pip install behave

project/
├── features/
│   ├── cart.feature                # Gherkin features
│   ├── steps/
│   │   ├── cart_steps.py            # step implementations
│   │   └── shared_steps.py
│   └── environment.py               # setup/teardown hooks
└── ...

# features/cart.feature
Feature: Apply promo code at checkout

  Background:
    Given a logged-in user
    And the cart contains 1 of "BOOK-001" at $24.99

  Scenario: Apply valid promo
    When I enter "WELCOME10" in the promo input
    And I click "Apply"
    Then the subtotal updates to $22.49

  Scenario Outline: Promo validation
    When I enter "<code>" in the promo input
    And I click "Apply"
    Then an error appears: "<error>"

    Examples:
      | code      | error                 |
      | EXPIRED50 | This code has expired |
      | NOTREAL   | Code not found        |

# features/steps/cart_steps.py
from behave import given, when, then
from app.cart import Cart
from app.checkout import CheckoutPage
from tests.fixtures import login_user

@given('a logged-in user')
def step_logged_in_user(context):
    context.user = login_user()
    context.page = CheckoutPage(context.user)

@given('the cart contains {qty:d} of "{sku}" at ${price:f}')
def step_cart_contains(context, qty, sku, price):
    context.cart = Cart()
    context.cart.add_item(sku, qty, price)
    context.page.set_cart(context.cart)

@when('I enter "{code}" in the promo input')
def step_enter_promo(context, code):
    context.page.enter_promo(code)

@when('I click "{label}"')
def step_click(context, label):
    context.page.click(label)

@then('the subtotal updates to ${expected:f}')
def step_subtotal(context, expected):
    assert abs(context.page.get_subtotal() - expected) < 0.01, \
        f"Expected {expected}, got {context.page.get_subtotal()}"

@then('an error appears: "{message}"')
def step_error(context, message):
    assert message in context.page.get_error_message()

# features/environment.py
def before_all(context):
    """Once before any scenario runs."""
    context.config.setup_logging()
    context.db = setup_test_database()

def after_all(context):
    """Once after all scenarios finish."""
    context.db.close()

def before_scenario(context, scenario):
    """Before each scenario."""
    context.db.start_transaction()

def after_scenario(context, scenario):
    """After each scenario (use scenario.status to check pass/fail)."""
    context.db.rollback()

def before_tag(context, tag):
    """Before scenarios with a specific tag."""
    if tag == 'requires_browser':
        context.browser = launch_browser()

def after_tag(context, tag):
    if tag == 'requires_browser':
        context.browser.quit()

@critical @regression
Scenario: Apply valid promo
  ...

@wip
Scenario: New checkout flow (work in progress)
  ...

# Run only critical
behave --tags=critical

# Skip wip
behave --tags=~wip

# Combine
behave --tags=critical --tags=~slow

# Plain text + JUnit XML for CI
behave --junit --junit-directory reports/junit/

# Per-feature output, no colors
behave --format=plain --no-color > test-results.txt

behave                         # all features
behave features/cart.feature   # one feature
behave --tags=@critical         # by tag
behave -i cart                  # match file pattern