Python with ... as

用法

with所求值的对象必须有一个__enter__()和__exit__()方法
with A as B:A被执行后，A所返回的对象的__enter__()方法被执行，这个方法返回的值传给了B，当with后面的代码全部完成后，将执行A所返回的对象的__exit__()

e.g. 1

file = open("test.txt")
try:
    data = file.read()
finally:
    file.close()

with open("test.txt") as file:
    data = file.read()

e.g. 2

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

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

def get_sample():
    return Sample()

with get_sample() as sample:
    print("sample:", sample)

# result
# In __enter__()
# sample: Foo
# In __exit__()

# Notes:
# __enter__()执行后，返回的值赋给了sample，然后执行with下面的代码，执行完毕后执行__exit__()

exit()的三个参数有什么用？

class Sample():
    def __enter__(self):
        return self

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

    def do_something(self):
        bar = 1/0
        return bar + 10

with Sample() as sample:
    sample.do_something()