Generating Tests with AI

# Bad prompt
"Write tests for the calculate_discount() function"

# Good prompt
"Write tests for calculate_discount(). The function:
- Returns 0% discount for orders under 100 EUR
- Returns 5% discount for orders 100-499 EUR
- Returns 10% discount for orders 500+ EUR
- Returns 15% discount for VIP customers regardless of amount
- Raises ValueError for negative amounts

Test each boundary case and VIP + amount combinations.
Use pytest, no mocks."

# Prompt: "Write pytest tests for a parse_csv_row() function
# that takes a CSV row as a string and returns a dict.
# Expected behavior:
# - Standard row: 'John,Smith,30' -> {'first': 'John', 'last': 'Smith', 'age': 30}
# - Empty field: 'John,,30' -> {'first': 'John', 'last': '', 'age': 30}
# - Quotes: '"John, Jr.",Smith,30' -> {'first': 'John, Jr.', 'last': 'Smith', 'age': 30}
# - Empty string: raises ValueError
# - Wrong field count: raises ValueError"

# AI generates:
import pytest
from myapp.parsers import parse_csv_row


def test_standard_row():
    result = parse_csv_row("John,Smith,30")
    assert result == {"first": "John", "last": "Smith", "age": 30}


def test_empty_field():
    result = parse_csv_row("John,,30")
    assert result == {"first": "John", "last": "", "age": 30}


def test_quoted_field_with_comma():
    result = parse_csv_row('"John, Jr.",Smith,30')
    assert result == {"first": "John, Jr.", "last": "Smith", "age": 30}


def test_empty_string_raises():
    with pytest.raises(ValueError):
        parse_csv_row("")


def test_wrong_field_count_raises():
    with pytest.raises(ValueError):
        parse_csv_row("John,Smith")

"Here is the implementation [paste code] and here are the
existing tests [paste tests]. The coverage report shows
that lines 45-67 are not covered. Write tests to cover
those lines. Focus on error handling and edge cases."

# Prompt for integration test:
# "Write an integration test for POST /api/payments/create/.
# The endpoint expects JWT authentication, a payload with course_slug,
# creates a Payment in the DB, and returns a redirect URL.
# Use pytest + Django test client. Real DB, no mocks
# except for the external Comgate API (use respx)."

# AI generates:
import pytest
import respx
from httpx import Response


@pytest.mark.django_db
def test_create_payment_success(authenticated_client, course):
    with respx.mock:
        respx.post("https://payments.comgate.cz/v1.0/create").mock(
            return_value=Response(200, json={
                "code": 0,
                "transId": "ABC-123",
                "redirect": "https://payments.comgate.cz/pay/ABC-123"
            })
        )

        response = authenticated_client.post(
            "/api/payments/create/",
            data={"course_slug": course.slug},
            content_type="application/json",
        )

    assert response.status_code == 200
    data = response.json()
    assert "redirect_url" in data
    assert Payment.objects.filter(user=authenticated_client.user).exists()

# 1. Generate coverage report
pytest --cov=myapp --cov-report=json

# 2. Let AI analyze coverage gaps
claude "Analyze the coverage report in coverage.json.
  Identify the 5 most important uncovered areas
  and write tests for them. Prioritize by:
  1. Business-critical functions
  2. Error handling paths
  3. Edge cases in data validation"