Arrange, Act, Assert

This is part of a series of posts on Python unit testing.

The tests that we’ve written so far have been simple enough to fit in a single line of code:

def test_validate_url_returns_an_http_url_unmodified():
    assert validate_url('') == ''

This line of code actually does three things:

  1. It creates a string to be passed to validate_url() as argument.
  2. It calls validate_url(), passing the string argument.
  3. It asserts something about the results of calling validate_url().

Here’s the function with the three steps separated out:

def test_validate_url_returns_an_http_url_unmodified():
    url = ''

    validated_url = validate_url(url)
    assert validated_url == ''

For tests as simple as this one we would normally collapse the three steps into a single line. But for more complex tests the test code will be clearer if it’s separated into three steps, with an empty line between each step:

  1. Arrange: create and setup any objects that you need for the test (for example, arguments that you need to pass to the function under test).

  2. Act: call the function under test, once only.

  3. Assert: use Python’s assert to test something about the result of calling the function.

For example, remove_nipsa_action() is a function that takes an Elasticsearch index name and an annotation and returns an Elasticsearch action that removes the NIPSA flag from the annotation. For the purposes of this tutorial it doesn’t really matter what a “remove nipsa action” is, just know that remove_nipsa_action() takes an annotation (in the form of a dictionary) as argument and returns an Elasticsearch action (also a dictionary) in an expected format.

Here’s one of the tests for this function:

def test_remove_nipsa_action():
    # 1. Arrange: create the annotation dict that we need to pass to
    #    remove_nipsa_action().
    annotation = {"_id": "test_id", "_source": {"nipsa": True, "foo": "bar"}}

    # 2. Act: call remove_nipsa_action(), once only.
    action = remove_nipsa_action("bar", annotation)

    # 3. Assert something about the result.
    assert action == {
        "_op_type": "index",
        "_index": "bar",
        "_type": "annotation",
        "_id": "test_id",
        "_source": {"foo": "bar"},

This test still only creates a single object in the arrange step, and only makes a single assertion in the assert step. Even more complex tests may need to create multiple objects before calling the function, and then make multiple assertions at the end.

Almost all Hypothesis tests follow the arrange, act, assert recipe. This three step recipe is usually a good way to write a test. The consistency makes the tests easier to understand, and the recipe also clearly separates setup, what is being tested (the function call), and verification.

Arrange act assert also discourages the writing of complex tests that try to do too many things at once. If a test is complicated it can become hard to understand what the test is for, what it’s supposed to be testing, especially when a future change to the code causes the test to start failing.

For more, see Arrange Act Assert on

Tip: Try writing the assert part of a test first, and then filling in the first two steps.

Naming tests

As well as following the arrange act assert recipe, it can really help to make the intent of your test clear if you give the test function a good name. The name of a test method should clearly explain the intent of the test. The names of failing tests are printed out when the test fails, if they’re named well then the developer can often tell what has gone wrong from the names of the failing tests alone, without having to look into the test code. When you do have to look into the code, a good name helps to communicate what the test is intended to be testing.

Long function names are fine for test functions, since we don’t write code that calls our test functions (pytest calls them automatically for us) there’s no benefit to excessive terseness.

test_validate_url() or test_validate_url_1() are poor test names. test_validate_url_with_a_valid_http_url() is better. test_validate_url_returns_a_valid_http_url_unmodified() may be even better.

In the next post we’ll cover how to use factories to easily create test objects.

Sean Hammond,