Lecture 0b100000

Testing and pytest

MCS 260 Fall 2021
Emily Dumas


  • Project 3 due at 6pm
  • Project 4 description coming soon
  • Homework 11 open, due Tuesday at 10am
  • assert

    Python's assert statement is simple: It is followed by an expression, usually a boolean expression. If that expression is truthy, nothing happens. If it is falsy, the AssertionError exception is raised.

    One use for assert is during debugging of a program under development: It stops execution if expected conditions are not met.

    For example,
            assert fact(0)==1
    is mostly equivalent to
            if not fact(0)==1:
                raise AssertionError()
    So a good use for assert is
            assert thing_expected_to_always_be_truthy


    In software development, testing refers to the process of checking that a program does what it is supposed to do, i.e. that it meets the desired specifications.

    Most often this takes the form of code that performs testing in an automated way.

    Some software development approaches rely very heavily on testing.

    Simplest testing approach

    Make some scripts that test your modules and programs.

    E.g. module dynam.py might tested by a script test_dynam.py.

    In this approach, finding, running, and interpreting results of tests is all manual.

    Automated testing in Python

    Many options for orchestrating tests.

    • unittest (part of the standard library)
    • pytest (our focus today)
    • nose
    • many others...


    Pytest is not typically included with Python. You need to install it before you can use it. This command should do that:

            python3 -m pip install pytest

    Then test to make sure it's working with

            python3 -m pytest

    pytest basics

    Put tests in files named test_????.py.

    Put each test in a function named test_????().

    Each test should return normally if it succeeds, or raise an exception if it fails.

    In a terminal, the command

            python3 -m pytest

    will find all tests (in current dir), run them, and report.


    Let's make tests for the module factorial.

    Recommended process

    Make tests that encode essential functions of the program, with most of them testing a single part (function, class, etc.) in isolation. These are unit tests.

    Every time you make a change to the program, run the tests again.

    • If all tests pass: Proceed.
    • If a test fails: Correct the code or correct the test.

    Asking for resources

    Most pytest test functions accept no arguments. But if you write one with an argument called tmpdir, pytest will create a temporary directory before the test runs, and delete it when the test ends.

    The tmpdir argument will be an object that contains data about the created directory. Calling str(tmpdir) will give the full path as a string.

    This feature is very helpful when a test involves writing and reading files.


    Revision history

    • 2021-11-05 Initial publication