[Python-Dev] PEP 343 - Abstract Block Redux

[Guido van Rossum]

[Nick Coghlan]
> Also, the call to __enter__() needs to be before the try/finally block (as it is
> in PEP 310). Otherwise we get the "releasing a lock you failed to acquire" problem.

I did that on purpose. There's a separate object ('abc' in the
pseudo-code of the translation) whose __enter__ and __exit__ methods
are called, and in __enter__ it can keep track of the reversible
actions it has taken.

Consider an application where you have to acquire *two* locks regularly:

    def lockBoth():
        got1 = got2 = False
        lock1.acquire(); got1 = True
        lock2.acquire(); got2 = True
        yield None
        if got2: lock2.release()
        if got1: lock1.release()

If this gets interrupted after locking lock1 but before locking lock2,
it still has some cleanup to do.

I know that this complicates simpler use cases, and I'm not 100% sure
this is the right solution; but I don't know how else to handle this
use case.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)