Being Assertive

Teaching 5 min

Exercises 5 min

Estimated 10 min

Questions

What happens with assertions in Python?

Objectives

Understand how assertions in Python correspond to exit codes
Figure out how pytest fits in

This is a relatively short section, but we need to connect some things you’ve learned from testing in Python with exit codes.

Assert Your Tests

An assertion is a sanity-check carried out by the assert statement, useful when debugging code.

Let’s create a file called python_assert.py with the following content:

x = "hello"

assert x == "hello"

assert x == "goodbye"

and then run it with python python_assert.py.

What happens when an assertion fails in Python?

Traceback (most recent call last):
  File "./python_assert.py", line 5, in <module>
    assert x == "goodbye"
AssertionError

An exception is raised: AssertionError. The nice thing about Python is that all unhandled exceptions return a non-zero exit code. If an exit code is not set, this defaults to 1.

> echo $?
1

Ignoring what would cause the assertion to be True or False, we can see that assertions automatically indicate failure in a script.

What about pytest?

Pytest, thankfully, handles these assertion failures intuitively. To try this out quickly, go ahead and modify python_assert.py as follows:

x = "hello"

def test_assert_success():
    assert x == "hello"

def test_assert_failure():
    assert x == "goodbye"

Running pytest python_assert.py will produce an expected exit code depending on whether the test passed or failed. You should be able to confirm that the exit codes are useful here.

Key Points

You can do whatever you like in a test, as long as you return the right exit code
Pytest, and other test utilities will propagate the exit codes correctly

Exit Codes Understanding YAML