pytest-dev/pytest-mock

feature request: fail on improper use of mock.side_effect

upstartjohnvandivier opened this issue · 2 comments

upstartjohnvandivier commented

What's the problem this feature will solve?

Mock.side_effect is tricky to use properly. Problem cases include:

setting the value to an array of length less than two, in which case return_value is more readable and debuggable
setting the value to an array of length n where the value doesn't change. This can be dangerous when the value is another mock or otherwise mutable byref
Having the test simply fail in these cases prevents other hard-to-trace errors from arising in the first place.

Describe the solution you'd like

two proposed options:

throw an error which includes an instructive message like "please ensure that a method under side_effect is called more than once, otherwise prefer return_value
whenever side_effect is set, automatically create an assertion that the method under side effect is not called only once
this behavior would only trigger when the side_effect has a non-None return value; ie it is exempt if it is actually creating a side effect like throwing an error instead of being used as a sort of dynamic return_value tool

Open to others, of course

Alternative Solutions

feature request also filed on ruff astral-sh/ruff#6199 and we have considered a custom internal linter (not ideal ofc)

nicoddemus commented

Hi @upstartjohnvandivier,

Thanks for writing!

Note that this is a thin-wrapper for easier pytest integration -- this library does not implement Mock.side_effect, this is implemented in the unittest.mock standard library.

I believe this issue should be brought up upstream.

Closing for now, feel free to follow up! 👍

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