Why python's monkeypatch doesn't work when importing a class instead of a module?

Question

I was having issues while using the code of the accepted answer here.

The code works depending on how I do the import of datetime. Why is that? Is it possible to mock it so it works both ways?

I am using Python 3.4. The following code illustrates the problem:

import pytest from datetime import datetime  mockdate = datetime(2000, 1, 1, 0, 0, 0)  @pytest.fixture(autouse=True) def patch_datetime_now(monkeypatch):     class mydatetime:         @classmethod         def now(cls):             return mockdate      monkeypatch.setattr('datetime.datetime', mydatetime)  def test_doesnt_work():     assert datetime.now() == mockdate  def test_works():     import datetime     assert datetime.datetime.now() == mockdate 

Answer 1

Even if you aren't using mock framework you should take a look to where to patch chapter. By writing

from datetime import datetime 

You are creating a new reference to datetime.datetime in your test module and call it datetime. That is the reference that you use in test_doesnt_work() test.

By writing

monkeypatch.setattr('datetime.datetime', mydatetime) 

You are patching datetime's absolute reference in datetime module: the one used in test_works().

Answer 2

@Michele d'Amico's answer explains why it doesn't work. This is how to make it work if you want to use "from datetime import datetime" instead of just "import datetime"

monkeypatch.setattr(__name__ + '.datetime', mydatetime)