I would prefer plain 'yield' to 'yield break' as a synonym for 'raise StopIteration'. But the raise stands out better visually as an exit point. The point about the raise not being explicitly caught is not valid since any generator could be used in the following code g = some_gen try: while True: process(g.next()) except StopIteration: whatever() where it is caught. Then under your proposal, there would be a catch without a throw ;-(. If there were a use case for empty generators, then iter() could be made to produce one, in analogy with int() returning 0, etc. But without a use case, 'iter()' is much more likely to be a bug and should be flagged as such. In the meanwhile, 'iter(())' is trivial to type. Terry Jan Reedy

In your example, why do you "raise StopIteration" instead just writing "return"? ----- Original Message ----- From: "Manuel Alejandro Cerón Estrada" Take a look at this example: def lines(): for line in my_file: if some_error(): raise StopIteration() yield line yield 'end' for line in lines(): do_something()

2007/12/8, Raymond Hettinger :

In your example, why do you "raise StopIteration" instead just writing "return"?

In the example would be better to use 'return' rather than 'raise StopIteration'. In most cases 'return' will have the same behavior than 'raise StopIteration', but not always. Acording to PEP 255: Note that return isn't always equivalent to raising StopIteration: the difference lies in how enclosing try/except constructs are treated. The trivial example is the empty generator where return is ambiguous with the same statement for normal functions. Maybe my proposal can change to "Replace 'return' with 'yield break' in generators" Again, PEP 255 talks about the possibility of removing 'return' from generators Q. Why allow "return" at all? Why not force termination to be spelled "raise StopIteration"? A. The mechanics of StopIteration are low-level details, much like the mechanics of IndexError in Python 2.1: the implementation needs to do *something* well-defined under the covers, and Python exposes these mechanisms for advanced users. That's not an argument for forcing everyone to work at that level, though. "return" means "I'm done" in any kind of function, and that's easy to explain and to use. Note that "return" isn't always equivalent to "raise StopIteration" in try/except construct, either (see the "Specification: Return" section). Of curse, the problem of low level details it's solved by 'yield break' Manuel.

2007/12/8, Raymond Hettinger :

...

...the proposal adds new syntax without adding functionality.

That is indeed the definition of syntactic sugar [1]. Python is full of that, for example the import statement. . . . The real problem is that raising StopIteration is not orthogonal. It is confusing for the reasons I exposed. Generators are something in the middle between a language feature and a framework/library. With 'yield break' they will be completely a language feature.

This personal notion of purity carries almost zero weight and is more a matter of personal taste/style than anything else. It certainly does not warrant a change to the grammar. I've been a big fan of generators and tend to use them everywhere. I've *never* had a use case for "yield break" and find it uncommon to even "raise StopIteration". The tone of your messages suggests that you're going to stick to this idea like glue, so to avoid further time wasting, this will likely be my last post on the subject. If for some reason, you persist through to writing a PEP, it will be almost mandatory to scan existing, published code to find examples of code fragments that would be improved by 'yield break' . My bet is that the examples will be rare and any "improvement" will be marginal at best and simply a matter of taste at worst. Raymond

At 12:30 PM 12/9/2007 +1300, Greg Ewing wrote:

Terry Reedy wrote:

...

I would prefer plain 'yield' to 'yield break' as a synonym for 'raise StopIteration'.

Don't we already have a plain yield these days meaning something else?

Yes. It's equivalent to 'yield None'.

2007/12/8, Greg Ewing :

I would put it the other way around -- the problem that 'yield break' is meant to solve is already solved by 'return'. So there's no need for change.

I have been re-thinking the problem and this is true. The only exception would be empty generators, but they are rare. 'return' works well for 99.99% of the cases. And the rest 0.01% does not worth a syntax change. Thank you very much for all your comments. Manuel.