[Python-Dev] [SPAM: 3.000] [issue11682] PEP 380 reference implementation for 3.3
greg.ewing at canterbury.ac.nz
Wed Nov 9 23:13:15 CET 2011
Guido van Rossum wrote:
> I see this as inevitable. By the time the parser sees 'yield' it has
> made its choices; the 'from' keyword cannot modify that. So whenever
> "yield expr" must be parenthesized, "yield from expr" must too.
This is patently untrue, because by version of the grammar
allows 'f(yield from x)', while disallowing 'f(yield x)'.
I made a conscious decision to do that, and I'm a bit alarmed
at this decision being overridden at the last moment with no
> At the same time, "yield expr, expr" works;
Um, no, that's a syntax error in any context, as far as I
> but does "yield from expr, expr" mean anything?
In my current grammar, it's a syntax error on its own,
but 'f(yield from x, y)' parses as 'f((yield from x), y)',
which seems like a reasonable interpretation to me.
What's not quite so reasonable is that if you have an
expression such as
f(x) + g(y)
and you decide to turn f into a generator, the obvious
way to rewrite it would be
yield from f(x) + g(y)
but that unfortunately parses as
yield from (f(x) + g(y))
If I'd thought about this more at the time, I would
probably have tried to make the argument to yield-from
something further down in the expression hierarchy,
such as a power. That might be tricky to achieve
while keeping the existing behaviour of 'yield',
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