1. Error for “fixture ‘mocker’ not found”
After running pytest, it reported:

E       fixture 'mocker' not found
>       available fixtures: cache, capfd, capsys, doctest_namespace, mock, mocker, monkeypatch, pytestconfig, record_xml_property, recwarn, request, requests_get, tmpdir, tmpdir_factory
>       use 'pytest --fixtures [testpath]' for help on them.

The solution is just installing the missing pip package:

pip install pytest-mock

2. How to make sure a function has been called without caring about its arguments?
There are two methods. The first method is using “.called”

    mocker.patch.object(EmailSender, "_email_spec")
    assert sender._email_spec.called

The second method is using “mocker.spy()”

    spy = mocker.spy(sender, "_email_spec")