There's nothing wrong with this at all, say:

    class MyClass:

        def __init__(self, my_dep1=None):
             if my_dep1 is None:
                 self.my_dep1 = get_my_dep1()  # Or potentially raise
             else:
                 self.my_dep1 = my_dep1

             [...]

    def test_my_class(): 
        mocked_dep1 = MagicMock()
        my_class = MyClass(my_dep1=mocked_dep1)
        [...]