python - With 语句将变量设置为 None

Question

我正在尝试运行这段代码:

class A:
    def __enter__(self):
        print "enter"
    def __exit__(self, *args):
        print "exit"
    def __init__(self, i):
        self.i = i
with A(10) as a:
    print a.i

我得到这个错误:

enter
exit
Traceback (most recent call last):
File ".last_tmp.py", line 9, in <module>
print a.i
AttributeError: 'NoneType' object has no attribute 'i'

我的语法有什么问题？

Answer 1

您需要从 __enter__ 返回 self:

def __enter__(self):
    print "enter"
    return self

您的 with 语句实际上等同于:

a = A(10).__enter__()  # with A(10) as a:
try:
    print a.i  # Your with body
except:
    a.__exit__(exception and type)
    raise
else:
    a.__exit__(None, None, None)

所以，你需要返回一些东西，否则a会有None的值(默认返回值)，而None不会有一个名为 i 的属性，所以你会得到一个 AttributeError。