Testing the Data Layer with Mocks

This is the Layer 5 gate. We test CountryRepository in complete isolation, using a small, controlled fake dataset — no real files, no real network, ever.

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 — Testing the Data Layer in Isolation

Building the repository around fake, in-memory data

Session 28's design — accepting any raw_data through the constructor — is exactly what makes this possible: a test can supply a tiny, purpose-built dataset instead of the real mock file or a real JSON file.

from country import Country, CountryRepository

def test_get_all_converts_raw_data_to_country_instances():
    fake_data = [
        {"name": "Testland", "region": "Testregion", "population": 1},
    ]
    repo = CountryRepository(raw_data=fake_data)

    result = repo.get_all()

    assert len(result) == 1
    assert isinstance(result[0], Country)
    assert result[0].name == "Testland"
    assert result[0].population == 1

Testing find_by_region against a known fake dataset

A small, deliberately crafted dataset — with known regions — makes it trivial to assert on exact results, following Session 33's exact-content discipline.

def test_find_by_region_filters_correctly():
    fake_data = [
        {"name": "Testland", "region": "Africa", "population": 1},
        {"name": "Otherland", "region": "Europe", "population": 2},
        {"name": "Thirdland", "region": "Africa", "population": 3},
    ]
    repo = CountryRepository(raw_data=fake_data)

    result = repo.find_by_region("Africa")

    assert [c.name for c in result] == ["Testland", "Thirdland"]

Testing validate_country_record with good and bad records

Session 30's validation function is tested the same way as any other function — normal case, and edge/error cases using pytest.raises.

import pytest
from country import validate_country_record

def test_validate_accepts_correct_record():
    data = {"name": "Kenya", "region": "Africa", "population": 54000000}
    assert validate_country_record(data) == data  # unchanged, valid

def test_validate_rejects_missing_field():
    with pytest.raises(ValueError):
        validate_country_record({"name": "Ghost Nation"})

def test_validate_rejects_wrong_type():
    with pytest.raises(TypeError):
        validate_country_record({"name": "Kenya", "region": "Africa", "population": "fifty-four"})

This is a unit test, not an integration test

Because everything happens in-memory, with no real file or network access, these tests run fast and are completely unaffected by whether a real file exists, is correctly formatted, or a real API happens to be reachable at test time.

4. Lab

What you will build

A file called tests/test_repository.py, extending country.py with CountryRepository and validate_country_record if not already present.

Step-by-step instructions

# country.py (additions if not already present)
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]

    def find_by_region(self, region):
        return [Country.from_dict(r) for r in self._raw_data if r["region"] == region]

def validate_country_record(data):
    required = {"name": str, "region": str, "population": int}
    for key, expected_type in required.items():
        if key not in data:
            raise ValueError(f"missing required field: {key}")
        if not isinstance(data[key], expected_type):
            raise TypeError(f"{key} must be {expected_type.__name__}, got {type(data[key]).__name__}")
    return data

# tests/test_repository.py
import pytest
from country import Country, CountryRepository, validate_country_record

def test_get_all_converts_to_country_instances():
    fake_data = [{"name": "Testland", "region": "Testregion", "population": 1}]
    repo = CountryRepository(raw_data=fake_data)
    result = repo.get_all()
    assert len(result) == 1
    assert isinstance(result[0], Country)
    assert result[0].name == "Testland"

def test_find_by_region_exact_matches():
    fake_data = [
        {"name": "Testland", "region": "Africa", "population": 1},
        {"name": "Otherland", "region": "Europe", "population": 2},
        {"name": "Thirdland", "region": "Africa", "population": 3},
    ]
    repo = CountryRepository(raw_data=fake_data)
    result = repo.find_by_region("Africa")
    assert [c.name for c in result] == ["Testland", "Thirdland"]

def test_find_by_region_no_matches():
    repo = CountryRepository(raw_data=[{"name": "Testland", "region": "Africa", "population": 1}])
    assert repo.find_by_region("Antarctica") == []

def test_validate_accepts_correct_record():
    data = {"name": "Kenya", "region": "Africa", "population": 54000000}
    assert validate_country_record(data) == data

def test_validate_rejects_missing_field():
    with pytest.raises(ValueError):
        validate_country_record({"name": "Ghost Nation"})

def test_validate_rejects_wrong_type():
    with pytest.raises(TypeError):
        validate_country_record({"name": "Kenya", "region": "Africa", "population": "fifty-four"})

# pytest -v

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.py tests/test_repository.py docs/sessions/session-35/index.html
git commit -m "session-35: test the repository and validation logic in complete isolation"

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