1
# 传统方式（不推荐，容易忘记 close）
2
f = open("test.txt", "r", encoding="utf-8")
3
content = f.read()
4
f.close()
5

6
# 推荐方式：with 上下文管理器（自动关闭，即使出错）
7
with open("test.txt", "r", encoding="utf-8") as f:
8
    content = f.read()
9
# 出了 with 块，文件自动关闭

1
# 写入
2
with open("note.txt", "w", encoding="utf-8") as f:
3
    f.write("第一行\n")
4
    f.write("第二行\n")
5
    f.writelines(["第三行\n", "第四行\n"])   # 写多行（不会自动加换行）
6

7
# 读取的三种方式
8
with open("note.txt", "r", encoding="utf-8") as f:
9
    all_content = f.read()          # 一次读全部（大文件慎用，占内存）
10

11
with open("note.txt", "r", encoding="utf-8") as f:
12
    lines = f.readlines()           # 读成列表，每行一个元素
13

14
# 推荐：逐行迭代（省内存，适合大文件）
15
with open("note.txt", "r", encoding="utf-8") as f:
16
    for line in f:                  # 文件对象是可迭代的
17
        print(line.strip())         # strip() 去掉行尾换行符

1
# 复制图片（二进制用 rb / wb）
2
with open("source.jpg", "rb") as src, open("copy.jpg", "wb") as dst:
3
    # 分块读取，避免大文件撑爆内存
4
    while chunk := src.read(4096):   # 海象运算符 := (Python 3.8+)
5
        dst.write(chunk)

1
try:
2
    num = int(input("输入一个整数："))
3
    result = 10 / num
4
except ValueError:                    # 捕获特定异常
5
    print("输入的不是有效整数")
6
except ZeroDivisionError:
7
    print("不能除以 0")
8
except (TypeError, KeyError) as e:    # 捕获多种，as e 拿到异常对象
9
    print(f"其他错误：{e}")
10
except Exception as e:                # 兜底捕获所有（放最后）
11
    print(f"未知错误：{e}")
12
else:
13
    print(f"没有异常，结果 = {result}")   # try 成功才执行
14
finally:
15
    print("无论如何都会执行（常用于清理资源）")

1
try 块
2
  ├─ 无异常 → 执行 else → 执行 finally
3
  └─ 有异常 → 匹配 except → 执行对应 except → 执行 finally

1
# 主动抛出异常
2
def set_age(age):
3
    if age < 0:
4
        raise ValueError("年龄不能为负数")
5
    return age
6

7
# 自定义异常类（继承 Exception）
8
class BusinessError(Exception):
9
    def __init__(self, code, message):
10
        self.code = code
11
        self.message = message
12
        super().__init__(message)
13

14
try:
15
    raise BusinessError(404, "用户不存在")
16
except BusinessError as e:
17
    print(f"错误码 {e.code}: {e.message}")
18

19
# 重新抛出 / 异常链
20
try:
21
    1 / 0
22
except ZeroDivisionError:
23
    print("记录日志...")
24
    raise                # 重新抛出原异常

1
# 方式1：类实现
2
class MyContext:
3
    def __enter__(self):
4
        print("进入")
5
        return self
6
    def __exit__(self, exc_type, exc_val, exc_tb):
7
        print("退出（自动清理）")
8
        return False     # False 表示不吞掉异常
9

10
with MyContext() as ctx:
11
    print("执行中")
12

13
# 方式2：contextlib 装饰器（更简洁）
14
from contextlib import contextmanager
15

16
@contextmanager
17
def timer():
18
    import time
19
    start = time.time()
20
    yield                       # yield 前是 __enter__，后是 __exit__
21
    print(f"耗时 {time.time()-start:.4f}s")
22

23
with timer():
24
    sum(range(1000000))

1
import requests
2
import json
3

4
def fetch_and_save(url, filename):
5
    try:
6
        resp = requests.get(url, timeout=10)
7
        resp.raise_for_status()
8
        data = resp.json()
9
        # 保存为 JSON 文件
10
        with open(filename, "w", encoding="utf-8") as f:
11
            json.dump(data, f, ensure_ascii=False, indent=2)
12
        print(f"已保存到 {filename}")
13
    except requests.RequestException as e:
14
        print(f"抓取失败：{e}")
15

16
fetch_and_save("https://api.github.com/users/jackfrued", "user.json")