This is my code

from mail_api import send_mail

def my_func():
    # do some work
    send_mail(to="test@example.com", content="data)
    # do some more work
    return result

Here is my unit test. I want to test for the return value but I do not want to send out any email at all from the unit test. That’s why I patch the send_mail function.

from mock import patch

  class MyTest(unittest.TestCase):

    @patch('mail_api.send_mail')
    def test_my_func(self, send_mail):
      self.assertEquals(my_func(), expected_result)

To my surprise an email is still sent out when the test is run!

It turns out my_func still retains a reference to the original send_mail function. If I want to successfully patch the mail api, I need to rewrite my_func like this:

import mail_api

def my_func():
    # do some work
    mail_api.send_mail(to="test@example.com", content="data)
    # do some more work
    return result