python - 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

element = element_source.get()
self.elements.append(element)
element_destination.send(elements)
self.remove_outdated_elements()

But when when i was trying to test this behavior, i've found that mocks don't copy their arguments.

>>> from unittest.mock import Mock
>>> m = Mock()
>>> a = [1]
>>> m(a)
<Mock name='mock()' id='139717658759824'>
>>> m.call_args
call([1])
>>> a.pop()
1
>>> m.assert_called_once_with([1])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.3/unittest/mock.py", line 737, in assert_called_once_with
    return self.assert_called_with(*args, **kwargs)
  File "/usr/lib/python3.3/unittest/mock.py", line 726, in assert_called_with
    raise AssertionError(msg)
AssertionError: Expected call: mock([1])
Actual call: mock([])

Is there a way to make Mock copy it's call arguments? If not, what is the best way to test this kind of behavior?

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='...'>