Modules

A module is simply a file of Python code. With import you use functions someone else wrote. There are three forms: the whole module, a part of it, or with a new name.

import math
from random import randint
import statistics as st

print(math.sqrt(81))        # 9.0
print(randint(1, 6))        # random 1..6
print(st.mean([4, 8, 9]))   # 7.0

Writing from module import * is discouraged: names get mixed up and you cannot tell where something came from.

Every .py file is a module. Write temp.py, define a function in it, and import it from another file. Being in the same folder is enough.

# temp.py
def c_to_f(c):
    return c * 9 / 5 + 32

if __name__ == "__main__":
    print(c_to_f(100))   # runs only when executed directly

# main.py
import temp
print(temp.c_to_f(37))

The if __name__ == "__main__": block runs when the module is executed directly but not when it is imported. This lets one file be both a program and a library.

A package is a folder containing modules (older versions needed an __init__.py file). Third-party packages that do not ship with Python are installed with pip. For projects it is wise to create a separate virtual environment.

python -m venv .venv
# Windows:  .venv\Scripts\activate
# macOS/Linux:  source .venv/bin/activate
pip install requests
pip list