Overview

Teaching: 5 min
Exercises: 5 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