python type_error ai_generated true

AttributeError: Mock object has no attribute 'return_value' when setting return_value on a non-callable mock

ID: python/unittest-mock-return-value-override

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80%Fix Rate
86%Confidence
0Evidence
2025-01-10First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
3.8 active — — —
3.9 active — — —

Root Cause

Attempting to set return_value on a Mock instance that was created with spec=None and not as a callable, but the mock is being used as if it were a function.

generic

中文

尝试在创建时spec=None且非可调用的Mock实例上设置return_value，但该mock被当作函数使用。

Workarounds

  1. 90% success Ensure the mock is created as a callable by using MagicMock or by passing spec=SomeCallable 
    from unittest.mock import MagicMock
mock = MagicMock()
mock.return_value = 42
result = mock()  # Works
Or: mock = Mock(spec=lambda: None)
mock.return_value = 42
  2. 0% success Use side_effect instead of return_value for non-callable mocks

Dead Ends

Common approaches that don't work:

  1. Creating a new MagicMock instead of Mock 50% fail

    MagicMock is callable by default, but the underlying issue may be that the mock is not intended to be callable.

  2. Setting return_value directly without checking if the mock is callable 70% fail

    This will still raise an AttributeError if the mock is not callable.