how to make a Python test driven development environment manually

requirements

start here

preview

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

mkdir
cd
touch
echo
cat
mv
rm
uv run pytest-watcher
source .venv/bin/activate
deactivate
history

questions about making a Python Test Driven Development Environment

Questions to think about as I go through the chapter

what is a Test Driven Development Environment?
how can I make a Python Test Driven Development Environment manually?
how can I change directories?
how can I make a directory?
how can I see directory structure?
how can I make an empty file?
how can I write text to a file?
how can I see what is inside a file?
how can I change the name of a file?
how can I run a Python Program?
how can I test for failure?
how can I make a Python package?
how can I run tests manually?
how can I run tests automatically?
how can I stop automated Python tests from running?
how can I set up a project with uv?
what is a Virtual Environment?
how can I activate a Virtual Environment?
how can I deactivate a Virtual Environment?
how can I document the Python programs my project needs?
how can I install the Python programs my project needs from a file?
how can I view all the commands I type in a terminal?

how to setup the project

I choose magic as the name of this project
I click Terminal in the menu bar at the top of the Integrated Development Environment (IDE), then click New Terminal to open a terminal
I change directory to where I will put all the projects from this book. I type cd in the terminal

Note

skip this step if you are already in the pumping_python directory or made it earlier
```
cd pumping_python
```
- if the terminal shows
```
cd: no such file or directory: pumping_python
```
  the folder (directory) does NOT exist. I need to make it. I use the mkdir program to make the pumping_python folder (directory)
```
mkdir pumping_python
```
  the terminal goes back to the command line
- I try changing directory again
```
cd pumping_python
```
  the terminal shows I am in the pumping_python folder (directory)
```
.../pumping_python
```
I type tree in the terminal to see what files and folders are in the pumping_python directory
```
tree
```
- if tree is not installed on the computer, the terminal shows
```
tree: command not found
```
  you can install it from here
- if tree is installed on the computer, the terminal shows
```
.

0 directories, 0 files
```
  Note
  
  If you have done other work in the pumping_python folder there will be files and folders not 0 directories and 0 files
I change directory to the magic project in the pumping_python folder
```
cd magic
```
the terminal is my friend, and shows
```
cd: no such file or directory: magic
```
there is no folder with the name magic in this folder, time to make magic

how to setup a project with uv

I use the uv Python Package Manager to setup the project
```
uv init magic
```
the terminal shows
```
Initialized project `magic` at `.../pumping_python/magic`
```
uv is a program that makes files and folders needed for a project. It also handles Python and Python Packages

how to change directory to the project

I try to change directory to the magic folder again
```
cd magic
```
the terminal shows
```
.../pumping_python/magic
```
uv made the directory for me
I use tree to see what uv added to the folder
```
tree -a -L 2
```
the terminal shows
```
.
├── .git
│   ├── HEAD
│   ├── branches
│   ├── config
│   ├── description
│   ├── hooks
│   ├── info
│   ├── objects
│   └── refs
├── .gitignore
├── .python-version
├── README.md
├── main.py
└── pyproject.toml
```
it added a few files and folders.
- 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
- Here is what uv made for me
  - .git this folder makes the project a git repository, it makes it easy to keep track of changes I make, and if I publish the repository I can work on the project from any computer anywhere
  - .gitignore is a file that tells git what files in the project to not keep track of, this is useful for things that I do not want or need to share
  - .python-version is a file that has the version of Python I am using, this helps if I do projects with different Python versions
  - README.md is a file that is used to describe the project
  - main.py is a Python module for me to write the code for the project
  - 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 look at what is inside .gitignore

cat .gitignore

the terminal shows

# Python-generated files
__pycache__/
*.py[oc]
build/
dist/
wheels/
*.egg-info

# Virtual environments
.venv

I use cat to see what is inside pyproject.toml

cat pyproject.toml

the terminal and shows

[project]
name = "magic"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.XY"
dependencies = []

I use cat to look at what is in .python-version
```
cat .python-version
```
the terminal is my friend, and shows
```
3.XY
```
I use cat to show what is inside README.md
```
cat README.md
```
the terminal goes back to the command line
```
.../pumping_python/magic
```
the file is empty

how to run a Python program

I use Python to run the magic program

python3 src/magic.py

the terminal is my friend, and shows

python3: can't open file '.../pumping_python/magic/src/magic.py': [Errno 2] No such file or directory

Python cannot find the file because it does not exist, and there is no folder named src, yet.

how to make a directory for the source code

I make a child folder in the magic directory for the program because I want to keep the files separate from the other files in the project
```
mkdir src
```
the terminal goes back to the command line

I use tree to see what changed in the magic directory

tree -a -L 1

the terminal shows

.
├── .git
│   ├── HEAD
│   ├── branches
│   ├── config
│   ├── description
│   ├── hooks
│   ├── info
│   ├── objects
│   └── refs
├── .gitignore
├── .python-version
├── README.md
├── main.py
├── pyproject.toml
└── src

I try to run the magic program again
```
python3 src/magic.py
```
the terminal is my friend, and shows the same error from before because there is no file named magic.py in the src folder

how to change the name of a file

I use the mv program to change the name of main.py to magic.py and move it to the src folder
```
mv main.py src/magic.py
```
the terminal goes back to the command line

I use tree to see what folders and files I now have

tree -a -L 2

the terminal shows

.
├── .git
│   ├── HEAD
│   ├── branches
│   ├── config
│   ├── description
│   ├── hooks
│   ├── info
│   ├── objects
│   └── refs
├── .gitignore
├── .python-version
├── README.md
├── pyproject.toml
└── src
    └── magic.py

main.py is now magic.py in the src folder

I try to run the magic program again
```
python3 src/magic.py
```
the terminal shows
```
Hello from magic!
```
Success! The uv Python package manager made the file with some Python code in it, I can successfully run it because the file is now in the folder.

I use cat to see what is in src/magic.py

cat src/magic.py

the terminal shows

def main():
    print("Hello from magic!")


if __name__ == "__main__":
    main()

magic!

test_failure

how to manually run tests

I use the unittest module from The Python Standard Library that comes with Python to run tests. I type this in the terminal
```
python3 -m unittest
```
the terminal shows
```
------------------------------------------------------
Ran 0 tests in 0.000s

NO TESTS RAN
```
because I do not have any tests, yet.
- python3 is the Python program
- -m is an option/switch passed when calling Python to run the module ( unittest in this case)
- a Python module is any file that ends with .py, this means somewhere on the computer there is a file named unittest.py, click here to see the source code for unittest

how to make a directory for the tests

I make a child folder to keep the tests in a different place from the actual program
```
mkdir tests
```
the terminal goes back to the command line

I use tree to see what my project looks like

tree -a -L 2

the terminal is my friend, and shows

.
├── .git
│   ├── HEAD
│   ├── branches
│   ├── config
│   ├── description
│   ├── hooks
│   ├── info
│   ├── objects
│   └── refs
├── .gitignore
├── .python-version
├── README.md
├── pyproject.toml
├── src
│   └── magic.py
└── 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/magic.py
```
the terminal goes back to the command line

I use tree to see what the project looks like now

tree -a -L 2

the terminal is my friend, and shows

.
├── .git
│   ├── HEAD
│   ├── branches
│   ├── config
│   ├── description
│   ├── hooks
│   ├── info
│   ├── objects
│   └── refs
├── .gitignore
├── .python-version
├── README.md
├── pyproject.toml
├── src
│   └── magic.py
└── tests
    └── magic.py

I run the test again
```
python3 -m unittest
```
the terminal is my friend, and shows
```
NO TESTS RAN
```
because I have not set up the test correctly, yet

RED: make it fail

I use the Integrated Development Environment (IDE) to open tests/magic.py in the editor
Tip

I can open a file from the terminal in the Integrated Development Environment (IDE) by typing the name of the program and the name of the file, for example with Visual Studio Code when I type this in the terminal
```
code tests/magic.py
```
magic.py from the tests folder opens in the editor.

I can also open a file from the terminal with ctrl (Windows/Linux) or command (MacOS) on the keyboard and a click with the mouse on the name of the file
I add the Python code below in tests/magic.py in the editor

Note

the line numbers below are a guide, no need to copy them
```
1import unittest
2
3
4class TestMagic(unittest.TestCase):
5
6    def test_failure(self):
7        self.assertFalse(True)
```
Here is an explanation of the code I typed in the file
- import unittest imports the unittest module from The Python Standard Library, this is what I am using for testing. It is like a toolbox that contains the tools I will use to test the code
- class TestMagic
  - class is the Python keyword for making classes - a group of attributes (values) and methods (functions) that belong together, see the classes chapter for more
  - TestMagic is the name I gave this class and will hold the test
    
    Tip
    
    I can use any name for the test class, it MUST start with Test or unittest will NOT run the tests in it
  - unittest.TestCase is a class from the unittest module that has methods for , this is the tool that contains the methods I will use to test code in this book
  - class TestMagic(unittest.TestCase) defines TestMagic as a “child” of unittest.TestCase which means I can use the methods and attributes of the unittest.TestCase class, I do not have to make them because someone else already did
- def test_failure
  - def is the Python keyword for making methods (functions), see functions for more
  - test_failure is the name of the method I used for this first test
    
    Tip
    
    I can use any name for the test method, it MUST start with test_ or unittest will NOT run the tests in it
  - self. lets me use attributes and methods of the TestMagic class which is a “child” of the unittest.TestCase class, instead of using TestMagic(). or unittest.TestCase(). when I want to use something from the unittest.TestCase class
    
    Tip
    
    the name self is Python convention. I can use any name though it is easier to stick with convention for this one
  - self.assertFalse(True) is an assertion (a way to make sure something is True or False)
    - assertFalse is a method in the unittest.TestCase class that checks if its input is False
    - True is given as the input
    I expect this line to fail because True is NOT False. If it does not fail, then Python and I have a problem
I turn on the Auto Save feature in the Integrated Development Environment (IDE) to automatically save files when I make a change so that I do not repeat myself. I do not want to use ctrl+s ((Windows/Linux) or command+s (MacOS)) on the keyboard every time I make a change, I want the computer to do that for me

Attention

Turn on the Auto Save feature in the Integrated Development Environment (IDE)

In Visual Studio Code it is under File in the menu
I try the command again to run the tests in the terminal
```
python3 -m unittest
```
the terminal is my friend, and shows
```
NO TESTS RAN
```
because the tests folder is NOT a Python package and unittest cannot find my test. I need to add a file named __init__.py to the tests folder, to make it a Python package for unittest to find the test.

how to make the tests a Python package

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

Danger

use 2 underscores (__) before and after init for __init__.py not _init_.py
```
touch tests/__init__.py
```
the terminal goes back to the command line

I run the tree command to see what changed

tree -a -L 2

the terminal is my friend, and shows

.
├── .git
│   ├── HEAD
│   ├── branches
│   ├── config
│   ├── description
│   ├── hooks
│   ├── info
│   ├── objects
│   └── refs
├── .gitignore
├── .python-version
├── README.md
├── pyproject.toml
├── src
│   └── magic.py
└── tests
    ├── __init__.py
    └── magic.py

I try to run the tests again
```
python3 -m unittest
```
the terminal does not feel like my friend, and shows
```
NO TESTS RAN
```
because unittest does not know that magic.py in the tests folder is a test file. I did not start the name with test_. I have to change the name
I close magic.py in the editor of the Integrated Development Environment (IDE)

Danger

if you do not close magic.py in the editor, 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 in the editor after you change its name
I use the mv program to change the name of magic.py in the tests folder to test_magic.py
```
mv tests/magic.py tests/test_magic.py
```
the terminal goes back to the command line

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

tree -a -L 2

the terminal is my friend, and shows

.
├── .git
│   ├── HEAD
│   ├── branches
│   ├── config
│   ├── description
│   ├── hooks
│   ├── info
│   ├── objects
│   └── refs
├── .gitignore
├── .python-version
├── README.md
├── pyproject.toml
├── src
│   └── magic.py
└── tests
    ├── __init__.py
    ├── __pycache__
    └── test_magic.py

if you do not see __pycache__ in the tree do not worry,

the important thing is that you renamed magic.py to test_magic.py for unittest to find the test

I run the tests again
```
python3 -m unittest
```
the terminal is my friend, and shows AssertionError
```
F
=========================================================
FAIL: test_failure (tests.test_magic.TestMagic.test_failure)
---------------------------------------------------------
Traceback (most recent call last):
  File "...pumping_python/magic/tests/test_magic.py", line 7, in test_failure
    self.assertFalse(True)
    ~~~~~~~~~~~~~~~~^^^^^^
AssertionError: True is not false

---------------------------------------------------------
Ran 1 test in A.XYZs

FAILED (failures=1)
```
Tip

I can use any name for the test file. It must start 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 (failures=1): the number of failures
- Ran 1 test in A.XYZs: the number of tests it ran and how long they took
- AssertionError: True is not false: the Error (Exception) that happened and its message, in this case AssertionError because True is NOT False
- self.assertFalse(True): the line of code that caused AssertionError
- ~~~~~~~~~~~~~~~~^^^^^^: points to the part of the line above, that Python thinks caused the error
- File ".../magic/tests/test_magic.py", line 7, in test_failure: the line number of the code that caused the error and the location of the file where it is
- 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
- FAIL: test_failure (tests.test_magic.TestMagic.test_failure) is a header with information in dot notation about the failing test method
  - tests.test_magic.TestMagic.test_failure is the location of the failing test
    - tests is the tests folder
    - test_magic is the test_magic.py file in the tests folder
    - TestMagic is the class defined on line 4 in test_magic.py in the tests folder
    - test_failure is the method (function) defined in the TestMagic class, on line 6 in test_magic.py in the tests folder
- F shows a failure
I hold ctrl (Windows/Linux) or option/command (MacOS) on the keyboard and use the mouse to click on File ".../pumping_python/magic/tests/test_magic.py", line 7 in the terminal, and the Integrated Development Environment (IDE) opens the file in the editor with the cursor at the line where the failure happened

GREEN: make it pass

I change True to False on line 7 of test_magic.py in the editor

import unittest

class TestMagic(unittest.TestCase):

    def test_failure(self):
        self.assertFalse(False)

I run the test again in the terminal

python3 -m unittest

the test passes! the terminal is my friend, and shows

.
------------------------------------------------------
Ran 1 test in A.XYZs

OK

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, it helps when I run into them later, because I will be more familiar with them. I add AssertionError to test_magic.py in the editor

import unittest

class TestMagic(unittest.TestCase):

    def test_failure(self):
        self.assertFalse(True)

# Exceptions seen
# AssertionError

Note

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

I ran python3 -m unittest to see the test fail
I ran python3 -m unittest every time I made a change until the test passed
I will run python3 -m unittest again when I add any code, to make sure tests that passed before do not fail and that the new code I add does what I want

This means I have to run python3 -m unittest for each part of the Test Driven Development Cycle or any time there is a code change.

I do not want to type python3 -m unittest again, I want the computer to do it for me.

how to run the tests automatically

I can use pytest-watcher to run tests automatically. It is a Python program that automatically runs pytest any time a Python file changes in the folder it is looking at, this means it will run the tests for me every time I make a change.

pytest is a Python package like unittest that is used for testing. It is not part of The Python Standard Library

I use uv to run pytest-watcher in the terminal

uv run pytest-watcher

the terminal is my friend, and shows

Using CPython 3.13.13
Creating virtual environment at: .venv
error: Failed to spawn: `pytest-watcher`
  Caused by: No such file or directory (os error 2)

because pytest-watcher is not installed on the computer. I can install it with the uv Python Package Manager.

how to write text to a file

I want to make a file where I list all the Python packages that my project needs as a way to document it and have uv install the programs listed in the file

I can write text to a file with the echo program, it shows whatever it is given as an argument, on the screen (standard output (stdout)) for example, if I type this in the terminal
```
echo "pytest"
```
it shows
```
pytest
```
I can also use echo to add text to a file. I use it to make the requirements file with pytest as what is inside it
```
echo "pytest" > requirements.txt
```
- > is an operator that is used to send output from a program to the given file
- pytest is a Python package like unittest, that is used for testing
- requirements.txt is the name of the file where I am adding Python packages for the uv Python Package Manager to install. The name requirements.txt is Python convention, I can use any name I want for the requirements file
I add pytest-watcher to the requirements file as well
```
echo "pytest-watcher" >> requirements.txt
```
- >> is an operator that is used to send output from a program to the given file, it adds to what is in the file without writing over it
- pytest-watcher is a Python program that automatically runs pytest when I change code in the project

I use tree to see what the project looks like now

tree -a -L 2

the terminal shows

.
├── .git
│   ├── HEAD
│   ├── branches
│   ├── config
│   ├── description
│   ├── hooks
│   ├── info
│   ├── objects
│   └── refs
├── .gitignore
├── .python-version
├── README.md
├── pyproject.toml
├── requirements.txt
├── src
│   └── magic.py
└── tests
    ├── __init__.py
    ├── __pycache__
    └── test_magic.py

requirements.txt is now in the magic folder

I use cat to make sure requirements.txt has pytest and pytest-watcher inside it
```
cat requirements.txt
```
the terminal shows
```
pytest
pytest-watcher
```
life is good!

how to install Python packages with uv

I use uv to install pytest-watcher from the requirements file
```
uv add --requirement requirements.txt
```
- --requirement is an option that can be given to the add argument for Python packages in a given file
- requirements.txt is the name of the given file. It helps to manage Python programs that are needed by the project. In this case I only have two programs. A project can have a big number of programs it needs and using one file with one command is easier than one command for each program
the terminal is my friend, and shows setup and installation
```
Resolved 9 packages in GHIms
░░░░░░░░░░░░░░░░░░░░ [0/7] Installing wheels...
...
Installed 7 packages in JKLms
 + iniconfig==A.B.C
 + packaging==DE.F
 + pluggy==G.H.I
 + pygments==J.K.L
 + pytest==M.N.O
 + pytest-watcher==P.Q.R
 + watchdog==S.T.U
```

I run tree to see what changed in the project

tree -a -L 1

the terminal shows

.
├── .git
├── .gitignore
├── .python-version
├── .venv
├── README.md
├── pyproject.toml
├── requirements.txt
├── src
├── tests
└── uv.lock

uv added 2 things

.venv a folder for a Virtual Environment
uv.lock a file that has the exact versions of the Python programs that were installed

I use cat to show what is now in pyproject.toml

cat pyproject.toml

the terminal is my friend, and shows

[project]
name = "magic"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.XY"
dependencies = [
    "pytest>=M.N.O",
    "pytest-watcher>=P.Q.R",
]

it added pytest and pytest-watcher to the dependencies of the project

what is a virtual environment?

A virtual environment is a separate folder where I can install Python packages that my project needs. This helps me keep things that belong to the project in one place, separate from other things on the computer.

It means I can have a separate virtual environment for every project with only the programs that the project needs. I do not have to keep every program I have ever used for projects that do not need them.

how to activate a virtual environment

When I want to work in a virtual environment, I make sure I am in the parent directory of it, for example, magic in this case. I activate the virtual environment in the terminal to use it
```
source .venv/bin/activate
```
the terminal is my friend, and shows
```
(magic) .../magic
```
(magic) on the far left of the command line in the terminal shows that I am in the virtual environment

how to deactivate a virtual environment

I leave the virtual environment by typing deactivate in the terminal
```
deactivate
```
the terminal goes back to the command line, (magic) is no longer on the left side
```
.../pumping_python/magic
```

how to run the tests automatically with uv and pytest-watcher

I try to run the tests again, this time with uv

uv run pytest-watcher . --now

the terminal is my friend, and shows results without going back to the command line

pytest-watcher version X.Y.Z
Runner command: pytest
Waiting for file changes in .../pumping_python/magic
=================== test session starts ====================
platform ____ -- Python 3.A.B, pytest-C.D.E, pluggy-F.G.H
rootdir: .../pumping_python/magic
configfile: pyproject.toml
collected 1 item

tests/test_magic.py .                                 [100%]

==================== 1 passed in X.YZs =====================
[pytest-watcher]
Current runner args: []
Press w to show menu

how to open the test file in the editor from the terminal

I hold ctrl (Windows/Linux) or option/command (MacOS) on the keyboard, then click on tests/test_magic.py to place the cursor in the editor of the Integrated Development Environment (IDE), then I change False to True on line 7

import unittest

class TestMagic(unittest.TestCase):

    def test_failure(self):
        self.assertFalse(True)

the terminal is my friend, and shows AssertionError

======================== FAILURES ==========================
_________________ TestMagic.test_failure ___________________

self = <tests.test_magic.TestMagic testMethod=test_failure>

    def test_failure(self):
>       self.assertFalse(True)
E       AssertionError: True is not false

tests/test_magic.py:7: AssertionError
=============== short test summary info ====================
FAILED tests/test_magic.py::TestMagic::test_failure - AssertionError: True is not false
=================== 1 failed in X.YZs ======================

I hold ctrl (Windows/Linux) or option/command (MacOS) on the keyboard, then click on tests/test_magic.py:7 to place the cursor in the editor of the Integrated Development Environment (IDE), then I change True back to False in test_magic.py
```
7      self.assertFalse(False)
```
the test passes. I can write the rest of the code for the project and get results back quickly because the tests run when I change the code

how to stop the automated tests

I go to the terminal and use q on the keyboard to stop the tests, the terminal goes back to the command line

.../pumping_python/magic

close the project

I close test_magic.py in the editor of the Integrated Development Environment (IDE)
I click in the terminal and change directory to the parent of magic
```
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

I gave the computer some commands to make a Python Test Driven Development environment
I made some folders
I made some files
I made a failing test
I made the failing test pass
I made the test run automatically with pytest-watcher

how to view all the commands typed in a terminal

I type history in the terminal to see all the commands I have typed so far
```
history
```
the terminal shows

the history program shows all the commands I typed in the terminal so far, and I use them to write the program that will automatically make a Python Test Driven Development environment for me

these are the commands I used to make a Python Test Driven Development environment

uv init NAME_OF_THE_PROJECT
cd NAME_OF_THE_PROJECT
mkdir src
mv main.py src/NAME_OF_THE_PROJECT.py
mkdir tests
touch tests/__init__.py
touch tests/test_NAME_OF_THE_PROJECT.py
echo "pytest" > requirements.txt
echo "pytest-watcher" >> requirements.txt
uv add --requirement requirements.txt
uv run pytest-watcher . --now

where NAME_OF_THE_PROJECT is the name I give the project

these are the steps I took to make a Python Test Driven Development environment

How many questions can you answer after going through this chapter?

what is next?

You have seen me make a Test Driven Development environment for a project called magic on any Linux, MacOS and Windows_computers. Would you like to test AssertionError next?

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