Python Testing

Unit testing and test coverage with pytest.

Installation

pip install pytest pytest-cov

Quick Start

# test_example.py
def add(a, b):
    return a + b

def test_add():
    assert add(2, 3) == 5
    assert add(-1, 1) == 0
    assert add(0, 0) == 0

Run tests:

pytest test_example.py
pytest -v  # verbose output

Test Organization

# Group related tests in classes
class TestLexer:
    def test_tokenize_number(self):
        lexer = Lexer("123")
        tokens = lexer.tokenize()
        assert tokens[0].type == "NUMBER"
        assert tokens[0].value == "123"

    def test_tokenize_identifier(self):
        lexer = Lexer("foo")
        tokens = lexer.tokenize()
        assert tokens[0].type == "IDENTIFIER"

Fixtures and Setup

import pytest

@pytest.fixture
def lexer():
    """Reusable lexer instance."""
    return Lexer()

@pytest.fixture
def sample_code():
    return "func add(a: int, b: int) -> int { return a + b; }"

def test_with_fixtures(lexer, sample_code):
    tokens = lexer.tokenize(sample_code)
    assert len(tokens) > 0

Test Coverage

# Run with coverage report
pytest --cov=mymodule tests/

# Generate HTML coverage report
pytest --cov=mymodule --cov-report=html tests/

# Check coverage threshold
pytest --cov=mymodule --cov-fail-under=80 tests/

Parametrized Tests

@pytest.mark.parametrize("input,expected", [
    ("123", "NUMBER"),
    ("foo", "IDENTIFIER"),
    ("+", "PLUS"),
    ("->", "ARROW"),
])
def test_token_types(input, expected):
    lexer = Lexer(input)
    tokens = lexer.tokenize()
    assert tokens[0].type == expected

Exception Testing

def test_syntax_error():
    parser = Parser(tokens)
    with pytest.raises(SyntaxError) as exc:
        parser.parse()
    assert "unexpected token" in str(exc.value)

Test Discovery

Pytest automatically discovers tests that follow naming conventions:

• Test files: test_*.py or *_test.py
• Test functions: test_*
• Test classes: Test*

Common Patterns

# Test multiple conditions
def test_parser_output():
    ast = parse("func main() -> int { return 42; }")
    assert ast["type"] == "Program"
    assert len(ast["functions"]) == 1
    assert ast["functions"][0]["name"] == "main"

# Test file output
def test_codegen_output(tmp_path):
    output_file = tmp_path / "output.wat"
    codegen = CodeGenerator()
    codegen.generate(ast, str(output_file))

    content = output_file.read_text()
    assert "(module" in content
    assert "(func $main" in content