Modules and import

A module is a file of Python code you can reuse. Python ships with hundreds of them, the standard library, and thousands more can be installed.

import

import math

print(math.sqrt(81))
print(math.pi)
print(math.ceil(4.2), math.floor(4.8))

import math loads the module; you then reach its contents with a dot: math.sqrt.

from ... import

Import specific names to use them without the module prefix:

from math import sqrt, pi

print(sqrt(16), pi)

Aliases

as gives a module or name a shorter name. Some aliases are universal conventions:

import statistics as stats
from datetime import date as Date

print(stats.mean([4, 8, 15, 16, 23, 42]))
print(Date(2026, 1, 1).strftime("%A"))

You'll see import numpy as np and import pandas as pd in every data-science script.

What's inside a module?

import random

print([name for name in dir(random) if not name.startswith("_")][:15])
print(random.choice.__doc__.splitlines()[0])

dir() lists a module's names and help(module.name) shows its documentation.

Your own modules

Any .py file is a module. If you had a file shop.py containing:

TAX_RATE = 0.18

def with_tax(price):
    return round(price * (1 + TAX_RATE), 2)

then another file in the same folder could use it:

import shop
print(shop.with_tax(100))     # 118.0

if __name__ == "__main__"

A file's code runs when it is imported. To have code that runs only when the file is executed directly, not when imported, use this guard:

def main():
    print("Running as a script")

if __name__ == "__main__":
    main()

Packages and pip

• A package is a folder of modules.
• pip installs packages from PyPI, the Python Package Index: pip install requests.

On CodeMIRAI, popular data packages (NumPy, pandas, matplotlib) load automatically when you import them.