Chapter 1: Pytest Fundamentals

Knowledge Points

Pytest Overview and Advantages

• Concise syntax: Uses standard assert statements, no special assertion methods needed
• Automatic test discovery: Automatically discovers and runs test files and test functions
• Rich plugin ecosystem: Hundreds of plugins to extend functionality
• Detailed failure information: Provides clear error reports and debugging information
• Fixture support: Powerful test data and resource management mechanism
• Parameterized testing: Easy implementation of data-driven tests

Installation and Environment Setup

# Install pytest using pip
pip install pytest

# Verify installation
pytest --version

Test Discovery Mechanism

Pytest automatically discovers tests following these naming conventions:

Type	Naming Convention
Test files	test_*.py or *_test.py
Test functions	test_*
Test classes	Test*
Test methods	test_*

Basic Commands

# Run all tests
pytest

# Run specific file
pytest test_example.py

# Run specific directory
pytest tests/

# Verbose output
pytest -v

# Display local variables
pytest -l

Example Code

First Test File

# test_basic.py
def add(a, b):
    """Simple addition function"""
    return a + b

def test_add_positive_numbers():
    """Test adding positive numbers"""
    result = add(2, 3)
    assert result == 5  # Use standard assert statement

def test_add_negative_numbers():
    """Test adding negative numbers"""
    result = add(-2, -3)
    assert result == -5

def test_add_zero():
    """Test adding with zero"""
    result = add(5, 0)
    assert result == 5

Running Tests

# Run from command line
$ pytest test_basic.py -v

# Example output:
# ========================= test session starts =========================
# collected 3 items
#
# test_basic.py::test_add_positive_numbers PASSED            [ 33%]
# test_basic.py::test_add_negative_numbers PASSED            [ 66%]
# test_basic.py::test_add_zero PASSED                        [100%]
#
# ========================= 3 passed in 0.01s =========================

Using Test Classes

# test_calculator.py
class Calculator:
    """Simple calculator class"""

    def add(self, a, b):
        return a + b

    def subtract(self, a, b):
        return a - b

class TestCalculator:
    """Calculator test class"""

    def setup_method(self):
        """Execute before each test method"""
        self.calc = Calculator()

    def test_addition(self):
        """Test addition functionality"""
        assert self.calc.add(2, 3) == 5
        assert self.calc.add(-1, 1) == 0

    def test_subtraction(self):
        """Test subtraction functionality"""
        assert self.calc.subtract(5, 3) == 2
        assert self.calc.subtract(0, 5) == -5

Test Failure Example

# test_failure_example.py
def divide(a, b):
    """Division function (intentionally contains error)"""
    return a / b  # No handling for division by zero

def test_divide_success():
    """Test normal division"""
    assert divide(10, 2) == 5

def test_divide_by_zero():
    """Test division by zero (this test will fail)"""
    # Here we expect an exception, but the function crashes directly
    result = divide(10, 0)
    assert result is None  # This line will never execute

Running failed tests will provide detailed error information:

$ pytest test_failure_example.py -v

# Output includes:
# FAILURES
# test_failure_example.py::test_divide_by_zero - ZeroDivisionError: division by zero
# along with detailed call stack information

Project Structure Best Practices

project/
├── src/
│   └── calculator.py          # Source code
├── tests/
│   ├── __init__.py           # Make tests a package
│   ├── test_calculator.py    # Test file
│   └── conftest.py           # pytest configuration file
├── pytest.ini               # pytest configuration
└── requirements.txt          # Dependencies file

Common Command Options

Option	Function
-v, --verbose	Display verbose output
-q, --quiet	Simplified output
-s	Display print output
-x, --exitfirst	Stop at first failure
--tb=short	Simplified error traceback
--collect-only	Only collect tests, don’t run

With this, we’ve completed the introduction to pytest fundamentals, laying a solid foundation for further learning of advanced features.