how to implement behaviour of 'with' blocks in python

withblock.py

class Exiter():
    def __enter__(self):
        print('i am in a block')
        return self # critical
    def __exit__(self, type, value, tb):
        print('goodbye cruel world')
    def add(self, a, b):
        return a + b

with Exiter() as exiter:
    val = exiter.add(5, 10)
    print('the sum of 5 and 10 is ' + str(val))