python type_error ai_generated true

TypeError: unsupported operand type(s) for +: 'Mock' and 'int'

ID: python/unittest-mock-magic-method-not-supported

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80%Fix Rate
89%Confidence
0Evidence
2024-02-14First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
3.8 active — — —
3.10 active — — —
3.12 active — — —

Root Cause

A plain unittest.mock.Mock does not implement magic methods like __add__, __len__, __iter__. Use MagicMock (or AsyncMock) when the code under test relies on dunder methods.

generic

中文

普通的 unittest.mock.Mock 不实现 __add__、__len__、__iter__ 等魔法方法。当被测代码依赖 dunder 方法时，应使用 MagicMock（或 AsyncMock）。

Workarounds

  1. 98% success 
    from unittest.mock import MagicMock
m = MagicMock()
m.__add__.return_value = 42
assert m + 1 == 42
  2. 90% success 
    from unittest.mock import AsyncMock
m = AsyncMock()
m.__aenter__.return_value = resource

Dead Ends

Common approaches that don't work:

  1. 90% fail

    Plain Mock ignores magic method assignments; you must use MagicMock for them to take effect.

  2. 95% fail

    int(Mock()) raises TypeError because Mock has no __int__; converting does not help.