pytest Basics

pytest is Python's standard testing framework. It is widely used for its concise syntax, rich plugin ecosystem, and detailed failure reports.

---

Installation and First Test

# pip install pytest

# test_hello.py
def add(a: int, b: int) -> int:
    return a + b


def test_add_positive():
    assert add(1, 2) == 3


def test_add_negative():
    assert add(-1, -2) == -3


def test_add_zero():
    assert add(0, 0) == 0


# Run: pytest test_hello.py
# Run: pytest -v (verbose)
# Run: pytest -v -s (include print output)

---

Test Discovery Rules

project/
├── src/
│   └── calculator.py
└── tests/
    ├── conftest.py          # shared fixtures
    ├── test_calculator.py   # test_ prefix files
    └── unit/
        └── test_utils.py

# pytest auto-discovery rules:
# 1. test_*.py or *_test.py files
# 2. Classes with Test prefix (TestCalculator)
# 3. Functions/methods with test_ prefix

# test_calculator.py
class TestCalculator:
    def test_add(self):
        assert 1 + 1 == 2

    def test_subtract(self):
        assert 5 - 3 == 2

    # No __init__ needed (recommended)

---

assert Rewriting — Detailed Failure Messages

# pytest rewrites assert statements to provide detailed info on failure

def test_list_comparison():
    result = [1, 2, 3, 4]
    expected = [1, 2, 3, 5]
    assert result == expected
    # On failure:
    # AssertionError: assert [1, 2, 3, 4] == [1, 2, 3, 5]
    #   At index 3 diff: 4 != 5


def test_string_comparison():
    result = "Hello World"
    assert "Python" in result
    # On failure:
    # AssertionError: assert 'Python' in 'Hello World'


def test_dict_comparison():
    result = {"name": "Alice", "age": 30}
    expected = {"name": "Alice", "age": 31}
    assert result == expected
    # On failure: dict diff is displayed


# Custom message
def test_with_message():
    value = 42
    assert value > 100, f"Value {value} must be greater than 100"

---

Exception Testing

import pytest


def divide(a: float, b: float) -> float:
    if b == 0:
        raise ZeroDivisionError("Cannot divide by zero")
    return a / b


def parse_age(value: str) -> int:
    age = int(value)
    if age < 0 or age > 150:
        raise ValueError(f"Invalid age: {age}")
    return age


# Confirm exception is raised
def test_divide_by_zero():
    with pytest.raises(ZeroDivisionError):
        divide(1, 0)


# Check exception message
def test_divide_by_zero_message():
    with pytest.raises(ZeroDivisionError, match="Cannot divide by zero"):
        divide(1, 0)


# Inspect exception object
def test_invalid_age():
    with pytest.raises(ValueError) as exc_info:
        parse_age("200")
    assert "200" in str(exc_info.value)


# Test that no exception is raised
def test_valid_divide():
    result = divide(10, 2)
    assert result == pytest.approx(5.0)


# Floating point comparison
def test_float_comparison():
    assert 0.1 + 0.2 == pytest.approx(0.3)
    assert 0.1 + 0.2 == pytest.approx(0.3, rel=1e-9)
    assert 1.0 == pytest.approx(1.0 + 1e-8, abs=1e-7)

---

Test Structure Patterns

import pytest


# AAA pattern: Arrange, Act, Assert
def test_user_full_name():
    # Arrange
    first_name = "Alice"
    last_name = "Smith"

    # Act
    full_name = f"{first_name} {last_name}"

    # Assert
    assert full_name == "Alice Smith"


# GWT pattern: Given, When, Then (BDD style)
def test_shopping_cart():
    # Given
    cart = []
    item = {"name": "Python Book", "price": 35000}

    # When
    cart.append(item)

    # Then
    assert len(cart) == 1
    assert cart[0]["name"] == "Python Book"


# Class-based tests (grouping related tests)
class TestBankAccount:
    def test_initial_balance(self):
        balance = 0
        assert balance == 0

    def test_deposit(self):
        balance = 0
        balance += 1000
        assert balance == 1000

    def test_withdraw(self):
        balance = 5000
        balance -= 2000
        assert balance == 3000

---

pytest Options and Execution

# Basic execution
pytest                        # entire current directory
pytest tests/                 # tests folder only
pytest tests/test_calc.py     # specific file
pytest tests/test_calc.py::test_add  # specific test

# Output options
pytest -v                     # verbose
pytest -s                     # allow print output
pytest -v --tb=short          # short traceback
pytest --tb=no                # no traceback

# Filtering
pytest -k "add or subtract"   # names containing add or subtract
pytest -k "not slow"          # exclude slow
pytest -m integration         # only marked tests

# Failure control
pytest -x                     # stop on first failure
pytest --maxfail=3             # stop after 3 failures
pytest --lf                   # only last failed tests
pytest --ff                   # failed tests first

# Parallel execution (pip install pytest-xdist)
pytest -n auto                # parallel by CPU count
pytest -n 4                   # 4 workers

---

pytest.ini / pyproject.toml Configuration

# pyproject.toml
[tool.pytest.ini_options]
testpaths = ["tests"]
python_files = ["test_*.py", "*_test.py"]
python_classes = ["Test*"]
python_functions = ["test_*"]
addopts = "-v --tb=short"
markers = [
    "slow: slow tests",
    "integration: integration tests",
    "unit: unit tests",
]

# pytest.ini (separate file)
[pytest]
testpaths = tests
addopts = -v --tb=short
markers =
    slow: marks tests as slow
    integration: marks integration tests

---

Summary

Feature	Usage
Basic assert	assert result == expected
Exception testing	pytest.raises(ExcType)
Floating point	pytest.approx(value)
Show print output	pytest -s
Specific test	pytest -k "test_name"
Stop on first failure	pytest -x

pytest's greatest strength is providing detailed diffs on failure while using standard assert statements as-is.