python type_error ai_generated true

TypeError: test_foo() missing 1 required positional argument: 'mock_open'

ID: python/unittest-patch-decorator-argument-order

Also available as: JSON · Markdown · 中文
80%Fix Rate
87%Confidence
0Evidence
2024-02-07First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
3.8 active — — —
3.12 active — — —

Root Cause

Multiple @patch decorators inject mocks in bottom-up order, but the test signature lists them top-down, so arguments are bound to the wrong parameters.

generic

中文

多个 @patch 装饰器按从下到上的顺序注入 mock，但测试签名按从上到下列出，导致参数绑定错误。

Workarounds

  1. 95% success 
    @patch('myapp.db.open')
@patch('myapp.fs.read')
def test_foo(mock_read, mock_open):
    # mock_read corresponds to the bottom decorator
    ...
  2. 90% success 
    def test_foo():
    with patch('myapp.fs.read') as mock_read, \
         patch('myapp.db.open') as mock_open:
        ...

Dead Ends

Common approaches that don't work:

  1. 50% fail

    Fixes one test but the convention is undocumented; other tests in the file break the same way.

  2. 60% fail

    Mocks are positional; *args swallows them but makes assertions awkward and hides binding bugs.