Catching exception in context manager __enter__()

Question

Is it possible to ensure the __exit__() method is called even if there is an exception in __enter__()?

>>> class TstCont         


        

Answer 1

I suggest you follow RAII (resource acquisition is initialization) and use the constructor of your context to do the potentially failing allocation. Then your __enter__ can simply return self which should never ever raise an exception. If your constructor fails, the exception may be thrown before even entering the with context.

class Foo:
    def __init__(self):
        print("init")
        raise Exception("booh")

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

    def __exit__(self, exc_type, exc_val, exc_tb):
        print("exit")
        return False


with Foo() as f:
    print("within with")

Output:

init
Traceback (most recent call last):
  File "", line 1, in 
  ...
    raise Exception("booh")
Exception: booh