Invoking a constructor in a ‘with’ statement

I have the following code:

class Test:

    def __init__(self, name):
        self.name = name

    def __enter__(self):
        print(f'entering {self.name}')

    def __exit__(self, exctype, excinst, exctb) -> bool:
        print(f'exiting {self.name}')
        return True

with Test('first') as test:
    print(f'in {test.name}')

test = Test('second')
with test:
    print(f'in {test.name}')

Running it produces the following output:

entering first
exiting first
entering second
in second
exiting second

But I expected it to produce:

entering first
in first
exiting first
entering second
in second
exiting second

Why isn’t the code within my first example called?

Answer

The __enter__ method should return the context object. with ... as ... uses the return value of __enter__ to determine what object to give you. Since your __enter__ returns nothing, it implicitly returns None, so test is None.

with Test('first') as test:
    print(f'in {test.name}')

test = Test('second')
with test:
    print(f'in {test.name}')

So test is none. Then test.name is an error. That error gets raised, so Test('first').__exit__ gets called. __exit__ returns True, which indicates that the error has been handled (essentially, that your __exit__ is acting like an except block), so the code continues after the first with block, since you told Python everything was fine.

Consider

def __enter__(self):
    print(f'entering {self.name}')
    return self

You might also consider not returning True from __exit__ unless you truly intend to unconditionally suppress all errors in the block (and fully understand the consequences of suppressing other programmers’ errors, as well as KeyboardInterrupt, StopIteration, and various system signals)