Python Modules

Every Python file is a module.

You can import a module from other files, and that’s the base of any program of moderate complexity, as it promotes a sensible organization and code reuse.

In the typical Python program, one file acts as the entry point. The other files are modules and expose functions that we can call from other files.

The file dog.py contains this code:

def bark():
    print('WOF!')

We can import this function from another file using import, and once we do, we can reference the function using the dot notation, dog.bark():

import dog

dog.bark()

Or, we can use the from .. import syntax and call the function directly:

from dog import bark

bark()

The first strategy allows us to load everything defined in a file.

The second strategy lets us pick the things we need.

Renaming what you import

You can give a different name to what you import, using as:

from dog import bark as woof

woof()

This helps when two modules expose functions with the same name, or when the original name is too long.

Importing standard library modules

The same syntax works for the modules Python ships with. The standard library is a big collection of modules, and you import them like your own:

import math

print(math.sqrt(16))  # 4.0

Nothing to install. If Python runs, the standard library is there.

Organizing modules in folders

Those modules are specific to your program, and importing depends on the location of the file in the filesystem.

Suppose you put dog.py in a lib subfolder.

In that folder, you need to create an empty file named __init__.py. This tells Python the folder contains modules.

Now you can choose, you can import dog from lib:

from lib import dog

dog.bark()

or you can reference the dog module specific function importing from lib.dog:

from lib.dog import bark

bark()

A pitfall to avoid

Be careful with file names. If you name one of your files math.py, then import math in the same folder loads your file, not the standard library module.

A call like math.sqrt(16) then fails with an AttributeError, because your file has no sqrt function.

This happens because Python searches the folder of the running script before the standard library. Rename your file, and the import works again.