How to mock a call to __next__

Question

I am trying to mock a coroutine. As such, this mock's __next__() and close() are called. While mocking close() works, I cannot mock __next__():

    mock_coroutine = mock.Mock()
    mock_coroutine.some_call(1, 2, 3)
    mock_coroutine.some_call.assert_called_with(1, 2, 3)
    mock_coroutine.close()
    mock_coroutine.close.assert_called_with()
    #this fails
    mock_coroutine.__next__()
    mock_coroutine.__next__.assert_called_with()

What am I missing? How to make sure my mock's __next__() method is called?

For now, I am using the following:

class MockCoroutine:
    def __next__(self):
        self.called_next = True

    def close(self):
        self.called_exit = True

However, I very much rather use a standard mock.

Answer 1

You need to use MagicMock, not Mock, to have magic methods like __next__ available by default:

>>> from unittest import mock
>>> mock_coroutine = mock.MagicMock()
>>> mock_coroutine.__next__()
<MagicMock name='mock.__next__()' id='4464126552'>
>>> mock_coroutine.__next__.assert_called_with()

Quoting from the documentation:

Mock allows you to assign functions (or other Mock instances) to magic methods and they will be called appropriately. The MagicMock class is just a Mock variant that has all of the magic methods pre-created for you (well, all the useful ones anyway).

So, alternatively, you could still use the regular Mock object, but then you need to explicitly add that attribute:

>>> mock_coroutine = mock.Mock()
>>> mock_coroutine.__next__ = mock.Mock()
>>> mock_coroutine.__next__()
<Mock name='mock.__next__()' id='4464139232'>

That's because although Mock creates attributes on the fly as you access them, any attributes with leading and trailing underscores are explicitly exempted from that. See this footnote:

唯一的例外是魔术方法和属性（具有前导和尾随双下划线的那些）。Mock 不会创建这些，而是​​会引发一个AttributeError. 这是因为解释器经常会隐式地请求这些方法，并且当它期望一个魔术方法时，会非常困惑地获取一个新的 Mock 对象。如果您需要魔术方法支持，请参阅魔术方法。

但请注意，魔术方法通常是在类上查找，而不是在实例上查找，因此直接将__next__属性添加到Mock实例仍然可能失败；该MagicMock课程会为您处理这个特定问题。