[Python-Dev] Extended Function syntax (original) (raw)

Guido van Rossum guido@python.org
Mon, 10 Feb 2003 08:13:20 -0500

Note that this was discussed endlessly when generators were first invented. IIRC, it appeared impossible to implement this in a reasonable manner.

Sorry to reply to myself, just wanted to mention that I found a real need for iterators to have an explicit "finally" action. For my program I will have to invent some convoluted way of doing it :-(

On Mon, Feb 03, 2003 at 05:10:42PM +0100, Armin Rigo wrote: > If no first-class thunks are wanted, then maybe the most direct > path from today to auto-closing files is to extend the iterator > protocol with an optional leave() method. It would be called > exactly once when the iterator is considered to be exhausted. The > semantics of 'for' would be modified to always call it at the end > of the loop, even when an exception or a return occurs within the > loop. This seems (to me) natural because in generators you could then put a 'yield' inside of a 'try-finally' pair. The semantics of def g(): try: yield 5 finally: are quite clear if it is guaranteed that for x in g(): ... always cause the 'finally' part of the generator to be called at the end of the loop. That's even something that is straightforward to implement, given that loops already set up a block stack entry, just like try-finally clauses. It just adds an extra function to the iterator protocol (we could even reasonably re-use tpclear, for example).

But since not all generators are used in for-loops, you can't really make this the for-loop's responsibility. And the for-loop might break out and the iterator might be used more later.

The only place where you could possibly do this to cover all cases would be in the generator's destructor, which isn't guaranteed to be called at a specific time in Jython. Not covering all cases seems worse to me than the current explicit rule that you can't do this.

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