Packages, __init__, relative imports

You will be able to

• ✓ See a package as a named folder of modules, not fifty loose files
• ✓ Know __init__.py runs when the package is imported
• ✓ Treat relative imports (from . import fees) as an inside-the-folder rule

A folder with a public name is a package

rsil_fees/ with late.py and gst.py is a package when the project treats that folder as one importable name. On a laptop, import rsil_fees looks for rsil_fees/__init__.py (in modern Python the file can be empty, but the folder still needs to be a package on the path). __init__.py runs at import time — so it must be as quiet as any module. It may re-export late_fee so callers write from rsil_fees import late_fee.

Dumping fifty .py files on PYTHONPATH is how names collide. A package is a fence: campus_gate, rsil_fees, lab_dht. Internships that skip the fence copy gst.py next to app.py until nobody knows which gst is which.

# rsil_fees/__init__.py
from .late import late_fee
from .gst import add_gst

# rsil_fees/late.py
def late_fee(days):
    if days <= 0:
        return 0
    return 25

# app.py — absolute name from the project root
from rsil_fees import late_fee
print(late_fee(4))

Relative imports are for inside that folder

from . import fees means “the fees sibling in this package,” not “whatever fees is on the USB.” from .. import tools walks up one package — easy to get wrong, so we name the rule: relative imports stay inside the package tree; application scripts use the public name (import rsil_fees). Running a file inside the package as a script is a common way to break relative imports. Launch the app from the project root.

This sandbox cannot create folders. You will practise a “package-shaped” pair of functions and a public report() that the fake __init__ would have exported.

Words that matter

Package

A named folder of modules with a public import path.

__init__

The file that runs when the package is imported — keep it quiet.

Relative import

from . import fees — sibling inside this package.

Common mistakes

On a full Python install — a package folder

__init__ is the package door. Relative imports (from . import fees) are for siblings, not for USB scripts.

No pip — a folder you named.

Public name rsil_fees; quiet __init__.

# rsil_fees/__init__.py  (keep this quiet)
from .late import late_fee

# rsil_fees/late.py
def late_fee(days):
    if days <= 0:
        return 0
    return 25

Example program — A fake init that only re-exports

No print in init. The app prints.

Your turn — Public package report

report() returns a dict with late 25 for 4 days. Print so 25 appears.

Self-assessment

Check your understanding before you mark the lesson complete.