python runtime_error ai_generated true

StopIteration: Caught StopIteration from side_effect; consider using side_effect with an iterable

ID: python/unittest-mock-side-effect-iteration

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80%Fix Rate
86%Confidence
0Evidence
2025-11-05First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
3.9 active — — —

Root Cause

Setting mock.side_effect to an iterable (e.g., a generator) that raises StopIteration when exhausted, and the test calls the mock more times than the number of items in the iterable.

generic

中文

将 mock.side_effect 设置为可迭代对象（如生成器），当迭代耗尽时引发 StopIteration，而测试调用模拟的次数超过了可迭代对象中的项数。

Workarounds

  1. 85% success 
    Ensure the number of mock calls matches the length of the side_effect iterable, or use side_effect with a function that handles arbitrary calls.
  2. 80% success 
    Set side_effect to a list with enough items to cover all expected calls, and use a sentinel value for extra calls if needed.

Dead Ends

Common approaches that don't work:

  1. 50% fail

    Using a list instead of a generator for side_effect still raises StopIteration if the list is exhausted, but the error may be more predictable.

  2. 40% fail

    Setting side_effect to a function that returns a default value after exhaustion may mask the fact that the mock is called too many times.