Arrange, Act and Assert (AAA) Pattern

A pattern for structuring unit tests, where tests are broken up into 3 sections:

  • Arrange: where you prepare any fixtures and setup the test.
  • Act: usually a single method call which calls the system under test.
  • Assert: the final assertions which are run after act.

The system under test is always stored in a variable called sut .

Khorikov writes, by conforming to a standard like this, anyone can easily read and understand a test reducing maintenance cost.

Khorikov (2020) (pg. 42-43)


def test_datestring_is_formatted():
    # Arrange
    date_time_instance = datetime.datetime(2008, 1, 2)
    sut = DateFormat(date_time_instance)

    # Act
    result = sut.format()

    # Assert
    assert result == "2008-1-2"

Similar to the Given-When-Then pattern.


Vladimir Khorikov. Unit Testing: Principles, Practices, and Patterns. Manning, Shelter Island, NY, 2020. ISBN 978-1-61729-627-7.