1、with as 简介

有一些操作，可能事先需要设置，事后做清理工作。对于这种场景，Python的with语句提供了一种非常方便的处理方式。

例如，

with open("/tmp/foo.txt") as file:
    data = file.read()

相当于 try finally代码：

file = open("/tmp/foo.txt")
try:
    data = file.read()
finally:
    file.close()

2、with as语句中调用构造函数

使用with as时，可以在with as语句中调用构造函数，也可以在之前调用，具体有什么区别，示例代码如下，

class Test:
    def __init__(self, name):
        self.name = name
    def __enter__(self):
        print(f'entering {self.name}')
    def __exit__(self, exctype, excinst, exctb) -> bool:
        print(f'exiting {self.name}')
        return True
with Test('first') as test:
    print(f'in {test.name}')
test = Test('second')
with test:
    print(f'in {test.name}')

输出：

entering first
exiting first
entering second
in second
exiting second

可以看出with as中没有调用构造函数，__enter__方法应该返回上下文对象。with as 使用__enter__的返回值来确定要给的对象。由于__enter__不返回任何内容，它隐式返回None，因此test为None。之后的test.name就引发了一个错误，调用了Test('first').__exit__返回True，可以改成如下：

class Test:
    def __init__(self, name):
        self.name = name
    def __enter__(self):
        print(f'entering {self.name}')
        return self
    def __exit__(self, exctype, excinst, exctb) -> bool:
        print(f'exiting {self.name}')
        return True
with Test('first') as test:
    print(f'in {test.name}')
test = Test('second')
with test:
    print(f'in {test.name}')