Modules and Imports

Our country data and our logic have been living in one file. Real projects split code into modules — this session is about how Python finds, loads, and shares code across files.

1. Learning Objective

2. Pre-Coding Quiz

Answer these before reading the concept explanation or writing any code. It is fine to get these wrong — that is the point. You need 4/5 to proceed to the lab.

3. The Concept — Modules and Imports

A module is just a .py file

Any Python file can be imported by another. The filename (without .py) becomes the module name.

# data.py
countries = [
    {"name": "Kenya", "region": "Africa"},
    {"name": "Ghana", "region": "Africa"},
]

def find_by_region(region):
    return [c for c in countries if c["region"] == region]

import module vs from module import name

Both forms load the same file. They differ in what name(s) end up available in the importing file.

# main.py — Option 1: import the whole module
import data
print(data.countries)
print(data.find_by_region("Africa"))

# main.py — Option 2: import specific names directly
from data import countries, find_by_region
print(countries)
print(find_by_region("Africa"))

Aliasing with as

You can rename what you import, either to shorten a long name or to avoid a clash with something already defined.

import data as d
print(d.countries)

from data import find_by_region as search
print(search("Africa"))

The if __name__ == "__main__": guard

Every Python file has a built-in variable __name__. When the file is run directly, it equals "__main__". When the file is imported by another file, it equals the module's own name instead. This guard lets a file define reusable functions AND include a demo that only runs when the file is executed directly.

# data.py
countries = [{"name": "Kenya", "region": "Africa"}]

def find_by_region(region):
    return [c for c in countries if c["region"] == region]

if __name__ == "__main__":
    # This block only runs when you execute: python data.py
    # It does NOT run when another file does: import data
    print(find_by_region("Africa"))

A module's top-level code runs once, and only once

Python caches every module after its first import. If ten different files import the same module, its top-level code still only executes a single time — every importer shares the same already-built module object.

# counter.py
print("data.py is loading...")
value = 0

# app.py
import counter          # prints "data.py is loading..." — first time
import counter as c2     # does NOT print again — already cached
print(counter is c2)     # True — same object

4. Lab

What you will build

Two files: country_data.py (the module) and explorer.py (imports and uses it).

Step-by-step instructions

# country_data.py
countries = [
    {"name": "Kenya", "region": "Africa", "population": 54000000},
    {"name": "Ghana", "region": "Africa", "population": 31000000},
    {"name": "Peru", "region": "Americas", "population": 33000000},
]

def find_by_region(region):
    return [c for c in countries if c["region"] == region]

if __name__ == "__main__":
    print("Running country_data.py directly")
    print(find_by_region("Africa"))

# explorer.py
import country_data

print(country_data.countries)
print(country_data.find_by_region("Africa"))

# explorer.py
from country_data import find_by_region as search

print(search("Africa"))

5. Expected Files Changed

6. Commit Checkpoint

Once the lab is complete and you can explain every line, make this exact commit:

git add country_data.py explorer.py docs/sessions/session-10/index.html
git commit -m "session-10: split data into a module, import it two ways, add a __main__ guard"

7. Code Review Checklist

Go through your code line by line and check each item:

8. Post-Coding Quiz

Same 5 questions. Take it again now that you have written and run the code. You need 4/5 to mark this session complete.

9. Reflection Questions

Think through these after the post-quiz. No right answer — they are for discussion.

10. What Breaks If This Knowledge Is Missing?

11. What We Learned