Python Modules and Packages

What Is A Module?

A module is a Python file that contains reusable code. If you have a file named math_tools.py, another file can import functions from it.

# math_tools.py
def double(number):
    return number * 2

# app.py
import math_tools

print(math_tools.double(5))

Modules help you split a growing program into smaller files.

Import Styles

You can import the whole module:

import math
print(math.sqrt(16))

You can import a specific name:

from math import sqrt
print(sqrt(16))

You can also use an alias:

import statistics as stats

Aliases are useful when a module name is long or has a common short form.

What Is A Package?

A package is a folder of modules. A simple package might look like this:

shop/
  __init__.py
  cart.py
  discounts.py

The __init__.py file marks the folder as a package in many Python project structures and can also run package setup code.

Avoiding Import Problems

Python searches for modules in certain locations, including the current project path and installed packages. Import errors often happen because of spelling mistakes, running a file from the wrong folder, or naming your file after a standard library module.

Avoid creating files named random.py, math.py, or json.py in small practice projects. Those names can shadow built-in modules.

The if __name__ == "__main__" Pattern

This pattern lets a file behave differently when run directly versus imported.

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

if __name__ == "__main__":
    main()

When another file imports this module, main() does not run automatically.

Common Mistakes

A common mistake is using from module import *. It can make code unclear because many names appear without showing where they came from.

Another mistake is creating circular imports, where file A imports file B and file B imports file A. This often means shared code should move to a third module.

Quick Summary

• A module is a Python file with reusable code.
• A package is a folder of modules.
• import module keeps names under the module namespace.
• from module import name imports one name directly.
• Aliases can make imports shorter.
• Avoid naming your files after standard library modules.
• Use if __name__ == "__main__" for direct-run code.

Practice Quiz

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