sample script would be as below :

from unittest import mock

class Foo:
    bar = None

patch = mock.patch.object(Foo, 'bar', 'x')
patch.start()
patch.stop()
patch.stop()

[voidspace]

It's almost certainly an oversight rather than a design decision. I'd be happy with the change you suggest Karthikeyan.

[tirkarthi]

Thanks for the confirmation. I will raise a PR for this.

[miss-islington]

New changeset 02b84cb by Miss Islington (bot) (Xtreak) in branch 'master':
bpo-36366: Return None on stopping unstarted patch object (GH-12472)
02b84cb

[brettcannon]

Thanks, everyone!