Thanks Vegard for the report. The docs say "Child mocks and the return value mock (if any) are reset as well." so it makes sense to me to reset the return_value and side_effect of children. But as mentioned in the PR it's a behaviour change that someone might be dependent on and can go on 3.9 . If we are not going ahead with the change then we can clarify the same in the docs and ask the user to reset mock recursively on their own so that it avoids confusion.