Writing Your Own Modules

15 min · Beginner

Any .py file you write is already a module. Put useful functions in one file, then import that file from another. Python also lets a file know whether it was started directly or imported.

A file is a module

Own module is a .py file you write yourself. Another file in the same folder can import it by its file name, without the .py ending.

Copy-pasting a function into five programs means five places to fix. Put it in one file, such as shop_tools.py, and import it. Both files sit in the same folder.

Text

1# shop_tools.py2def total_with_tax(price, rate):3    return price * (1 + rate)45# main.py  (same folder)6import shop_tools78print(shop_tools.total_with_tax(200, 0.18))

The file name without .py becomes the module name. Running main.py prints 236.0. To run it yourself, save the two files side by side and run python main.py.

The main guard

Main guard is the line if __name__ == "__main__":. Code under it runs only when the file is started directly, not when another file imports it.

When you import a file, Python runs it from top to bottom. A test line at the bottom would then run on every import. Python sets __name__ to tell the two cases apart.

Python

1def greet(name):2    return f"Welcome, {name}"3
4print(__name__)5if __name__ == "__main__":6    print(greet("Asha"))

Example 1 · A reusable function with a guard

Python

1def total_with_tax(price, rate):2    return round(price * (1 + rate), 2)3
4
5if __name__ == "__main__":6    print(total_with_tax(200, 0.18))7    print(total_with_tax(99.5, 0.05))

Output

236.0
104.48

What this code does

The function is the part other files can import and reuse.
The two print lines run only when this file is started directly.
round(..., 2) keeps two decimal places.

Example 2 · What __name__ holds

Python

import math
print(__name__)print(math.__name__)

Output

__main__
math

What this code does

The file you run is __main__.
An imported module reports its own name. math.__name__ is math.

Run directly, __name__ is "__main__", so the guarded line runs. When another file imports this one, __name__ is the file name instead, and the guarded block is skipped.

Where Python looks for a module

Import path is the ordered list of places Python searches for a module. It is kept in sys.path. The first match wins, and no match is an error.

When you write import report, Python searches in a fixed order: the folder of your script first, then the standard library, then installed packages. The first match wins.

Python

try:    import report_2026except ModuleNotFoundError as err:    print(err)

Example 3 · A missing module is an ImportError

Python

1try:2    import billing_reports3except ImportError as err:4    print(type(err).__name__)5    print(err.name)6
7print(issubclass(ModuleNotFoundError, ImportError))

Output

ModuleNotFoundError
billing_reports
True

What this code does

ModuleNotFoundError is a more specific kind of ImportError, so except ImportError catches it.
The error's name attribute holds the module that was missing.

If no place has the module, Python raises ModuleNotFoundError. Do not name your own file random.py or math.py: your folder is searched first, so your file would hide the real module.

Next step

Check your understanding

9 quick questions

The main guard

Main guard is the line if __name__ == "__main__":. Code under it runs only when the file is started directly, not when another file imports it.

When you import a file, Python runs it from top to bottom. A test line at the bottom would then run on every import. Python sets __name__ to tell the two cases apart.

Where Python looks for a module

Import path is the ordered list of places Python searches for a module. It is kept in sys.path. The first match wins, and no match is an error.

When you write import report, Python searches in a fixed order: the folder of your script first, then the standard library, then installed packages. The first match wins.