Main guards and packages

Python sets __name__ to "__main__" for a directly run file and to its module name when imported. Put demo or command-line work behind a main guard. Create report.py and main.py in the same folder, then run python main.py.

Worked example

def label(n):
    return f"Total: {n}"


if __name__ == "__main__":
    print(label(5))

import report

print(report.label(8))

Total: 8

Why this works

Importing report defines label but skips the guarded print. A package is a directory of modules; an __init__.py file makes an ordinary package explicit.

Another case

# This file marks tools as a package.

def upper(text):
    return text.upper()

from tools.labels import upper

print(upper("python"))

PYTHON

Practice

Run report.py directly after the example. Explain why its output differs from importing it.

Try another case

report.py also has print("loading") above its main guard. What does import report display?