Modules — Importing Modules

A module is simply a .py file containing Python code (functions, classes, variables) that can be reused in other programs.

Ways to Import a Module

1. Import the whole module

import math
print(math.sqrt(16))    # 4.0
print(math.pi)            # 3.141592653589793

2. Import specific names

from math import sqrt, pi
print(sqrt(25))    # 5.0
print(pi)           # 3.141592653589793

3. Import with an alias

import math as m
print(m.factorial(5))   # 120

4. Import everything (generally discouraged)

from math import *
print(sqrt(9))   # 3.0 -- works, but pollutes the namespace

How Python finds modules

Python searches, in order:

1. The current directory
2. Directories in the PYTHONPATH environment variable
3. The standard library installation directories

import sys
print(sys.path)   # list of directories Python searches for modules

dir() — list names defined in a module

import math
print(dir(math))   # lists all functions/constants available in the math module

Module caching

A module is only executed once per program run, even if imported multiple times (from different files); subsequent imports reuse the cached module object.

import sys
print('math' in sys.modules)   # False, before first import
import math
print('math' in sys.modules)   # True, after import