Functions & Modules

Turn repeated steps into small functions and split a project into importable files.

By the end of this lesson

You will be able to

Define functions with clear inputs and outputs.
Return results rather than relying on hidden state.
Import code from another Python file.
Use a main guard to separate imports from script execution.

A function should do one job

Give a function a clear name, explicit inputs and a useful return value. Prefer returning a result over printing from inside every function; the caller decides whether to print, save or test it.

def mean(values):
    if not values:
        raise ValueError("values cannot be empty")
    return sum(values) / len(values)

result = mean([10, 20, 30])
print(result)

Separate reusable code from script entry

Put reusable functions in a module and call them from a small entry-point file. The if __name__ == "__main__" guard prevents the script-only workflow from running when the module is imported elsewhere.

# summaries.py
def describe_count(values):
    return {"total": sum(values), "rows": len(values)}

# main.py
from summaries import describe_count

if __name__ == "__main__":
    print(describe_count([10, 20, 30]))

Apply what you learned

Exercise

Split the project into modules

Move the project setup and summary logic into functions that can be imported independently.

Create data_io.py with a function that reads a CSV path.
Create audit.py with a function that reports row and column counts.
Create main.py as the entry point.
Return dictionaries or DataFrames from functions instead of printing inside them.
Add a main guard to main.py.

Done when

Importing data_io or audit does not run the full report.

Free references

Study resources

3 links

01
Defining functions Article
Read function definitions, arguments, return statements and default values.

Done
02
Functions Video
Watch the single 22-minute video for a worked function example.

Done
03
Modules Article
Read how modules are imported and searched.

Done

Check your understanding

Self-test

2 questions

Question 1

Why return a result instead of printing inside a reusable function?
Question 2

What does the __main__ guard protect?

Completion check

Move on when

Your project has reusable functions in separate modules and a small import-safe entry point.

Mark passed and continue Back to module