How do Python modules and imports work?

A module is any .py file. Importing it executes the file (once per interpreter session; subsequent imports reuse the cached version from sys.modules) and makes its names available in the importing namespace.

# Importing the whole module — access via module.name
import math
print(math.sqrt(16))   # 4.0

# Importing specific names — available without prefix
from math import sqrt, pi
print(sqrt(25))   # 5.0

# Import with alias — avoid name clashes or shorten long names
import numpy as np
import pandas as pd

# Star import — pulls all public names (avoid in production code)
from math import *

Python looks for modules in this order: (1) built-in modules compiled into the interpreter, (2) sys.modules cache, (3) directories listed in sys.path — which includes the directory of the script being run, PYTHONPATH env var locations, and site-packages.

A package is a directory containing an __init__.py file (can be empty). Nested packages create a hierarchy: from mypackage.utils import helper. Python 3.3+ introduced namespace packages (no __init__.py needed), but regular packages with __init__.py are still the norm.

The if __name__ == '__main__': guard at the bottom of a module lets you write code that runs when the file is executed directly but not when imported as a module. It is the standard way to write both importable modules and runnable scripts in the same file.

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