example_with_mock.py

from unittest import TestCase
from unittest.mock import patch
from email.mime.text import MIMEText
import os
import smtplib


def send_email(email_from, email_to, email_subject, email_msg):

    with smtplib.SMTP(os.getenv('SMTP_SERVER','localhost')) as smtpObj:
        to_msg = MIMEText(email_msg)
        to_msg['Subject'] = email_subject
        to_msg['From'] = email_from
        to_msg['To'] = email_to
        smtpObj.sendmail(email_from, email_to.split(','), to_msg.as_string())


class TestFunction(TestCase):

    @patch.object(smtplib.SMTP, 'sendmail')
    @patch.object(smtplib.SMTP, 'close')
    def test_send_email(self, mock_close, mock_sendmail):
        test_message = MIMEText('hello')
        test_message['Subject'] = 'TEST'
        test_message['From'] = 'testuser@test.com'
        test_message['To'] = 'testuser@test.com,testuser2@test.com'
        
        send_email(test_message['From'],
                   test_message['TO'],
                   test_message['Subject'],
                   'hello')
        mock_close.assert_called()
        mock_sendmail.assert_called_with(test_message['From'],
                                         test_message['To'].split(','),
                                         test_message.as_string())


if __name__ == '__main__':
    TestFunction()