11.1. What are Modules?

In the last chapter, we learned that functions allow us to reuse the same block of code many times throughout a program. Once we create a useful function, we might find a use for it in other programs as well. We could easily copy and paste that function into new projects, but this leads to a couple of problems:

  1. Copy/pasting code does NOT keep our work DRY (Don’t Repeat Yourself),

  2. If we make an update to the function, we need to apply that update to ALL copies of the function. If the function is super-useful, we probably put it into lots of projects. Tracking down and changing every single one would take lots of time, and we would likely miss a few.

Fortunately, we have a way to share functions and other useful tools across different programs WITHOUT copy/paste.

A module is a file containing Python values, functions, etc. that can be used in other Python programs. Instead of pasting a new copy of a function into every new project, we store the original in a file and then import that file when we need it. Changing the code in the module affects all of the programs that imported it. This makes applying updates much more efficient.

A module being imported into three different programs.

The same module code can be imported and used by completely different programs.


Modules are NOT designed to run on their own. Their purpose is to support many different programs by storing the code for commonly used functions, values, tools, and tasks.

11.1.1. Why Use Modules?

There are many modules that come with Python as part of the standard library. These offer us lots of ready-made tools that we can use to speed up the time it takes us to build our own projects.

We can also create our own modules to store data, functions, and tools that we have built on our own and want to use again later.

Even better, we can SHARE our modules with other programmers, or combine someone else’s work (with permission) to our own. Instead of writing every single part of a new project from scratch, another coder might have already written some of the code we need. If that person makes their work available, we can immediately import their modules into our own project—saving us time and effort.

Modules keep us from reinventing the wheel.

We do not need to worry about HOW a module works. We just need to be able to pull it into a project and use its functions.

Finally, modules allow us to keep different parts of our program in separate, smaller pieces. We code these smaller chunks and then connect the modules together to create the big project. Just like keeping functions small and simple helps with debugging, splitting up a large project into smaller pieces makes it easier to fix. We can also add more modules to the project to quickly give the whole program new abilities.

11.1.2. A List of Python Modules

Python comes with LOTS of modules, and we have already used one of these quite a bit—the turtle module. Once we import the module, we gain access to everything it contains.


import turtle           # Allows us to use the turtle module

bob = turtle.Turtle()   # Create a turtle named bob

bob.forward(150)        # Move bob forward 150 pixels
bob.left(90)            # Turn bob left by 90 degrees
bob.circle(75)          # Draw a circle with a radius of 75 pixels

In this example, we use several tools defined in the module—Turtle(), forward(), left(), and circle().

What if no one told us about turtle? How would we know it exists? How would we know what it can do for us?

The Python Documentation site is an extremely useful source of information. The site includes a list of all the standard modules that come with Python (called the Global Module Index).

The main Python Documentation page, with Global Module Index highlighted.

The Global Module Index gives an alphabetical listing of all the modules that are available as part of the standard library.

The Python Module Index page.

Clicking on the turtle option opens a page that describes all of the tools the module contains.

The webpage that describes the Python turtle module.


For now, do not worry about wading through all of the possible modules. In this book, we point out the most useful ones for beginning Python coders.

As your skills grow, you can explore other options on your own.

11.1.3. Check Your Understanding


In Python a module is:

  1. A file containing Python code for use in other Python programs.
  2. A separate block of code within a program.
  3. One line of code in a program.
  4. A file that contains information about functions in Python.