Working with JSON Files

We give the repository its first genuinely external data source: a real JSON file on disk, read and parsed with Python's standard library.

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 — Reading and Writing JSON

Writing data to a JSON file

Python's standard library json module converts Python data structures to and from JSON text — no external installation required.

import json
from mock_countries import MOCK_COUNTRIES

with open("countries.json", "w") as f:
    json.dump(MOCK_COUNTRIES, f, indent=2)

# countries.json now contains readable, formatted JSON text on disk

Reading it back

json.load() parses a JSON file directly back into native Python data structures — a JSON array of objects becomes exactly the list of dicts we've used since Session 06.

import json

with open("countries.json") as f:
    data = json.load(f)

print(type(data))        # <class 'list'>
print(type(data[0]))     # <class 'dict'>
print(data[0]["name"])   # "Kenya"

Handling a missing or malformed file

A file that does not exist, or contains invalid JSON, raises an exception — Session 11's discipline applies directly.

import json

def load_countries_file(path):
    try:
        with open(path) as f:
            return json.load(f)
    except FileNotFoundError:
        print(f"{path} does not exist — using an empty dataset")
        return []
    except json.JSONDecodeError as e:
        print(f"{path} contains invalid JSON: {e}")
        return []

data = load_countries_file("countries.json")
data_missing = load_countries_file("does_not_exist.json")  # handled gracefully

Pointing the repository at the file — nothing else changes

This is Session 28's payoff, delivered: only the constructor argument changes, because get_all() was never coupled to WHERE raw_data came from.

class CountryRepository:
    def __init__(self, raw_data):
        self._raw_data = raw_data

    def get_all(self):
        return [Country.from_dict(r) for r in self._raw_data]

# Before: built from the in-memory mock list
# repo = CountryRepository(raw_data=MOCK_COUNTRIES)

# Now: built from a real file on disk — get_all() is completely unchanged
repo = CountryRepository(raw_data=load_countries_file("countries.json"))
print(len(repo.get_all()))

4. Lab

What you will build

A file called json_lab.py, plus a generated countries.json.

Step-by-step instructions

# json_lab.py
import json
from mock_countries import MOCK_COUNTRIES

with open("countries.json", "w") as f:
    json.dump(MOCK_COUNTRIES, f, indent=2)

print("Wrote countries.json")

def load_countries_file(path):
    try:
        with open(path) as f:
            return json.load(f)
    except FileNotFoundError:
        print(f"{path} does not exist — using an empty dataset")
        return []
    except json.JSONDecodeError as e:
        print(f"{path} contains invalid JSON: {e}")
        return []

real_data = load_countries_file("countries.json")
missing_data = load_countries_file("does_not_exist.json")
print("Real records:", len(real_data))
print("Missing-file fallback:", missing_data)

class Country:
    def __init__(self, name, region, population):
        self.name = name
        self.region = region
        self.population = population

    @classmethod
    def from_dict(cls, data):
        return cls(**data)

class CountryRepository:
    def __init__(self, raw_data):
        self._raw_data = raw_data

    def get_all(self):
        return [Country.from_dict(r) for r in self._raw_data]

repo = CountryRepository(raw_data=load_countries_file("countries.json"))
print(len(repo.get_all()))

5. Expected Files Changed

6. Commit Checkpoint

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

git add json_lab.py countries.json docs/sessions/session-29/index.html
git commit -m "session-29: read and write JSON files, point the repository at real disk data"

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