Python with as 读写文件详解

1 概述

• "传统方法"： 读写文件时，总是要写关闭语句，不但容易忘记，而且写法也比较 繁琐
• "with open() as 方法" ：实现的功能相同，但语句更加 简洁

2 详解

2.1 区别

2.1.1 传统方法

path = r"C:\Users\wangyou\Desktop\1"

full_file_name = path + r'\1.txt'

file = open(full_file_name, 'r')
try:
    file_content = file.read()
    print(file_content)
finally:
    file.close()

2.1.2 with open() as 方法

• 与上述 “传统方法” 完全一致（异常时，自动关闭文件）

path = r"C:\Users\wangyou\Desktop\1"

full_file_name = path + r'\1.txt'

with open(full_file_name, 'r') as file:  # 自定义名称，如：file
    print(file.read())

2.2 with open() as 原理

• 基本思想是 with 所求值的对象必须有一个 enter() 方法，一个 exit() 方法。
• enter() : 该方法的返回值被赋值给 as 后面的变量
• exit() : 当 with 后面的代码块全部被执行完之后，再执行该方法（如：file.close()）

建议 Debug 下列代码：

class Sample:
    def __enter__(self):
        print("In __enter__()")
        return "Foo"

    def __exit__(self, type, value, trace):
        print("In __exit__()")


def get_sample():
    return Sample()


if __name__ == '__main__':
    with get_sample() as sample:
        print("sample:", sample)

测试结果：

In __enter__()
sample: Foo
In __exit__()

3 示例

def write_file(path):
    with open(path, 'w', encoding='utf-8') as file:  # 'a': 追加
        file.write("Hello, Python!")


def read_file(path):
    with open(path, 'r', encoding='utf-8') as file:
        print(file.read())


if __name__ == '__main__':
    path = r"C:\Users\wangyou\Desktop\1"

    full_file_name = path + r'\1.txt'

    write_file(full_file_name)
    read_file(full_file_name)