PEP 255: Simple Generators

[Russell E. Owen]

In article <mailman.992537422.15240.python-list at python.org>,
 "Tim Peters" <tim at digicool.com> wrote:

>You can view an HTML version of PEP 255 here:
>
>    http://python.sourceforge.net/peps/pep-0255.html
...
>
>    This PEP introduces the concept of generators to Python, as well
>    as a new statement used in conjunction with them, the "yield"
>    statement.

This sounds like an interesting and really useful proposal. However,a 
few details are worrisome to me:

* A generator looks like a function until you find the "yield" statement 
inside. I think this will make the code much harder to read. One keyword 
buried anywhere in a function totally changes its behavior. If a 
generator acted like a function until it hit "yield", this wouldn't be 
so bad, but it sounds as if generators have enough significant 
differences (such as restrictions on and a changed meaning for return) 
that they are very different beasts.

I'd much rather see a declaration up front that this is going to be a 
generator. E.g. add a new keyword such as "generator".


* The unusual use of "return" and the associated restriction of no 
expression list. What "return" means in a generator is "raise StopIter", 
not return. I personally really dislike using a keyword for multiple 
vaguely similar purposes. Look at "static" in C++ for an example of 
where this can lead.

Several proposals come to mind. They are all compatible with each other:

1) Have users explicitly say "raise StopIter" when they mean it. This is 
my first choice. Sure it's two words, but has the huge advantages of:
-- saying exactly what it means
-- not cluttering up "return" with multiple meanings
-- not having to explain to users why no expression list is allowed on 
"return"
-- it leaves the door open for a Better Way, should one be proposed; 
meanwhile, no damage has been done

2) Add a keyword, such as "stop". It could always raise StopIter, or 
raise a new exception Stop in a non-generator and StopIter in a 
generator. On the plus side, stop might be handy in other contexts. On 
the minus side, I doubt it's worth the fuss.

3) Have "return" mean exactly what it means now, but also have a 
generator raise StopIter for the next call following a return. This is a 
natural meaning for return (and allows an expression list), but has the 
disadvantage that it doesn't permit one to say "I'm done and have no 
more data to return".

-- Russell