Creating A Module

Last updated on 2026-06-23 | Edit this page

Overview

Questions

  • How do I create a Python module?
  • How do I import a local module into my code?

Objectives

  • Create a Python module with multiple files
  • Import functions from a local module into a script

Project Organization

In order to keep our project organized, we’ll start by creating some directories to put our code in. So that we can keep the “source” code of our project separate from other aspects, we’ll start by creating a directory called “src”. In this directory, we’ll create a second directory with the name of our module, in this case “vehicle_module”. We can also delete the “main.py” file that was generated automatically by uv. Your project folder should now look like this:

vehicle-module/
├── src/
│   └── vehicle_module/
├── .venv
├── .gitignore
├── .python-version
├── pyproject.toml
└── README.md
Callout

Note that the interior folder has an underscore instead of a hyphen. We will import the module using the name of the interior folder, vehicle_module. This is important as hyphens are not a valid character in Python module names.

Next, we’ll create a file called __init__.py in the src/vehicle_module directory. This file will make Python treat the directory as a package. Next to the __init__.py file, we can create other Python files that will contain the code for our module.

Callout

The __init__.py file is a special filename in python that indicated that the directory should be treated as a package. Often these files are simply blank, however we can also include some additional code to initialize the package or set up any necessary imports, as we will see later.

Note that this not only applies to the top-level directory of the package, but also to any subdirectories that we want to include as part of the package! Basically, anything that we want to be able to easily import into our code should have an __init__.py file in its directory.

If you have a directory without an __init__.py file, you can still import code from it, but you will need to use the full path to the file in your import statement.

Let’s create a code file now, called horn_noises.py and put a simple function in it:

PYTHON 

def honk_horn(times = 1):
    return "Honk! " * times

Our project folder should now look like this:

vehicle-module/
├── src/
│   └── vehicle_module/
│       ├── __init__.py
│       └── horn_noises.py
├── .gitignore
├── .python-version
├── pyproject.toml
└── README.md

We want to be able to use this function in our code, so let’s make it accessible by adding a line to our __init__.py file:

PYTHON 

from . import horn_noises

This tells python that when we import the vehicle_module package, it should also import the horn_noises module that we have in the same directory.

Previewing Our Module

It’s all well and good to write some code in here, but how can we actually use it? Let’s create a python script to test our module.

Let’s create a directory called “tests”, and start a new file called vehicle_module_tests.py in it.

Add the following code to it:

PYTHON 

import vehicle_module

result = vehicle_module.horn_noises.honk_horn(2)

if result == "Honk! Honk! ":
    print("Test passed!")
else:
    print("Test failed!")

Our project folder should now look like this:

vehicle-module/
├── src/
│   └── vehicle_module/
│       ├── __init__.py
│       └── horn_noises.py
├── tests/
│   └── vehicle_module_tests.py
├── .gitignore
├── .python-version
├── pyproject.toml
└── README.md
Callout

One of the really nice things about using uv is that we can replace python in our commands with uv run and it will use the environment we have created for the project to run the code. At the moment, this doesn’t make a difference, but once we start adding dependencies to our project we’ll see how useful this is.

Let’s run the script from our command line. If you’re in the root directory of the project, your command will look something like uv run tests/vehicle_module_tests.py.

Aaaand… It doesn’t work!

PYTHON 

E:\Documents\Projects\vehicle-module>uv run tests/vehicle_module_tests.py
Traceback (most recent call last):
  File "E:\Documents\Projects\vehicle-module\tests\vehicle_module_tests.py", line 1, in <module>
    from vehicle_module.horn_noises import honk_horn
ModuleNotFoundError: No module named 'vehicle_module'

The reason for this is that we never actually told python where it can find our code!

The Python PATH

When you run a command like import pandas, what python actually does is search the a series of directories in order looking for a module file called pandas.py. We can see what directories will be checked by printing the sys.path variable. Start up a python shell in the terminal and run the following code:

PYTHON 

import sys
sys.path

Exactly what you see will depend on your specific machine, but what you should see is a list of directories that python will check when you try to import a module. This includes the current working directory (’’), as well as any directories that are included in the PYTHONPATH environment variable.

We are only interested in checking our current code, not in installing it as a package. However because we have the __init__.py file in our package directory, if we add the exact or relative path to our package directory to our sys.path variable, python will look there for modules as well.

Let’s add a quick line to the top of our testing script to add our specific module directory to the path:

PYTHON 

import sys
sys.path.insert(0, "./src")

import vehicle_module

result = vehicle_module.horn_noises.honk_horn(2)

if result == "Honk! Honk! ":
    print("Test passed!")
else:
    print("Test failed!")

This time it works! And if we modify the honk_horn function to print out something slightly different, we can see that running the script changes the output right away!

Callout

We used sys.path.insert instead of sys.path.append because we want to give our module directory the highest priority when searching for modules. This way, if there are any naming conflicts with other installed packages, our local module will be found first.

Ideally you would never be working with a package name that has a conflict elsewhere in the path directories, but just in case, this avoids some potential issues.

Dot Notation in Imports

You probably noticed that our function call mimics the file and directory structure of the project. We have the project directory (vehicle_module), then the filename (horn_noises), and finally the function name (honk_horn). Python treats all of these similarly when trying to locate a function or module. We can also modify our import to make the code a little neater:

PYTHON 

import sys
sys.path.insert(0, "./src")

from vehicle_module import horn_noises

result = horn_noises.honk_horn(2)

if result == "Honk! Honk! ":
    print("Test passed!")
else:
    print("Test failed!")

or even better:

PYTHON 

import sys
sys.path.insert(0, "./src")

from vehicle_module.horn_noises import honk_horn

result = honk_horn(2)

if result == "Honk! Honk! ":
    print("Test passed!")
else:
    print("Test failed!")
Callout

For simplicity here, we are using the from ... import ... syntax to import only the function we need from the module. This way we don’t have to include the module name every time we call the function.

This is a common practice in python, however the absolute best practice would be to import the entire module and then call the function with the module name, as in the first example. This way we avoid potential naming conflicts with functions from other modules, as well as providing clarity about where the function is coming from when reading the code.

If the shorter import syntax is so bad, why would I want to type it all out? Can’t I just use the shorter syntax? And maybe add a comment to clarify where the function is coming from?

Yes, you absolutely can! The dot-notation used in python pathing can use .. to refer to the parent directory, or even ... to refer to a grandparent directory.

The reason we’re not doing this here is for clarity, as recommended in PEP8.

Challenge

Challenge 1: Add a Sub Module

Create a directory under src/vehicle_module called efficiency and add a file called fuel.py with a function called “calculate_liters_per_100km that prints out the following:

calculate_liters_per_100km(km=50, liters=2.5):
>>> 5.0

How do you call this function from your testing script and check that it is working properly?

The function should look something like this:

PYTHON 

def calculate_liters_per_100km(km, liters):
    return (liters / km) * 100

Don’t forget to add a __init__.py file to the efficiency directory as well!

Add a line to the __init__.py file in the vehicle_module directory to import the efficiency submodule, as well as a line to the __init__.py file in the efficiency directory to import the fuel module.

Just like we used the dot notation to specify the file and function we wanted earlier, we can also use it to specify the directory.

PYTHON 

from vehicle_module.efficiency.fuel import calculate_liters_per_100km

result = calculate_liters_per_100km(km=50, liters=2.5)
if result == 5.0:
    print("Test passed!")
else:
    print("Test failed!")
Challenge

Challenge 2: Inter-Module imports

We have some tests with some simple functions, but what happens when we have functions in two files that need to call each other? Start a new file in the vehicle_module directory called engine_noise.py and add the following code to it:

PYTHON 

def play_engine_sound(rpm):
    return honk_horn(1) + f"\nVroom! Engine at {rpm} RPM"

We’re using the honk_horn function from the horn_noises.py file, but how do we import it into this new file?

In our .py file:

PYTHON 

from vehicle_module.horn_noises import honk_horn

def play_engine_sound(rpm):
    return honk_horn(1) + f"\nVroom! Engine at {rpm} RPM"
Challenge

Challenge 3: Check that our engine noise function is working

We never added a test to check that our play_engine_sound function is working properly. Add a test for this function in our testing script.

Make sure that you’ve added the file to the __init__.py file in the vehicle_module directory so that it can be imported!

Our test script should now look something like this:

PYTHON 

import sys

sys.path.insert(0, "./src")

import vehicle_module

result = vehicle_module.horn_noises.honk_horn(2)
if result == "Honk! Honk! ":
    print("Test passed!")
else:
    print("Test failed!")

result = vehicle_module.engine_noise.play_engine_sound(3000)
if result == "Honk! \nVroom! Engine at 3000 RPM":
    print("Test passed!")
else:
    print("Test failed!")

result = vehicle_module.efficiency.fuel.calculate_liters_per_100km(km=50, liters=2.5)
if result == 5.0:
    print("Test passed!")
else:
    print("Test failed!")

If you followed along with the entire episode and did both challenges, your project directory should now look something like this:

vehicle-module/
├── src/
│   └── vehicle_module/
│       ├── __init__.py
│       ├── horn_noises.py
│       ├── engine_noise.py
│       └── efficiency/
│           └── fuel.py
├── tests/
│   └── vehicle_module_tests.py
...
Key Points
  • Python modules are simply directories with an __init__.py file in them
  • You can add the path to your module directory to sys.path to make it available for import
  • You can use dot notation in your imports to specify the module, file, and function you want to use