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Generators versus Coroutines

Lenard Lindstrom len-1 at telus.net
Sun Aug 15 11:59:25 EDT 2004 
Nick Patavalis <npat at efault.net> writes:

> On 2004-08-14, Paul Rubin <> wrote:
> >
> > You can't yield across multiple levels of function calls.  That's why
> > they're called "simple generators" instead of just "generators".
> >
> 
> For me it would be *tremendously* usefull if one could yield across
> multiple levels of function calls. If this was supported, then most
> uses of O/S treads could be replaced by generators.
> 
> Does anyone know if such a feature is planned?
> 
Having followed the development of Prothon it is my understanding that
such a thing is not possible in CPython for two reasons. The first is
that the yield statement itself defines a function declaration as a
generator. Without a specific "gen" declarator keyword as in Prothon:

gen evens(n):    # This is a generator
    for i in n:  # "for i in range(n)" in Python
        doubleit(i)

def doubleit(i)  # This is a helper that yields
    yield(2 * x) # "yield 2 * x"

for i in evens(5):
    print(i)     # "print i"

the yield does not know how many function calls up it must go to return
a value. In CPython it simply saves one frame, that of the current function
call. The second reason is a python function call in CPython involves an
actual recursive C call of the interpreter. So to have a yield span more
than one python function would require saving and restoring part of the
actual C stack - coroutines at the C level - or a rewrite of the interpreter
to not make the recursive C calls as in Stackless Python.

Lenard Lindstrom
<len-l at telus.net>
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