Modules and Imports

Big idea

A module is a file of Python code you can reuse. import brings a module into your program so you can use its functions.

import

import math

print(math.sqrt(16))
print(math.pi)
print(math.floor(3.7), math.ceil(3.2))

output
4.0
3.141592653589793
3 4

import math gives you the name math; everything inside is reached with a dot: math.sqrt, math.pi. The dot keeps names organised — math.floor can't clash with a floor of your own.

Other ways to import

from math import sqrt, pi     # just these names, used without "math."
import statistics as stats    # a shorter nickname

from math import sqrt
import statistics as st

print(sqrt(81))
print(st.mean([2, 4, 9]))
print(st.median([5, 1, 3]))

9.0
5
3

Avoid from math import * — it dumps every name into your program, and you can no longer tell where a name came from.

Your own modules

Any .py file is a module. If you have helpers.py next to your program:

# helpers.py
def greet(name):
    return f"Hello, {name}!"

then in main.py:

import helpers
print(helpers.greet("Ada"))

if __name__ == "__main__"

When Python imports a file, it runs it. To have code that runs only when the file is run directly — not when it's imported — use this guard:

def greet(name):
    return f"Hello, {name}!"

if __name__ == "__main__":
    print(greet("test"))     # only when you run helpers.py itself

__name__ is "__main__" in the file you ran, and the module's name in a file that was imported.

Installing more

Python ships with a huge standard library (next lesson). Thousands more packages — like requests for the web or numpy for maths — are installed from the terminal with pip install requests. (The exercises here use only the standard library.)

print(math.sqrt(49))

output
7.0

Watch it

Think of it like

A toolbox in the garage. import math brings the whole toolbox into the room; from math import sqrt brings just the one tool you need.

Key terms

• Module — A .py file whose code you can import.
• import — Load a module so you can use its names.
• Package — A folder of modules; installed with pip.