课程 16：文件操作（File I/O）

1. 文件操作基础

1.1 什么是文件操作？

# 文件路径可以是相对路径或绝对路径
# 常见模式：'r' 只读，'w' 写入（覆盖），'a' 追加，'b' 二进制

1.2 打开与关闭文件

# 打开文件（推荐使用with自动关闭文件）
with open('data.txt', 'r', encoding='utf-8') as f:
    # 文件操作
    pass
# 传统方式
f = open('data.txt', 'r', encoding='utf-8')
f.close()

2. 文件读取操作

2.1 读取全部内容

with open('data.txt', 'r', encoding='utf-8') as f:
    content = f.read()
    print(content)

2.2 逐行读取

with open('data.txt', 'r', encoding='utf-8') as f:
    for line in f:
        print(line.strip())

2.3 读取部分内容

with open('data.txt', 'r', encoding='utf-8') as f:
    first10 = f.read(10)  # 读取前10个字符
    print(first10)

with open('data.txt', 'r', encoding='utf-8') as f:
    lines = f.readlines()
    print(lines)

3. 文件写入操作

3.1 覆盖写入

with open('output.txt', 'w', encoding='utf-8') as f:
    f.write('Hello, world!\n')

3.2 追加写入

with open('output.txt', 'a', encoding='utf-8') as f:
    f.write('追加内容\n')

3.3 写入多行

lines = ['第一行\n', '第二行\n', '第三行\n']
with open('output.txt', 'w', encoding='utf-8') as f:
    f.writelines(lines)

3.4 JSON 读写

import json

# 写入 JSON 文件（推荐：ensure_ascii=False 保留中文，indent 美化）
data = {
    'name': '小明',
    'age': 18,
    'scores': [95, 88, 92],
    'active': True
}
with open('data.json', 'w', encoding='utf-8') as f:
    json.dump(data, f, ensure_ascii=False, indent=2)

# 读取 JSON 文件
with open('data.json', 'r', encoding='utf-8') as f:
    loaded = json.load(f)
print(loaded)

# 在内存中进行 JSON 与字符串互转
json_str = json.dumps(data, ensure_ascii=False, indent=2)
print(json_str)
parsed = json.loads(json_str)
print(parsed['name'])

# 序列化非内置类型（示例：将无法序列化对象用 str 处理）
from datetime import datetime
payload = { 'time': datetime(2025, 1, 1, 8, 0, 0) }
json_str_safe = json.dumps(payload, default=str)
print(json_str_safe)  # {"time": "2025-01-01 08:00:00"}

4. 二进制文件操作

4.1 读取与写入二进制文件

# 读取图片、音频等二进制文件
with open('image.png', 'rb') as f:
    data = f.read()
# 写入二进制文件
with open('copy.png', 'wb') as f:
    f.write(data)

5. 文件操作的常见错误与调试

try:
    with open('nofile.txt', 'r', encoding='utf-8') as f:
        content = f.read()
except FileNotFoundError:
    print('文件不存在！')

6. 实际应用案例

6.1 日志记录

def log_message(msg):
    with open('log.txt', 'a', encoding='utf-8') as f:
        f.write(msg + '\n')
log_message('程序启动')

6.2 数据持久化

# 保存和读取用户分数
score = 95
with open('score.txt', 'w', encoding='utf-8') as f:
    f.write(str(score))
with open('score.txt', 'r', encoding='utf-8') as f:
    saved_score = int(f.read())
print(saved_score)

6.3 文本文件批量处理

# 统计文本文件的行数和单词数
def count_file(filename):
    lines = 0
    words = 0
    with open(filename, 'r', encoding='utf-8') as f:
        for line in f:
            lines += 1
            words += len(line.split())
    return lines, words
print(count_file('data.txt'))