Trying to figure out how the 'with..as' construct works in python

Question

I am trying to learn python and I landed on the

with..as

construct, that used like this:

with open("somefile.txt", 'rt') as file:
    print(file.read()) 
    # at the end of execution file.close() is called automatically.

So as a learning strategy I tried to do the following:

class Derived():

    def __enter__(self):
        print('__enter__')

    def __exit__(self, exc_type, exc_value, traceback):
        print('__exit__')

with  Derived() as derived:
    print(derived)

and I got this output:

__enter__
None
__exit__

My question is then:

• why did print(derived) return a None object and not a Derived object?

Answer 1

The name derived is bound to the object returned by the __enter__ method, which is None. Try:

def __enter__(self):
    print('__enter__')
    return self

Docs:

object.__enter__(self)

Enter the runtime context related to this object. The with statement will bind this method’s return value to the target(s) specified in the as clause of the statement, if any.