Python Modules and import

Python modules let you use code that someone else already wrote, tested and packaged for you. Instead of writing your own square root function or random number generator, you load a module with the import statement and start using its tools straight away. In this lesson, you'll learn every common way to import a module, how to explore what's inside one, and which modules from Python's standard library are worth knowing first.

What is a module in Python?

A module is simply a Python file (ending in .py) that contains functions, variables and other code you can reuse. When you import a module, Python runs that file once and gives you access to everything it defines.

Modules come from three places:

• The standard library: modules that come with Python, like math, random and datetime. Nothing to install.
• Your own files: any .py file you write can be imported. You'll do this in How to Create Your Own Python Module.
• Third-party packages: code shared by other developers, installed with pip. See pip and Virtual Environments.

How do you import a module in Python?

The simplest form is import followed by the module name. After that, you reach the module's tools with a dot: module_name.tool_name.

import math

print(math.sqrt(81))
print(math.pi)

9.0
3.141592653589793

The dot is like saying "the sqrt function from math". It keeps names organized: you always know where a function came from, and a module's sqrt can never clash with a variable called sqrt in your own code.

How Python imports work

When Python reaches an import line, it finds the module, runs it once, and creates a module object that holds all its names. Your program then gets a name pointing to that object (with import math) or names pointing straight at the tools you picked (with from math import sqrt).

Importing specific names with from ... import

If you only need one or two tools, use from module import name. The names land directly in your program, so you use them without the module prefix:

from math import sqrt, pi

print(sqrt(49))
print(round(pi, 2))

7.0
3.14

This is shorter to type, but you lose the reminder of where sqrt came from. A good rule: use import module for modules you use a lot, and from ... import for a few well-known names.

Giving a module a nickname with as

Some module names are long, or clash with your own names. The as keyword creates an alias (a nickname):

import statistics as stats

test_scores = [72, 85, 90, 64]
print(stats.mean(test_scores))
print(stats.median(test_scores))

77.75
78.5

You can alias single names too:

from statistics import mean as average

daily_steps = [8200, 10450, 6900]
print(average(daily_steps))

8516.666666666666

In real projects you'll see popular aliases everywhere, such as import numpy as np and import pandas as pd. Those are third-party libraries, but the as trick is exactly the same.

What does import * do?

from module import * copies every public name from the module into your program:

from math import *

print(floor(2.7))
print(ceil(2.1))

2
3

How do you see what's inside a module?

The built-in dir() function lists all the names a module contains. The math module has a lot, so here we keep only the names that start with "f" using a list comprehension:

import math

f_names = [name for name in dir(math) if name.startswith("f")]
print(f_names)

['fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum']

To read what one of those tools does, call help() on it, for example help(math.floor). It prints the function's built-in documentation right in your terminal.

A quick tour of the Python standard library

Python is famous for coming "with batteries included". Here are standard library modules you'll use again and again:

You'll dig into math and random and datetime later in this chapter, and json and csv in the File Handling chapter.

Real-world example: picking a password alphabet

Here, two modules work together. string provides ready-made groups of characters, and math helps calculate how many possible 8-character passwords those characters allow:

import math
import string

alphabet = string.ascii_letters + string.digits
print("Characters available:", len(alphabet))

combinations = math.pow(len(alphabet), 8)
print("8-character passwords possible:", f"{combinations:,.0f}")

Characters available: 62
8-character passwords possible: 218,340,105,584,896

Without modules, you'd have to type out all 62 characters yourself. With them, it takes one line.

Common mistakes with Python modules

1. Forgetting the import

print(math.sqrt(16))

NameError: name 'math' is not defined. Did you forget to import 'math'?

Every file must import the modules it uses, even if another file already imported them.

2. Misspelling the module name

import maths

ModuleNotFoundError: No module named 'maths'

3. Using the prefix after from ... import

from math import sqrt

print(math.sqrt(25))

NameError: name 'math' is not defined. Did you forget to import 'math'?

from math import sqrt gives you the name sqrt only, not math. Call it as sqrt(25).

4. Naming your own file after a module

If you save your practice script as random.py or math.py, then import random loads your file instead of the real module, and you'll get confusing errors like AttributeError: module 'random' has no attribute 'randint'. Give your files unique names like dice_game.py.

Practice exercise

from math import pi
import statistics as st

radius = 30 / 2
area = pi * radius * radius
print("Pizza area:", round(area, 1), "sq cm")

slice_prices = [2.5, 3.0, 3.5, 4.0]
print("Average slice price:", st.mean(slice_prices))

Pizza area: 706.9 sq cm
Average slice price: 3.25

What's next?

You can now use any module that comes with Python. Next, learn how to create your own Python module and share code between your own files.