Python 文件操作

12.1 打开与关闭文件

open() 函数

# 基本语法
file = open("filename", mode)

# 使用完毕后关闭
file.close()

文件模式

模式	说明
`r`	读取（默认）
`w`	写入（覆盖）
`a`	追加
`x`	创建（存在则报错）
`b`	二进制模式
`t`	文本模式（默认）
`+`	读写模式

12.2 with 语句（推荐）

with 语句自动管理文件关闭，是最佳实践。

# 读取文件
with open("file.txt", "r") as f:
    content = f.read()
# 文件自动关闭

# 写入文件
with open("file.txt", "w") as f:
    f.write("Hello, World!")

12.3 读取文件

读取方法

with open("example.txt", "r") as f:
    # read() - 读取全部内容
    content = f.read()
    print(content)

with open("example.txt", "r") as f:
    # readline() - 读取一行
    line1 = f.readline()
    line2 = f.readline()
    print(line1, line2)

with open("example.txt", "r") as f:
    # readlines() - 读取所有行（列表）
    lines = f.readlines()
    for line in lines:
        print(line.strip())

逐行读取（推荐大文件）

1
2
3

with open("large_file.txt", "r") as f:
    for line in f:
        print(line.strip())

12.4 写入文件

write() 方法

# 写入（覆盖）
with open("output.txt", "w") as f:
    f.write("第一行\n")
    f.write("第二行\n")

# 追加
with open("output.txt", "a") as f:
    f.write("追加的内容\n")

writelines() 方法

lines = ["Line 1\n", "Line 2\n", "Line 3\n"]

with open("output.txt", "w") as f:
    f.writelines(lines)

12.5 文件路径操作

pathlib 模块（推荐）

from pathlib import Path

# 创建路径
p = Path("data/file.txt")

# 路径拼接
full_path = p.parent / "new_file.txt"

# 检查存在
print(p.exists())

# 检查类型
print(p.is_file())
print(p.is_dir())

# 获取信息
print(p.name)      # file.txt
print(p.stem)      # file
print(p.suffix)    # .txt
print(p.parent)    # data

# 创建目录
Path("new_dir").mkdir(exist_ok=True)

# 列出文件
for file in Path(".").iterdir():
    print(file.name)

os.path 模块

import os

path = "data/file.txt"

print(os.path.exists(path))
print(os.path.isfile(path))
print(os.path.isdir(path))
print(os.path.basename(path))  # file.txt
print(os.path.dirname(path))   # data
print(os.path.join("data", "file.txt"))  # data/file.txt

12.6 JSON 文件处理

写入 JSON

import json

data = {
    "name": "Alice",
    "age": 25,
    "skills": ["Python", "JavaScript"]
}

with open("data.json", "w") as f:
    json.dump(data, f, indent=2, ensure_ascii=False)

读取 JSON

import json

with open("data.json", "r") as f:
    data = json.load(f)

print(data["name"])  # Alice
print(data["skills"])  # ['Python', 'JavaScript']

JSON 字符串

import json

# 字典转 JSON 字符串
data = {"name": "Alice", "age": 25}
json_str = json.dumps(data, ensure_ascii=False)
print(json_str)

# JSON 字符串转字典
parsed = json.loads(json_str)
print(parsed)

12.7 CSV 文件处理

写入 CSV

import csv

data = [
    ["Name", "Age", "City"],
    ["Alice", 25, "Beijing"],
    ["Bob", 30, "Shanghai"],
    ["Charlie", 22, "Guangzhou"]
]

with open("data.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerows(data)

读取 CSV

import csv

with open("data.csv", "r", encoding="utf-8") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

# 使用 DictReader
with open("data.csv", "r", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["Name"], row["Age"])

12.8 异常处理

文件操作可能抛出异常，需要处理。

try:
    with open("nonexistent.txt", "r") as f:
        content = f.read()
except FileNotFoundError:
    print("文件不存在")
except PermissionError:
    print("没有权限")
except Exception as e:
    print(f"发生错误: {e}")

12.9 实用示例

统计文件行数

def count_lines(filename):
    with open(filename, "r") as f:
        return sum(1 for _ in f)

print(count_lines("example.txt"))

复制文件

def copy_file(src, dst):
    with open(src, "rb") as f_src:
        with open(dst, "wb") as f_dst:
            f_dst.write(f_src.read())

copy_file("source.txt", "copy.txt")

查找文件中包含关键词的行

def search_file(filename, keyword):
    results = []
    with open(filename, "r") as f:
        for i, line in enumerate(f, 1):
            if keyword in line:
                results.append((i, line.strip()))
    return results

matches = search_file("log.txt", "ERROR")
for line_num, content in matches:
    print(f"第 {line_num} 行: {content}")

12.10 本章小结

使用 with open() 自动管理文件关闭
读取方法：read()、readline()、readlines()
写入方法：write()、writelines()
pathlib 是现代路径操作推荐方式
JSON 使用 json.dump() 和 json.load()
CSV 使用 csv.writer 和 csv.reader
文件操作需要异常处理

练习题

编写程序读取文件，统计单词出现频率
将字典数据保存为 JSON 文件，再读取并打印
创建 CSV 文件存储学生成绩，计算平均分
编写程序合并两个文本文件的内容