Modules and Packages

Import useful code from modules, understand packages, and recognize a script's main entry point.

11 min read 10 quiz questions

Reuse code from a module

A module is a Python file that contains code you can import. Python's standard library includes many ready-to-use modules, including math and random.

Use import to bring in a module, then use its name and a dot to access its tools.

Example 1: Import the math module

import math

print(math.sqrt(81))
print(math.ceil(2.1))

math.sqrt and math.ceil are names inside the math module. Keeping the module name makes it clear where the function came from.

Example 2: Import one name with an alias

from math import ceil as round_up

print(round_up(4.2))

Use random predictably in examples

Example 3: Seed random before using it

import random

random.seed(0)
print(random.randint(1, 10))
print(random.choice(["red", "blue", "green"]))

from ... import ... imports one name directly. as gives an imported name an alias. The random.seed(0) call makes this example repeatable; real programs often leave randomness unseeded.

Scripts, packages, and pip

Example 4: Run code only when a file is a script

def main():
    print("Starting the program")

if __name__ == "__main__":
    main()

When Python runs a file directly, its __name__ is "__main__", so the guarded code runs. When another file imports it, that guarded code does not run automatically.

A package is a way to organize related modules in a directory. pip is Python's package installer; it can install packages made by other people when your project needs them.

Key points

A module is a Python file with reusable code.
import module keeps module names visible.
from ... import ... can import one name and as can rename it.
Seed random in repeatable examples.
Packages organize modules, and pip installs external packages.

Worked example

Splitting a bill fairly per head needs ceiling maths — import it from the standard library instead of reinventing it.

Example

import math

total = 1000
people = 3
per_head = math.ceil(total / people)
print(per_head)

How it works, step by step

import math brings the module into scope under its name.
1000 / 3 is 333.33…; ceil rounds UP so the host is not short-changed.
Standard library = batteries included, tested by thousands.

References & Further Reading

Related entries from PyLabs's own reference library.

dir() (List an object's available attribute names.)
help() (Open Python's built-in help system for a topic.)
vars() (Get an object's attribute dictionary when it has one.)

Your notes

Practice exercise

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Import the math module. Use math.floor to round 6.9 down, then print the result.

Expected output: 6

# Write your code below

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