[Python-Dev] Simpler finalization semantics (was Re: PEP 343 - Abstract Block Redux)

[Phillip J. Eby]

At 11:03 PM 5/17/2005 +1000, Nick Coghlan wrote:
>Do you mean translating this:
>
>    with EXPR1 as VAR1:
>        BLOCK1
>
>To something along the lines of:
>
>    the_stmt = EXPR1
>    stmt_enter = getattr(the_stmt, "__enter__", None)
>    stmt_exit = getattr(the_stmt, "__exit__", None)
>
>    if stmt_enter is None:
>        VAR1 = the_stmt
>    else:
>        VAR1 = stmt_enter()
>
>    if stmt_exit is None:
>        BLOCK1
>    else:
>        exc = (None, None, None)
>        try:
>            try:
>                BLOCK1
>            except:
>                exc = sys.exc_info()
>                raise
>        finally:
>            stmt_exit(*exc)

Essentially, yes; although I was actually suggesting that there be 
PyResource_Enter() and PyResource_Exit() C APIs that would supply the 
default behaviors if the tp_resource_enter and tp_resource_exit slots were 
missing from the object's type.  But that's an implementation detail.


>It has a certain elegance - you can switch from using an object that needs
>finalisation to one that doesn't without having to change your code.
>
>And library code can safely ensure finalisation without having to check 
>whether
>the object needs it or not - that check becomes an inherent part of the with
>statement.

Precisely.