文件操作-with和上下文管理器

代码：

# -*- coding:utf-8 -*-
# 普通版 如果写入的过程中出错 则不会释放资源
def m1():
    f = open("test.txt","w")
    f.write("hello python")
    f.close()
# 进阶版
def m2():
    f = open("test2.txt","w")
    try:
        f.write("hello python2")
    except IOError:
        print("oops error")
    finally:
        f.close()
# 高级
def m3():
    with open("test3.txt","w") as f:
        f.write("hello python3")
m1()
m2()
m3()

# 上下文管理器
# 任何实现了 __enter__() 和 __exit__() 方法的对象都可称之为上下文管理器，上下文管理器对象可以使用 with 关键字。显然，文件（file）对象也实现了上下文管理器。
class File(object):
    def __init__(self,filename,model):
        self.filename = filename
        self.model = model

    def __enter__(self):
        print("--enter--")
        self.f = open(self.filename,self.model)
        return self.f

    def __exit__(self, *args):
        print("--exit--")
        self.f.close()

with File("test4.txt","w") as f:
    f.write("hello python4")   # 输出  --enter--
                                    # --exit - -
# 装饰器实现上下文管理器
from contextlib import contextmanager

@contextmanager
def my_open(path,model,encod):
    f = open(path,model,encoding=encod)
    yield f
    f.close()

with my_open("test5.txt","w",'utf-8') as f:
    f.write("contextmanager实现上下文管理器")