#!/usr/bin/env python3 """ Day 017 — 断言与防御式编程示例 演示 assert 的正确用法和防御式编程的多种策略。 运行方式： python3 07-assert-defensive.py python3 -O 07-assert-defensive.py # 看断言被禁用的效果 """ import sys import math SEP = "=" * 60 # ═══════════════════════════════════════════════════════════════ # Part 1：assert 的正确用法 # ═══════════════════════════════════════════════════════════════ def demo_assert_preconditions(): """断言用于检查前置条件（开发期）""" print(SEP) print("1️⃣ assert 前置条件检查") print(SEP) def sqrt_safe(x): """安全开方 — 前置条件断言""" assert x >= 0, f"sqrt_safe: x={x} 必须 >= 0" return math.sqrt(x) # 正常 result = sqrt_safe(9.0) print(f" ✅ sqrt_safe(9.0) = {result}") # 断言失败 try: sqrt_safe(-1.0) except AssertionError as e: print(f" ❌ 断言触发: {e}") print("\n ⚠️ Python -O 模式下 assert 被禁用，检查失效！") def demo_assert_invariants(): """断言用于检查内部不变量""" print(SEP) print("2️⃣ assert 内部不变量检查") print(SEP) class BankAccount: def __init__(self, balance: float): assert balance >= 0, "初始余额不能为负" self.balance = balance def deposit(self, amount: float): assert amount > 0, "存款金额必须为正" old = self.balance self.balance += amount # 后置条件：余额应该增加 assert self.balance > old, f"存款后余额应增加: {old} → {self.balance}" def withdraw(self, amount: float): assert amount > 0, "取款金额必须为正" assert self.balance >= amount, "余额不足" self.balance -= amount assert self.balance >= 0, f"取款后余额不能为负: {self.balance}" account = BankAccount(100.0) account.deposit(50.0) print(f" ✅ 存款后余额: {account.balance}") account.withdraw(30.0) print(f" ✅ 取款后余额: {account.balance}") def demo_assert_postconditions(): """断言用于检查后置条件""" print(SEP) print("3️⃣ assert 后置条件检查") print(SEP) def sort_and_validate(items: list) -> list: """排序并验证结果""" result = sorted(items) # 后置条件：结果已排序 assert all(result[i] <= result[i + 1] for i in range(len(result) - 1)), \ "排序结果不满足升序要求" # 后置条件：元素数量不变 assert len(result) == len(items), "元素数量变化" return result data = [3, 1, 4, 1, 5, 9, 2, 6] sorted_data = sort_and_validate(data) print(f" ✅ 排序验证通过: {sorted_data}") # ═══════════════════════════════════════════════════════════════ # Part 2：防御式编程 # ═══════════════════════════════════════════════════════════════ def demo_type_guarding(): """类型守卫""" print(SEP) print("4️⃣ 防御式编程 — 类型守卫") print(SEP) def safe_divide(a, b): """带类型守卫的除法""" # 类型守卫 if not isinstance(a, (int, float)): raise TypeError(f"a 必须是数字，收到 {type(a).__name__}") if not isinstance(b, (int, float)): raise TypeError(f"b 必须是数字，收到 {type(b).__name__}") # 值守卫 if b == 0: raise ValueError("除数不能为零") return a / b # ✅ 正常 print(f" ✅ safe_divide(10, 3) = {safe_divide(10, 3):.4f}") # ❌ 类型错误 try: safe_divide("10", 3) except TypeError as e: print(f" ❌ {e}") # ❌ 值错误 try: safe_divide(10, 0) except ValueError as e: print(f" ❌ {e}") def demo_guard_clause(): """Guard Clause 模式""" print(SEP) print("5️⃣ 防御式编程 — Guard Clause") print(SEP) def get_user_score(data: dict) -> float: """带多层守卫的分数获取函数""" # Guard 1: data 为 None if data is None: raise ValueError("数据不能为空") # Guard 2: 键不存在 if "score" not in data: raise KeyError("缺少 score 字段") # Guard 3: 类型不正确 score = data["score"] if not isinstance(score, (int, float)): raise TypeError(f"score 必须是数字，收到 {type(score).__name__}") # Guard 4: 值范围不正确 if score < 0 or score > 100: raise ValueError(f"score 必须在 [0, 100] 范围内，收到 {score}") return float(score) # 测试 test_cases = [ (None, "None"), ({}, "空字典"), ({"score": "abc"}, "字符串分数"), ({"score": 150}, "超范围"), ({"score": 85}, "✅ 正常"), ] for data, desc in test_cases: try: result = get_user_score(data) print(f" ✅ {desc}: {result}") except (ValueError, KeyError, TypeError) as e: print(f" ❌ {desc}: {e}") def demo_result_object(): """Result Object 模式 — 返回成功/失败""" print(SEP) print("6️⃣ 防御式编程 — Result Object 模式") print(SEP) from typing import NamedTuple, Optional class Result(NamedTuple): """表示操作结果""" success: bool value: Optional[object] = None error: Optional[str] = None def parse_age(text: str) -> Result: """安全解析年龄，返回 Result 而不是抛异常""" try: age = int(text.strip()) if age < 0 or age > 150: return Result(False, error=f"年龄 {age} 超出 [0, 150]") return Result(True, value=age) except (ValueError, AttributeError) as e: return Result(False, error=f"解析失败: {e}") tests = ["25", "abc", "-5", "200", ""] for t in tests: r = parse_age(t) if r.success: print(f" ✅ '{t}' → {r.value}") else: print(f" ❌ '{t}' → {r.error}") # ═══════════════════════════════════════════════════════════════ # Part 3：断言 vs 异常 对比 # ═══════════════════════════════════════════════════════════════ def demo_assert_vs_exception(): """断言 vs 异常 场景对比""" print(SEP) print("7️⃣ 断言 vs 异常 — 场景对比") print(SEP) scenarios = [ ("用户输入验证", "❌ assert", "✅ 异常"), ("内部不变量", "✅ assert", "❌ 异常"), ("函数前置条件（开发期）", "✅ assert", "✅ 可选"), ("外部数据验证", "❌ assert", "✅ 异常"), ("后置条件验证", "✅ assert", "❌ 异常"), ("可恢复错误", "❌ assert", "✅ 异常"), ("单元测试", "✅ assert", "❌ 异常"), ] print(f" {'场景':<30s} {'用 assert':<15s} {'用异常':<15s}") print(f" {'-'*30} {'-'*15} {'-'*15}") for scene, a, e in scenarios: print(f" {scene:<30s} {a:<15s} {e:<15s}") print() print(" 📌 黄金法则:") print(" assert: 开发期调试、内部不变量") print(" 异常: 用户输入、外部数据、可恢复错误") # ═══════════════════════════════════════════════════════════════ # 入口 # ═══════════════════════════════════════════════════════════════ def main(): print("🐍 Python 断言与防御式编程") print() demo_assert_preconditions() demo_assert_invariants() demo_assert_postconditions() demo_type_guarding() demo_guard_clause() demo_result_object() demo_assert_vs_exception() print(SEP) print("✅ 演示完毕！") print("📌 总结:") print(" assert → 开发期调试、内部不变量") print(" raise → 用户输入、外部数据验证") print(" Guard Clause → 多层防御") print(" Result Object → 优雅处理预期内错误") if __name__ == "__main__": main()