pytest fixtures and parametrize for a pricing rule

# ======================================================================
# Q: Show me pytest with fixtures and parametrize for a pricing rule.
#
# The fixture provides shared data, and parametrize turns the business table
# into tests. I often take the table of examples from the product owner as is:
# it also becomes documentation of the rule.
# ======================================================================

import pytest
from decimal import Decimal as D
from pricing import Line, order_total

@pytest.fixture
def lines():
    return [Line(D('10.00'), 3), Line(D('2.50'), 4)]       # 30 + 10 = 40

@pytest.mark.parametrize('discount, expected', [
    (D('0'),     D('40.00')),
    (D('0.10'),  D('36.00')),
    (D('0.333'), D('26.68')),      # 26.68 after ROUND_HALF_UP
    (D('1'),     D('0.00')),
], ids=['none', '10pct', 'odd', 'full'])
def test_order_total(lines, discount, expected):
    assert order_total(lines, discount) == expected

def test_empty_cart_is_zero():
    assert order_total([], D('0.1')) == D('0.00')