python type_error ai_generated true

TypeError: test_x() missing 1 required positional argument: 'mock_b' During handling of the above exception, another exception occurred: TypeError: test_x() takes 1 positional argument but 3 were given

ID: python/pytest-unittest-mock-patch-decorator-order

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80%Fix Rate
90%Confidence
0Evidence
2024-02-14First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
3.8 active — — —
3.11 active — — —
3.12 active — — —

Root Cause

Stacked @patch decorators inject mocks in bottom-up order, but the test function parameters were written top-down (or vice versa). This mismatches mock-to-parameter assignment.

generic

中文

堆叠的 @patch 装饰器按自下而上的顺序注入 mock，但测试函数参数按自上而下（或相反）编写。这导致 mock 与参数对应关系错位。

Workarounds

  1. 95% success 
    Remember: decorators apply bottom-up, so the LAST decorator is the FIRST parameter:
  @patch('a.foo')
  @patch('a.bar')
  def test_x(mock_bar, mock_foo): ...
  2. 93% success 
    Use context managers instead of decorators to avoid ordering confusion:
  def test_x():
      with patch('a.foo') as mf, patch('a.bar') as mb:
          ...

Dead Ends

Common approaches that don't work:

  1. 85% fail

    unittest passes mocks positionally; *args accepts them but the names inside the body no longer refer to the intended mocks.

  2. 60% fail

    Non-deterministic across refactors; masks the real order rule.