#!/usr/bin/env python3 """ Day 017 — 异常处理基础用法示例 涵盖： 1. try/except 基本捕获 2. 多个 except 子句 3. try/except/else 4. try/except/finally 5. 完整结构：try/except/else/finally 6. 异常链与 raise from 7. 重抛异常 8. 断言 assert 运行方式： python3 01-exception-basics.py """ import sys import json import traceback SEP = "=" * 60 # ═══════════════════════════════════════════════════════════════ # 1. try/except 最简形式 # ═══════════════════════════════════════════════════════════════ def demo_basic_try_except(): """基本 try/except 捕获异常""" print(SEP) print("1️⃣ 基本 try/except") print(SEP) try: result = 10 / 0 print("这行不会执行") except ZeroDivisionError: print("捕获到: 不能除以零！") # 捕获多个异常 try: values = {"a": 1} print(values["b"]) # KeyError except (KeyError, IndexError) as e: print(f"捕获到多个异常类型: {e}") # 使用 as 获取异常对象 try: int("不是数字") except ValueError as e: print(f"ValueError: {e} (类型: {type(e).__name__})") # ═══════════════════════════════════════════════════════════════ # 2. 分层捕获多个不同异常 # ═══════════════════════════════════════════════════════════════ def demo_hierarchical_except(): """分层捕获不同异常""" print(SEP) print("2️⃣ 分层捕获多个异常") print(SEP) def risky_calc(x): """根据 x 的值触发不同异常""" if x == 0: return 10 / 0 # ZeroDivisionError elif x == 1: return int("abc") # ValueError elif x == 2: d = {} return d["missing"] # KeyError return 42 for x in [0, 1, 2, 3]: try: print(f"\nrisky_calc({x}) = ", end="") result = risky_calc(x) print(result) except ZeroDivisionError: print("❌ ZeroDivisionError — 除零错误") except ValueError: print("❌ ValueError — 值无效") except KeyError as e: print(f"❌ KeyError — 键不存在: {e}") else: print(f"✅ 成功: {result}") finally: print(f" finally: 清理操作 (x={x})") # ═══════════════════════════════════════════════════════════════ # 3. try/except/else — 无异常时的分支 # ═══════════════════════════════════════════════════════════════ def demo_else_clause(): """else 子句 — 仅当无异常时执行""" print(SEP) print("3️⃣ else 子句 (无异常时执行)") print(SEP) def safe_divide(a, b): """安全除法""" try: result = a / b except ZeroDivisionError: print(f" {a}/{b} = ❌ 除零错误") return None except TypeError: print(f" {a}/{b} = ❌ 类型错误") return None else: # 仅当 try 完成无异常时执行 print(f" {a}/{b} = ✅ {result}") return result finally: print(f" [finally] 除法运算结束") safe_divide(10, 2) safe_divide(10, 0) safe_divide(10, "a") # ═══════════════════════════════════════════════════════════════ # 4. try/except/finally — 始终执行清理 # ═══════════════════════════════════════════════════════════════ def demo_finally(): """finally 始终执行 — 即使 return 也会执行""" print(SEP) print("4️⃣ finally — 始终执行") print(SEP) def read_file(path): """模拟文件读取，演示 finally""" f = None try: print(f" 尝试打开: {path}") f = open(path, "r") return f.read() except FileNotFoundError: print(f" ❌ 文件不存在: {path}") return "FALLBACK_DATA" finally: print(f" ✅ finally: 确保关闭文件...") if f and not f.closed: f.close() print(f" ✅ 文件已关闭: {f.closed}") print("\n--- 情况 1: 文件存在 ---") # 创建临时文件 with open("/tmp/_demo_file.txt", "w") as tf: tf.write("Hello, 异常处理!") print(f" 内容: {read_file('/tmp/_demo_file.txt')!r}") print("\n--- 情况 2: 文件不存在 ---") print(f" 内容: {read_file('/nonexistent.txt')!r}") # ═══════════════════════════════════════════════════════════════ # 5. 异常链与 raise from # ═══════════════════════════════════════════════════════════════ class ConfigError(Exception): """配置错误""" pass def demo_exception_chaining(): """异常链 — raise from""" print(SEP) print("5️⃣ 异常链 — raise ... from") print(SEP) config_data = '{"name": "test", "version": }' # 无效 JSON def load_config(data: str) -> dict: """加载配置，将底层异常包装为业务异常""" try: return json.loads(data) except json.JSONDecodeError as e: # 使用 from 建立异常链 raise ConfigError("配置文件 JSON 格式无效") from e def load_config_suppressed(data: str) -> dict: """使用 from None 抑制异常链""" try: return json.loads(data) except json.JSONDecodeError: raise ConfigError("配置文件错误") from None # 展示异常链 print("\n▶ 使用 raise ... from (保留因果链):") try: load_config(config_data) except ConfigError as e: print(f" ConfigError: {e}") cause = e.__cause__ if cause: print(f" ├── 由 {type(cause).__name__} 引发: {cause}") print(f" └── traceback:\n{''.join(traceback.format_exception_only(type(cause), cause)).rstrip()}") print("\n▶ 使用 raise ... from None (抑制因果链):") try: load_config_suppressed(config_data) except ConfigError as e: print(f" ConfigError: {e}") print(f" __cause__ = {e.__cause__}") # None # ═══════════════════════════════════════════════════════════════ # 6. 重抛与保留追踪 # ═══════════════════════════════════════════════════════════════ def demo_reraise(): """重抛异常（保留完整 traceback）""" print(SEP) print("6️⃣ 重抛异常 — 保留完整 traceback") print(SEP) def low_level(): """底层函数""" raise PermissionError("底层: 没有权限访问资源") def mid_level(): """中层函数""" try: low_level() except PermissionError as e: print(f" [mid_level] 记录日志: {e}") # 记录后重抛，保留原始 traceback print(f" [mid_level] 重抛异常...") raise # 裸 raise = 重抛当前异常 def high_level(): """高层函数""" try: mid_level() except PermissionError as e: print(f" [high_level] 最终处理: {e}") print("\n▶ 重抛流程:") high_level() # ═══════════════════════════════════════════════════════════════ # 7. 断言 assert # ═══════════════════════════════════════════════════════════════ def demo_assert(): """断言用法与注意事项""" print(SEP) print("7️⃣ 断言 — assert 语句") print(SEP) def withdraw(balance: float, amount: float) -> float: """模拟取款（内部不变量检查）""" assert amount > 0, f"取款金额 ({amount}) 必须为正数" assert balance >= amount, f"余额 ({balance}) 不足 ({amount})" return balance - amount # 正常情况 result = withdraw(100.0, 30.0) print(f" ✅ 取款成功, 余额: {result}") # 断言失败 try: withdraw(50.0, -10.0) except AssertionError as e: print(f" ❌ 断言触发: {e}") try: withdraw(50.0, 100.0) except AssertionError as e: print(f" ❌ 断言触发: {e}") # 注意：断言不等于输入验证 print("\n ⚠️ 注意: 断言可被 -O 禁用，") print(" 请勿用 assert 替代用户输入的验证！") # ═══════════════════════════════════════════════════════════════ # 8. sys.exc_info 与 traceback 模块 # ═══════════════════════════════════════════════════════════════ def demo_exc_info(): """使用 sys.exc_info() 与 traceback 模块""" print(SEP) print("8️⃣ sys.exc_info() 与 traceback 模块") print(SEP) try: 1 / 0 except ZeroDivisionError: exc_type, exc_value, exc_tb = sys.exc_info() print(f" 异常类型: {exc_type.__name__}") print(f" 异常值: {exc_value}") print(f" traceback: {exc_tb}") print(f"\n 格式化 traceback:") fmt = traceback.format_exc() for line in fmt.strip().split("\n"): print(f" {line}") # ═══════════════════════════════════════════════════════════════ # 主函数 # ═══════════════════════════════════════════════════════════════ def main(): print("🐍 Python 异常处理基础用法示例") print() demo_basic_try_except() demo_hierarchical_except() demo_else_clause() demo_finally() demo_exception_chaining() demo_reraise() demo_assert() demo_exc_info() print(SEP) print("✅ 所有演示完成！") if __name__ == "__main__": main()