Pytest — Fundamentals

Test Discovery

Pytest auto-discovers tests by convention:

Convention	Pattern
File names	test_*.py or *_test.py
Function names	test_*
Class names	Test* (no __init__)
Directory	tests/ (configurable via testpaths)

---

Assertions

Plain assert with automatic detailed diffs on failure.

import pytest


def test_greeting():
    assert greet("Alice") == "Hello, Alice!"


def test_membership():
    user = get_user(1)
    assert user["role"] in ("admin", "editor")


def test_exception_message():
    # match= checks the string representation of the exception
    with pytest.raises(ValueError, match="must be positive"):
        withdraw(-1)


def test_float_comparison():
    # approx avoids floating-point precision failures
    assert 0.1 + 0.2 == pytest.approx(0.3)
    assert [1.01, 2.02] == pytest.approx([1.0, 2.0], abs=0.05)

---

Fixtures

Fixtures provide test dependencies via dependency injection. yield splits setup from teardown — cleanup runs even if the test fails.

import pytest
from myapp.db import Database


@pytest.fixture
def db():
    conn = Database(":memory:")
    conn.create_tables()
    yield conn         # ← test runs here
    conn.close()       # ← always runs after test


def test_insert_user(db):
    db.insert_user("Alice")
    assert db.count_users() == 1

Fixture Scope

Scope	Lifetime	Use case
function (default)	Per test	Maximum isolation
class	Per test class	Shared setup within a class
module	Per .py file	Expensive resources per file
session	Entire run	DB pool, API auth token

@pytest.fixture(scope="session")
def api_token():
    """Created once, reused across all tests in the run."""
    return authenticate("test-user", "test-pass")


@pytest.fixture
def user_repo(db):
    """Depends on db fixture — pytest resolves the graph automatically."""
    return UserRepository(db)

autouse

Runs for every test in scope without explicit request:

@pytest.fixture(autouse=True)
def reset_db(db):
    """Rollback after each test to prevent state leakage."""
    yield
    db.rollback()

---

conftest.py

Special file for shared fixtures. Pytest discovers it automatically — no import needed.

Location	Available to
tests/conftest.py	All tests
tests/api/conftest.py	Only tests/api/

# tests/conftest.py — shared across all tests
import pytest
from myapp import create_app


@pytest.fixture(scope="session")
def app():
    """Application instance shared for the entire test run."""
    return create_app(testing=True)


@pytest.fixture
def client(app):
    """Fresh test client per test."""
    return app.test_client()

---

Parametrize

Run one test function with multiple inputs.

import pytest


@pytest.mark.parametrize("email, is_valid", [
    ("alice@example.com", True),
    ("bob@test.org", True),
    ("invalid", False),
    ("", False),
    ("@no-local.com", False),
])
def test_email_validation(email, is_valid):
    assert validate_email(email) == is_valid

Named Test IDs

@pytest.mark.parametrize("a, b, expected", [
    pytest.param(2, 3, 5, id="positives"),
    pytest.param(-1, 1, 0, id="zero-sum"),
    pytest.param(0, 0, 0, id="zeros"),
])
def test_add(a, b, expected):
    assert add(a, b) == expected

---

Markers

Tag tests to run selectively or modify behavior.

Built-in Markers

Marker	Purpose
@pytest.mark.skip(reason="...")	Always skip
@pytest.mark.skipif(cond, reason="...")	Conditional skip
@pytest.mark.xfail(reason="...")	Expected failure
@pytest.mark.usefixtures("name")	Apply fixture without requesting its value

Custom Markers

Register in pyproject.toml — --strict-markers catches typos:

[tool.pytest.ini_options]
markers = ["slow: long-running tests", "integration: needs external service"]

Run: pytest -m "not slow" · pytest -m integration.

Naming: test_<unit>_<scenario>_<expected> — e.g. test_withdraw_negative_amount_raises_error.

Output flags: -v verbose · -s show stdout · --tb=short short tracebacks · -ra summary.