我如何检查调用参数是否会随 unittest.mock 改变

Question

我的一个类在一个列表中累积值，将该列表用作另一个对象的方法的参数，并删除该列表中的一些值。类似的东西

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

但是当我尝试测试这种行为时，我发现模拟不会复制它们的参数。

>>> 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([])

有没有办法让 Mock 复制它的调用参数？如果不是，那么测试这种行为的最佳方法是什么？

Answer 1

文档中有一个章节“Coping with mutable arguments”，它为您的问题提出了几种解决方案。

我会选择这个：

>>> 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='...'>