Lecture 0b100000

Testing and pytest

MCS 260 Fall 2020
David Dumas

Reminders

assert

Python's assert statement is simple: It is followed by an 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.

        assert fact(0)==1
    

        if not fact(0)==1:
            raise AssertionError()
    

        assert thing_expected_to_always_be_truthy
    

Testing

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.

Testing in Python

Many options for orchestrating tests.

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

pytest

I hope you've installed pytest, a popular Python module for testing. Reminder: These shell commands should install and run pytest for most of you.

        python -m pip install pytest  # install pytest
        python -m pytest              # run pytest
    

pytest basics

To use pytest, you need some Python source files with names that begin test_ in which there are functions that begin test_.

Each such function should run a single test, returning if it succeeds and raising AssertionError (or another Exception) if it fails.

First example: Let's make tests for fact.

Running tests

In the directory containing your program, run the shell command

        python -m pytest
    

to find and run all tests. A helpful report is displayed.

It's best to have just one program in a directory when using pytest.

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.