At 10:22 PM 3/24/2009 -0400, Steve Holden wrote:
There is one non-trivial extension that I've been chewing over for a
while. What if you want to yield not the values from the generator but
some function of those values? The present proposal appears to have no
way to specify that. What about extending the syntax somewhat to
yield expr for x from X
The idea is that x should be a a bound variable in expr, but the "expr
for x" could be optional to yield the existing proposal as a degenerate
case.
That would be spelled:
yield from (expr for x in X)
And the compiler could optionally optimize away the
genexpr. Assuming, of course, that this is considered valuable
enough to implement in the first place, which I don't think it
is... especially not with the return bit factored in.
Now, if somebody came up with a different way to spell the extra
value return, I wouldn't object as much to that part. I can just see
people inadvertently writing 'return x' as a shortcut for 'yield x;
return', and then having what seem like mysterious off-by-one errors,
or being confused by receiving a generator object instead of their
desired non-generator return value.
It also seems weird that the only syntactically-supported way to get
the generator's "return value" is to access it inside *another*
generator... which *also* can't return the return value to anyone!
But if it were spelled 'raise Return(value)' or 'raise
StopIteration(value)' or something similar (or even had its own
syntax!), I wouldn't object, as it would then be obvious how to get
the value, and there could be no possible confusion with a regular
return value.
The unusual spelling would also signal that something unusual (i.e.,
multitasking) is taking place, similar to the way some frameworks use
things like 'yield Return(value)' to signal the end of a task and its
return value, in place of a value in the stream.
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