The program door, hiding names, import as

You will be able to

• ✓ Put launch prints behind if __name__ == "__main__" so import stays silent
• ✓ Know _name and __all__ as hints for from *
• ✓ Treat import pandas as pd as a nickname, not a second library

Importing fees.py must not print a receipt

When Python loads a file, it sets __name__ to "__main__" if a human (or a scheduler) launched that file, and to the module name if someone imported it. if __name__ == "__main__": is the door: the only block that prints a hostel receipt or talks to a CSV path. Helpers above the door stay importable. Tests import late_fee and never hit the door.

_late_table is a convention: please do not from fees import * that name. __all__ = ["late_fee"] tells from * what to copy on a laptop. You still should not use star imports in app code. The underscore is a hint to humans and to star; it is not a lock.

__all__ = ["late_fee"]

def late_fee(days):
    if days <= 0:
        return 0
    return 25

def _debug_table():
    return [0, 25, 50]

if __name__ == "__main__":
    print(late_fee(4))

import as is a nickname

import pandas as pd does not install a second pandas. It binds the same module to a shorter name. import rsil_fees.late as late is the same idea for your package. Nicknames belong in one style: pd, np, plt — or the full name. Mixing both in one file is how a reviewer loses the thread.

This sandbox cannot set __name__. You practise the shape: functions first, then a print block you would indent under the door.

Words that matter

Program door

if __name__ == "__main__" — runs only when this file was launched.

__all__

Laptop list of names from * should copy.

import as

A nickname for the same module — not a second library.

Common mistakes

On a full Python install — __main__ and aliases

The door keeps helpers importable. import fees as f is a nickname for the same file.

stdlib — no pip

Quiet import, loud door, one alias.

# app.py
import fees as hostel

if __name__ == "__main__":
    print(hostel.late_fee(4))

Example program — Nickname as data, same function

pd-style alias is a name you choose. The function is unchanged.

Your turn — Door prints the fee

Define late_fee as 25 for 4 days. Print Launch and then the fee so 25 appears.

Self-assessment

Check your understanding before you mark the lesson complete.