copyingmock 0.1

MagicMock has problems when used on a function with mutable arguments. This is called out in the documentation:

Another situation is rare, but can bite you, is when your mock is called with mutable arguments. call_args and call_args_list store references to the arguments. If the arguments are mutated by the code under test then you can no longer make assertions about what the values were when the mock was called.

They then go on to propose a workaround, using side_effect, but it’s not very likable. There is also an elegant recipe offered which copies arguments at call-time. It’s simply a subclass of MagicMock which copies the arguments, instead of storing references. I’m not sure why the recipe wasn’t included directly in mock, so here it is as a third-party package.