Writing Tests with Claude Code

Writing tests is essential but often tedious. Claude Code can generate comprehensive test suites that cover your code’s behavior, edge cases, and error scenarios.

Generating Unit Tests

Point Claude Code at a function and ask for tests:

> Write unit tests for the validate_email function in src/utils/validators.py

Claude Code will generate tests covering:

Valid email addresses
Invalid formats (missing @, no domain, etc.)
Edge cases (empty string, very long emails, special characters)
Boundary conditions

Example generated test:

import pytest
from src.utils.validators import validate_email


class TestValidateEmail:
    def test_valid_email(self):
        assert validate_email("user@example.com") is True

    def test_valid_email_with_subdomain(self):
        assert validate_email("user@mail.example.com") is True

    def test_invalid_email_no_at(self):
        assert validate_email("userexample.com") is False

    def test_invalid_email_no_domain(self):
        assert validate_email("user@") is False

    def test_empty_string(self):
        assert validate_email("") is False

Generating Integration Tests

> Write integration tests for the /api/users endpoint. Test creating, reading, updating, and deleting users.

Testing Existing Code

Ask Claude to analyze code and determine what needs testing:

> Look at the OrderService class and write tests for any untested business logic

Test-Driven Development (TDD)

Claude Code supports TDD workflows:

# Step 1: Write the test first
> Write a failing test for a function that calculates shipping cost based on weight and destination

# Step 2: Implement the code
> Now implement the calculate_shipping function to make the tests pass

# Step 3: Run and verify
> Run the tests to confirm they pass

Running Tests

Ask Claude to run your test suite:

# Run all tests
> Run the full test suite

# Run specific tests
> Run only the authentication tests

# Run with coverage
> Run tests with coverage report

Fixing Failing Tests

When tests fail, Claude Code can diagnose and fix them:

> The test_create_user test is failing. Can you figure out why and fix it?

Best Practices

Specify your testing framework — “Use pytest” or “Use Jest” helps Claude generate the right code
Ask for edge cases — “Include edge cases and error scenarios”
Request mocking when needed — “Mock the database calls in the tests”
Match existing test patterns — “Follow the testing style used in test_auth.py”

Summary

Claude Code can write unit tests, integration tests, and support TDD workflows. It generates comprehensive test coverage and helps fix failing tests. Next, we’ll learn about configuring Claude Code with CLAUDE.md and memory.

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Next: Project Configuration – CLAUDE.md & Memory