With statement sets variable to None

Question

I am trying to run this code:

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

And I get this error:

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'

What is wrong with my syntax?

Answer 1

You will need to return self from __enter__:

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

Your with statement is effectively equivalent to:

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)

So, you need to return something, otherwise a will have the value of None (default return value), and None does not have an attribute named i, so you get an AttributeError.

Answer 2

The return value of the object's __enter__ method is what is assigned to the name following the as keyword. Your method (implicitly) returns None, resulting in the error you see. Instead, __enter__ should return self, so that the object created by A(10) is assigned to a.