Invoking a constructor in a 'with' statement

Question

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 1

The problem is that your __enter__ method returns None. Hence, test is assigned None.

Then you try to access (None).name, which raises an error. Since your __exit__ method returns True always, it will suppress any errors. According to the docs:

Returning a true value from this method will cause the with statement to suppress the exception and continue execution with the statement immediately following the with statement.

Answer 2

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)