Chapter 4: Python Modules

Modules #

A module is a file that allows other modules to use definitions within that file.

To import definitions from other modules:

# my_math.py

def add(a, b):
  return a + b
  
def sub(a, b):
  return a - b
  
def mul(a, b):
  return a * b
  
def div(a, b):
  return a / b

import my_math

my_math.add(2, 3)
my_math.sub(5, 2)
my_math.mul(10, 2)
my_math.div(5, 3)

To import a definition directly in a module:

from my_math add, sub, mul, div

add(2, 3)
sub(5, 2)
mul(10, 2)
div(5, 3)

Imports can also be aliased using the as keyword:

import my_math as some_other_name

from my_math import add as my_math_add

Executing modules as scripts #

The module's file name is provided through the __name__ variable.

If a module is executed as a script; when it is provided as an argument to the python interpreter, the __name__ global variable will be set to '__main__'.

This is useful when you want to execute certain things within the module only when it is executed as a script:

# my_math.py

def add(a, b):
  return a + b
  
if __name__ == '__main__':
  # This will only execute if the module is 
  # ran as a script.
  print(add(10, 3))

When used as a module, the module will not execute the statements within the if statement.

>>> import my_math
>>> my_math.add(2, 3)
5

Ran as a script however, it will:

$ python my_math.py
13

Module resolution #

Python resolves modules through the following:

1. Check if there is a builtin with the given module's name.
2. If not found, python then looks for <module_name>.py within a list of directories given by sys.path.

Packages #

A package is a collection of modules within a given directory:

webapp/
    __init__.py
    routes/
          __init__.py
          foo_route.py
          bar_route.py
    controllers/
          __init__.py
          foo_controller.py
          bar_controller.py
    services/
          __init__.py
          foo_service.py
          bar_service.py

The __init__.py files are required and allows python to treat
directories as a package that contains modules.

The file can be empty or have content that execute certain things ie. initialization of the modules.

To use modules within packages:

import webapp.services

webapp.services.foo_service.thing_in_foo_service()