how to make a Python Test Driven Development environment manually

This is one way to make a Python Test Driven Development project. I walk through making the folders (directories) and files for the environment, including setting up the first test.

By the end of the chapter you will know these commands better

cd
tree
mkdir
touch
cat
mv
python3 -m unittest
history

cd
tree
mkdir
New-Item
cat
Move-Item
python -m unittest
history

requirements

questions about making a Python Test Driven Development Environment

how to setup the project

how to setup a project with uv

how to change directory to the project

  • I try to change directory to the person folder again

    cd person

    the terminal shows

    .../pumping_python/person

    uv made the directory for me.

  • I use tree to see what uv added to the folder

    tree -a -L 2

    the -L option tells tree how deep to go when showing the folders and files, I use 2 to make it show only the first level of contents of the child folders.

    tree

    uv added a few files and folders:

    • .git this folder makes the project a git repository, which makes it easy to keep track of changes I make. If I publish the repository I can work on the project from any computer anywhere (as long as it is has access to the repository).

    • .gitignore is a file that tells git what files in the project to not keep track of, which helps if there are things I do not want or need to share.

    • .python-version is a file that has the version of Python I am using, it helps if I do projects with different Python versions.

    • README.md is a file that is used to describe the project.

    • src is a folder that contains the code for the project (the source code).

    • pyproject.toml is a file that is used to configure Python projects for packaging see pyproject.toml for more

how to see what is inside a file

  • I can use the cat program to see what is inside .gitignore

    cat .gitignore

  • I use cat to see what is inside pyproject.toml

    cat pyproject.toml

  • I use cat to see what is in .python-version

    cat .python-version

    the terminal is my friend, and shows

    3.XY

    where XY are numbers like 14 depending on what version of Python is installed.

  • I use cat to show what is inside README.md

    cat README.md

how to run a Python program

  • I use Python to run the person program

    python3 src/person/__init__.py

    python src/person/__init__.py

    No errors! The uv Python package manager made the file with some Python code in it that I can run.

  • I can also use uv to run the program

    uv run person

    the terminal shows

    Hello from person!

    Success!

  • I use cat to see what is in src/person/__init__.py

    cat src/person/__init__.py

test_failure

how to manually run tests

how to make a directory for the tests

  • I make a child folder to keep the tests separate from the other files

    mkdir tests

  • I use tree to see what my project now looks like

    tree -a -L 1

    tree

    there is a new folder named tests.

how to make a Python file for the tests in the ‘tests’ directory

  • I use touch to add an empty file to the tests directory for the actual test

    touch tests/person.py

  • I use tree to see what the project looks like now

    tree -a -L 2

  • I run the test again

    python3 -m unittest

  • I use New-Item to add an empty file to the tests directory for the actual test

    New-Item tests/person.py

  • I use tree to see what the project looks like now

    tree

  • I run the test again

    python -m unittest

the terminal is my friend, and shows

NO TESTS RAN

because I have not set up the test correctly.

RED: make it fail

how to make the tests a Python package

Danger

use 2 underscores (__) before and after init for __init__.py not _init_.py

  • I use touch to add an empty file with the name __init__.py to the tests folder

    touch tests/__init__.py

  • I run the tree command to see what changed

    tree -a -L 2

  • I try to run the test again

    python3 -m unittest

  • I use New-Item to add an empty file with the name __init__.py to the tests folder

    New-Item tests/__init__.py

  • I run the tree command to see what changed

    tree

  • I try to run the test again

    python -m unittest

the terminal does not feel like my friend, and shows

NO TESTS RAN

because unittest does not know that tests/person.py is a test file.

I have to change the name to make sure it starts with test_.

how to change the name of a file

  • I close tests/person.py

    Danger

    if you do not close tests/person.py, there will be 3 files in the tests folder after the next step (instead of 2), because the Auto Save feature (enabled earlier) will save the original file if it is still open after you change its name.

  • I use the mv program to change the name of tests/person.py to tests/test_person.py

    mv tests/person.py tests/test_person.py

  • I use tree with the -L option to see what I have so far

    tree -a -L 2

  • I run the test again

    python3 -m unittest

  • I use Move-Item to change the name of tests/person.py to tests/test_person.py

    Move-Item tests/person.py tests/test_person.py

  • I use tree to see what I have so far

    tree

  • I run the test again

    python -m unittest

the terminal still shows NO TESTS RAN

  • I add assert before False is True in tests/test_person.py

    1# False is True
2assert False is True

  • I try to run the test again

    python3 -m unittest

    python -m unittest

    the terminal is my friend, and shows AssertionError

    E
======================================================
ERROR: tests.test_person (unittest.loader._FailedTest.tests.test_person)
------------------------------------------------------
ImportError: Failed to import test module: tests.test_person
Traceback (most recent call last):
  File "/usr/local/lib/python3.XY/unittest/loader.py",
    line ABC, in _find_test_path
    module = self._get_module_from_name(name)
  File "/usr/local/lib/python3.XY/unittest/loader.py",
    line DEF, in _get_module_from_name
    __import__(name)
    ~~~~~~~~~~^^^^^^
  File ".../pumping_python/person/tests/test_person.py",
    line 2, in <module>
    assert False is True
           ^^^^^^^^^^^^^
AssertionError


------------------------------------------------------
Ran 1 test in 0.001s

FAILED (errors=1)

    Success! I have my first failure.

    I can use any name for the test file as long as it starts with test or unittest will NOT run the tests in the file.

    This is the RED part of the Test Driven Development Cycle. The message in the terminal is about the failure, I like to read these from the bottom up. Here is an explanation of each line, starting from the last line on the screen

    • FAILED (errors=1): the number of failures or errors

    • Ran 1 test in A.XYZs: the number of tests it ran and how long they took

    • AssertionError: the Error (Exception) that happened. Since I used an assert statement I get AssertionError because the statement after assert is False - False is NOT True

    • assert False is True: the line of code that caused AssertionError

    • the arrows (^^^^^^^^^^^^^): point to the part of the line above, that Python thinks caused the error

    • File ".../pumping_python/person/tests/test_person.py", line 2, in <module>: the line number of the code that caused the error and the location of the file where the error happened and again the question - what is a module?

    • __import__(name) shows another error that is triggered by the one from assert False is True

    • File "/usr/local/lib/python3.XY/unittest/loader.py", line 367, in _get_module_from_name shows the line, method and file where the error triggered by my assert False is True happened

    • module = self._get_module_from_name(name) a failure triggered by the failure triggered by my assert False is True

    • File "/usr/local/lib/python3.XY/unittest/loader.py", line 426, in _find_test_path shows the line, method and file where the error triggered by the one triggered by my assert False is True happened

    • Traceback (most recent call last):: all the information shown after this line that is indented to the right shows the calls that led to the failure. The last line is usually the most important one that points to what caused the failure, this is why I like to read it from the bottom up. In this case it is the only one I care about because it is the one I added to cause the failure.

    • ERROR: tests.test_person (unittest.loader._FailedTest.tests.test_person) is a header with information in dot notation about the failing test

      I think of tests.test_person as an address

      • tests is the tests folder.

      • test_person is the test_person.py file in the tests folder.

      tests
│   __init__.py
└── test_person.py

GREEN: make it pass

  • I hold ctrl (Windows/Linux) or option/command (MacOS) on the keyboard and use the mouse to click on File ".../pumping_python/person/tests/test_person.py", line 2 in the terminal, and the Integrated Development Environment (IDE) opens the file with the cursor at the line where the failure happened.

  • I change True to False in test_person.py

    1# False is True
2# assert False is True
3assert False is False

  • I go back to the terminal to run the test

    python3 -m unittest

    python -m unittest

    the test passes! The terminal shows

    ------------------------------------------------------
Ran 0 tests in 0.000s

NO TESTS RAN

    This is confusing, since the only way I know the test passed, is because I saw it fail. There has to be a better way. It is why the RED part of the cycle is important, it shows that the test works. For now cue CELEBRATION MUSIC AND DANCE! I am GREEN!!

REFACTOR: make it better

  • I keep a list of Errors/Exceptions that show up in the terminal as I go through this book to help me know them better. I add AssertionError to tests/test_person.py

    1# False is True
2# assert False is True
3assert False is False
4
5
6# Exceptions seen
7# AssertionError

    comments in Python are written with # at the beginning, they do not do anything, they are notes for me.

  • I add the new files and folders to git for tracking

    git add .

    the terminal goes back to the command line.

  • I add a git commit message

    git commit --all --message \
'setup project'

    the terminal shows a summary of the changes then goes back to the command line.

close the project

  • I close tests/test_person.py

  • I click in the terminal and change directory to the parent of person

    cd ..

    .. is for the parent of any directory I am in. the terminal shows

    .../pumping_python

    I am back in the pumping_python folder.

review

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