with statement work on class

问题

{class foo(object):
    def __enter__ (self):
        print("Enter")
    def __exit__(self,type,value,traceback):
        print("Exit")
    def method(self):
        print("Method")
with foo() as instant:
    instant.method()}

Execute this py file and console shows these message:

Enter
Exit

instant.method()
AttributeError: 'NoneType' object has no attribute 'method'

unable to find methods?

回答1:

__enter__ should return self:

class foo(object):
    def __enter__ (self):
        print("Enter")
        return self
    def __exit__(self,type,value,traceback):
        print("Exit")
    def method(self):
        print("Method")
with foo() as instant:
    instant.method()

yields

Enter
Method
Exit

If __enter__ does not return self, then it returns None by default. Thus, instant is assigned the value None. This is why you get the error message

'NoneType' object has no attribute 'method'

(my emphasis)

回答2:

The problem is that your __enter__ method does not return self.

来源：https://stackoverflow.com/questions/16533794/with-statement-work-on-class