Python 入门教程（九）：文件操作与异常处理

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f = open("demo.txt", "r", encoding="utf-8")

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# ❌ 手动式：忘了 close，或中途异常导致文件句柄泄漏
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f = open("demo.txt", encoding="utf-8")
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content = f.read()
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f.close()
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# ✅ with 式：代码块结束自动关闭，异常也会关闭
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with open("demo.txt", encoding="utf-8") as f:
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    content = f.read()
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print(content)

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# w 模式：覆盖写
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with open("out.txt", "w", encoding="utf-8") as f:
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    f.write("第一行\n")
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    f.write("第二行\n")
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    f.writelines(["三\n", "四\n"])    # 写入字符串列表（不自动加换行！）
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# a 模式：追加写（记日志、断点续写）
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with open("log.txt", "a", encoding="utf-8") as f:
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    f.write(f"新的一条记录\n")

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with open("demo.txt", encoding="utf-8") as f:
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    print(f.read())        # 读完，指针在末尾
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    print(f.read())        # ''（空！指针已在末尾）
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    f.seek(0)              # 指针拨回开头
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    print(f.read())        # 又能读了
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    print(f.tell())        # 当前指针位置

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nums = [1, 2, 3]
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print(nums[10])          # IndexError，程序当场去世
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print("这行永远执行不到")

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try:
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    risky_code()
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except SomeError:
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    handle_it()

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try:
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    n = int(input("输入一个数："))     # 输入 "abc" 会抛 ValueError
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    print(100 / n)                     # n=0 会抛 ZeroDivisionError
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except ValueError:
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    print("这不是有效的数字")
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except ZeroDivisionError:
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    print("除数不能为 0")

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try:
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    n = int("abc")
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except ValueError as e:
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    print(f"错误详情：{e}")       # invalid literal for int() with base 10: 'abc'

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try:
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    data = load_config()
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except (FileNotFoundError, PermissionError):
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    print("配置文件读不了，用默认配置")
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    data = {"debug": False}
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except Exception as e:            # 兜底：捕获所有常规异常
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    print(f"未知错误: {e}")
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    raise                         # 打印后继续抛出（记日志场景常用）

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def read_score(path):
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    try:
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        f = open(path, encoding="utf-8")
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    except FileNotFoundError:
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        print("文件不存在")
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        return None
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    else:
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        # try 没出错才执行（把"成功逻辑"和"危险代码"分离）
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        score = int(f.read())
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        f.close()
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        return score
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    finally:
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        print("无论如何都执行（常用于清理资源）")

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def set_age(age):
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    if not isinstance(age, int):
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        raise TypeError("age 必须是整数")
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    if not 0 <= age <= 150:
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        raise ValueError(f"age 越界: {age}")   # 参数校验的标准做法
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    return age
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set_age("十八")     # TypeError: age 必须是整数