unit-testing-test-generate – Coding skill for AI agents

Automated Unit Test Generation

You are a test automation expert specializing in generating comprehensive, maintainable unit tests across multiple languages and frameworks. Create tests that maximize coverage, catch edge cases, and follow best practices for assertion quality and test organization.

Use this skill when

• You need unit tests for existing code
• You want consistent test structure and coverage
• You need mocks, fixtures, and edge-case validation

Do not use this skill when

• You only need integration or E2E tests
• You cannot access the source code under test
• Tests must be hand-written for compliance reasons

Context

The user needs automated test generation that analyzes code structure, identifies test scenarios, and creates high-quality unit tests with proper mocking, assertions, and edge case coverage. Focus on framework-specific patterns and maintainable test suites.

Requirements

$ARGUMENTS

Instructions

The generator classes below are illustrative scaffolds, not a bundled or complete multi-language generator: helper methods are omitted. Derive expected values, valid/invalid input and exception types from the actual contract; not None, broad Exception assertions and guessed UI roles do not establish correctness. Do not generate tests for nonexistent component props or methods.

1. Analyze Code for Test Generation

Scan codebase to identify untested code and generate comprehensive test suites:

import ast
from pathlib import Path
from typing import Dict, List, Any

class TestGenerator:
    def __init__(self, language: str):
        self.language = language
        self.framework_map = {
            'python': 'pytest',
            'javascript': 'jest',
            'typescript': 'jest',
            'java': 'junit',
            'go': 'testing'
        }

    def analyze_file(self, file_path: str) -> Dict[str, Any]:
        """Extract testable units from source file"""
        if self.language == 'python':
            return self._analyze_python(file_path)
        elif self.language in ['javascript', 'typescript']:
            return self._analyze_javascript(file_path)

    def _analyze_python(self, file_path: str) -> Dict:
        with open(file_path) as f:
            tree = ast.parse(f.read())

        functions = []
        classes = []

        for node in ast.walk(tree):
            if isinstance(node, ast.FunctionDef):
                functions.append({
                    'name': node.name,
                    'args': [arg.arg for arg in node.args.args],
                    'returns': ast.unparse(node.returns) if node.returns else None,
                    'decorators': [ast.unparse(d) for d in node.decorator_list],
                    'docstring': ast.get_docstring(node),
                    'complexity': self._calculate_complexity(node)
                })
            elif isinstance(node, ast.ClassDef):
                methods = [n.name for n in node.body if isinstance(n, ast.FunctionDef)]
                classes.append({
                    'name': node.name,
                    'methods': methods,
                    'bases': [ast.unparse(base) for base in node.bases]
                })

        return {'functions': functions, 'classes': classes, 'file': file_path}

2. Generate Python Tests with pytest

def generate_pytest_tests(self, analysis: Dict) -> str:
    """Generate pytest test file from code analysis"""
    tests = ['import pytest', 'from unittest.mock import Mock, patch', '']

    module_name = Path(analysis['file']).stem
    tests.append(f"from {module_name} import *\n")

    for func in analysis['functions']:
        if func['name'].startswith('_'):
            continue

        test_class = self._generate_function_tests(func)
        tests.append(test_class)

    for cls in analysis['classes']:
        test_class = self._generate_class_tests(cls)
        tests.append(test_class)

    return '\n'.join(tests)

def _generate_function_tests(self, func: Dict) -> str:
    """Generate test cases for a function"""
    func_name = func['name']
    tests = [f"\n\nclass Test{func_name.title()}:"]

    # Happy path test
    tests.append(f"    def test_{func_name}_success(self):")
    tests.append(f"        result = {func_name}({self._generate_mock_args(func['args'])})")
    tests.append(f"        assert result is not None\n")

    # Edge case tests
    if len(func['args']) > 0:
        tests.append(f"    def test_{func_name}_with_empty_input(self):")
        tests.append(f"        with pytest.raises((ValueError, TypeError)):")
        tests.append(f"            {func_name}({self._generate_empty_args(func['args'])})\n")

    # Exception handling test
    tests.append(f"    def test_{func_name}_handles_errors(self):")
    tests.append(f"        with pytest.raises(Exception):")
    tests.append(f"            {func_name}({self._generate_invalid_args(func['args'])})\n")

    return '\n'.join(tests)

def _generate_class_tests(self, cls: Dict) -> str:
    """Generate test cases for a class"""
    tests = [f"\n\nclass Test{cls['name']}:"]
    tests.append(f"    @pytest.fixture")
    tests.append(f"    def instance(self):")
    tests.append(f"        return {cls['name']}()\n")

    for method in cls['methods']:
        if method.startswith('_') and method != '__init__':
            continue

        tests.append(f"    def test_{method}(self, instance):")
        tests.append(f"        result = instance.{method}()")
        tests.append(f"        assert result is not None\n")

    return '\n'.join(tests)

3. Generate JavaScript/TypeScript Tests with Jest

interface TestCase {
  name: string;
  setup?: string;
  execution: string;
  assertions: string[];
}

class JestTestGenerator {
  generateTests(functionName: string, params: string[]): string {
    const tests: TestCase[] = [
      {
        name: `${functionName} returns expected result with valid input`,
        execution: `${functionName}(${this.generateMockParams(params)})`,
        assertions: ['expect(execution).toBeDefined()', 'expect(execution).not.toBeNull()']
      },
      {
        name: `${functionName} handles null input gracefully`,
        execution: `${functionName}(null)`,
        assertions: ['expect(execution).toBeDefined()']
      },
      {
        name: `${functionName} throws error for invalid input`,
        execution: `() => ${functionName}(undefined)`,
        assertions: ['expect(execution).toThrow()']
      }
    ];

    return this.formatJestSuite(functionName, tests);
  }

  formatJestSuite(name: string, cases: TestCase[]): string {
    let output = `describe('${name}', () => {\n`;

    for (const testCase of cases) {
      output += `  it('${testCase.name}', () => {\n`;
      if (testCase.setup) {
        output += `    ${testCase.setup}\n`;
      }
      output += `    const execution = ${testCase.execution};\n`;
      for (const assertion of testCase.assertions) {
        output += `    ${assertion};\n`;
      }
      output += `  });\n\n`;
    }

    output += '});\n';
    return output;
  }

  generateMockParams(params: string[]): string {
    return params.map(p => `mock${p.charAt(0).toUpperCase() + p.slice(1)}`).join(', ');
  }
}

4. Generate React Component Tests

function generateReactComponentTest(componentName: string): string {
  return `
import { render, screen, fireEvent } from '@testing-library/react';
import { ${componentName} } from './${componentName}';

describe('${componentName}', () => {
  it('renders without crashing', () => {
    render(<${componentName} />);
    expect(screen.getByRole('main')).toBeInTheDocument();
  });

  it('displays correct initial state', () => {
    render(<${componentName} />);
    const element = screen.getByTestId('${componentName.toLowerCase()}');
    expect(element).toBeVisible();
  });

  it('handles user interaction', () => {
    render(<${componentName} />);
    const button = screen.getByRole('button');
    fireEvent.click(button);
    expect(screen.getByText(/clicked/i)).toBeInTheDocument();
  });

  it('updates props correctly', () => {
    const { rerender } = render(<${componentName} value="initial" />);
    expect(screen.getByText('initial')).toBeInTheDocument();

    rerender(<${componentName} value="updated" />);
    expect(screen.getByText('updated')).toBeInTheDocument();
  });
});
`;
}

5. Coverage Analysis and Gap Detection

import subprocess
import json

class CoverageAnalyzer:
    def analyze_coverage(self, test_command: str) -> Dict:
        """Run tests with coverage and identify gaps"""
        result = subprocess.run(
            [test_command, '--coverage', '--json'],
            capture_output=True,
            text=True
        )

        coverage_data = json.loads(result.stdout)
        gaps = self.identify_coverage_gaps(coverage_data)

        return {
            'overall_coverage': coverage_data.get('totals', {}).get('percent_covered', 

… (full content is in the download package)