[New-bugs-announce] [issue43478] Disallow Mock spec arguments from being Mocks

[Matthew Suozzo]

New submission from Matthew Suozzo <msuozzo at google.com>:

An unfortunately common pattern over large codebases of Python tests is for spec'd Mock instances to be provided with Mock objects as their specs. This gives the false sense that a spec constraint is being applied when, in fact, nothing will be disallowed.

The two most frequently observed occurrences of this anti-pattern are as follows:

* Re-patching an existing autospec.

  def setUp(self):
    mock.patch.object(mod, 'Klass', autospec=True).start()
    self.addCleanup(mock.patch.stopall)

  @mock.patch.object(mod, 'Klass', autospec=True)  # :(
  def testFoo(self, mock_klass):
    # mod.Klass has no effective spec.

* Deriving an autospec Mock from an already-mocked object

  def setUp(self):
    mock.patch.object(mod, 'Klass').start()
    ...
    mock_klass = mock.create_autospec(mod.Klass)  # :(
    # mock_klass has no effective spec

This is fairly easy to detect using _is_instance_mock at patch time however it can break existing tests.

I have a patch ready and it seems like this error case is not frequent enough that it would be disruptive to address.

Another option would be add it as a warning for a version e.g. 3.10 and then potentially make it a breaking change in 3.11. However considering this is a test-only change with a fairly clear path to fix it, that might be overly cautious.

----------
components: Tests
messages: 388532
nosy: msuozzo
priority: normal
severity: normal
status: open
title: Disallow Mock spec arguments from being Mocks
type: enhancement
versions: Python 3.10

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43478>
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