How can i check call arguments if they will change with unittest.mock

Question

One of my classes accumulates values in a list, uses the list as an argument to a method on another object and deletes some of the values in this list. Something like

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Answer 1

There is a chapter "Coping with mutable arguments" in the documentation, which suggests several solutions to your problem.

I'd go with this one:

>>> from copy import deepcopy
>>> class CopyingMock(MagicMock):
...     def __call__(self, *args, **kwargs):
...         args = deepcopy(args)
...         kwargs = deepcopy(kwargs)
...         return super(CopyingMock, self).__call__(*args, **kwargs)
...
>>> c = CopyingMock(return_value=None)
>>> arg = set()
>>> c(arg)
>>> arg.add(1)
>>> c.assert_called_with(set())
>>> c.assert_called_with(arg)
Traceback (most recent call last):
    ...
AssertionError: Expected call: mock(set([1]))
Actual call: mock(set([]))
>>> c.foo
<CopyingMock name='mock.foo' id='...'>