Lab 004：Agent 自动写测试，是增强质量还是制造幻觉？

flowchart TD
  A["输入规格和源码"] --> B["Agent 生成测试计划"]
  B --> C["人工或规则检查测试计划"]
  C --> D["Agent 写测试代码"]
  D --> E["运行测试"]
  E --> F{"是否能失败"}
  F -->|否| G["怀疑空测试或弱断言"]
  F -->|是| H["修复实现或确认缺陷"]
  H --> I["变异检查"]
  I --> J["纳入回归测试"]

test_generation_input:
  feature_name: "订单优惠券计算"
  source_files:
    - "app/order/discount.py"
  spec:
    - "百分比优惠券最多抵扣订单金额的 50%"
    - "固定金额优惠券不能让订单金额小于 0"
    - "过期优惠券必须被拒绝"
  known_bugs:
    - "曾经出现 fixed coupon 导致 total=-1 的问题"
  test_framework: "pytest"
  constraints:
    - "不要 mock 被测函数本身"
    - "必须包含边界值"
    - "每个测试必须有明确断言"

## 测试计划

1. 百分比优惠券正常抵扣
   - 输入：total=100, percent=20
   - 期望：final_total=80

2. 百分比优惠券超过 50% 上限
   - 输入：total=100, percent=80
   - 期望：final_total=50

3. 固定金额优惠券超过订单金额
   - 输入：total=30, fixed=50
   - 期望：final_total=0

4. 过期优惠券
   - 输入：expires_at < now
   - 期望：抛出 CouponExpired

from datetime import datetime, timedelta, timezone

import pytest

from app.order.discount import Coupon, CouponExpired, apply_coupon


def test_percent_coupon_applies_discount():
    coupon = Coupon(kind="percent", value=20, expires_at=datetime.now(timezone.utc) + timedelta(days=1))

    assert apply_coupon(total=100, coupon=coupon) == 80


def test_percent_coupon_is_capped_at_half_total():
    coupon = Coupon(kind="percent", value=80, expires_at=datetime.now(timezone.utc) + timedelta(days=1))

    assert apply_coupon(total=100, coupon=coupon) == 50


def test_fixed_coupon_never_makes_total_negative():
    coupon = Coupon(kind="fixed", value=50, expires_at=datetime.now(timezone.utc) + timedelta(days=1))

    assert apply_coupon(total=30, coupon=coupon) == 0


def test_expired_coupon_is_rejected():
    coupon = Coupon(kind="fixed", value=10, expires_at=datetime.now(timezone.utc) - timedelta(days=1))

    with pytest.raises(CouponExpired):
        apply_coupon(total=100, coupon=coupon)

import ast
from pathlib import Path


def test_generated_tests_have_assertions():
    for path in Path("tests/generated").glob("test_*.py"):
        tree = ast.parse(path.read_text())
        test_funcs = [n for n in ast.walk(tree) if isinstance(n, ast.FunctionDef) and n.name.startswith("test_")]
        assert test_funcs, f"{path} has no test functions"

        for func in test_funcs:
            has_assert = any(isinstance(n, ast.Assert) for n in ast.walk(func))
            has_pytest_raises = any(
                isinstance(n, ast.With)
                and any("pytest.raises" in ast.unparse(item.context_expr) for item in n.items)
                for n in ast.walk(func)
            )
            assert has_assert or has_pytest_raises, f"{func.name} has no assertion"

cp app/order/discount.py /tmp/discount.py.bak
python scripts/mutate_discount_cap.py

if pytest tests/generated/test_discount.py -q; then
  echo "generated tests did not catch mutation"
  cp /tmp/discount.py.bak app/order/discount.py
  exit 1
fi

cp /tmp/discount.py.bak app/order/discount.py

test_generation_result:
  generated_files:
    - "tests/generated/test_discount.py"
  test_count: 4
  assertion_count: 4
  covers_known_bugs: true
  includes_boundary_cases: true
  initial_run:
    passed: true
    command: "pytest tests/generated/test_discount.py -q"
  mutation_check:
    passed: true
    killed_mutations: 3
    survived_mutations: 0
  reviewer_notes:
    - "测试验证业务规格，没有 mock 被测函数"