test-writer

name	test-writer
description	How to write pytest tests for modules in this workspace. Load whenever you are about to write or extend tests.

Test writer

When writing pytest tests in this workspace:

• One test file per module. foo.py lives next to foo_test.py (suffix, not prefix).
• Cover the happy path AND at least one edge case. Empty input, duplicates, boundary values — pick what matters for the unit under test.
• Use pytest.mark.parametrize when the same assertion runs over a small table of inputs. Keep IDs descriptive.
• Name tests test_<unit>_<scenario>_<expected>. Examples: test_total_empty_cart_returns_zero, test_add_duplicate_item_merges_quantities.
• Arrange / Act / Assert. Three clear blocks. No setup hidden in fixtures unless it's reused across at least two tests.
• Assert behavior, not implementation. Don't reach into private attributes or count function calls; check the observable result.
• Imports at module top. Don't import inside test functions.