On Sat, Mar 15, 2008 at 3:21 PM, Guido van Rossum <[EMAIL PROTECTED]> wrote:
> This post does point out an inconistency in Thomas's patch: > > def f(): > yield 1, 2, 3 > > Yields a single three-tuple. > > But > > def f(): > yield *[1, 2], 3 > > Yields three separate values. Uhm, what? It doesn't, and it shouldn't, and I'm not sure what makes you think it does. >>> def f(): ... yield *[1, 2], 3 ... >>> list(f()) [(1, 2, 3)] The comma makes the argument to 'yield' a single item (a tuple) and the '*' is part of the item in the tuple. In other words, the * binds more tightly than the comma (which is not something I changed; the parser already worked this way and it's necessary for '*a, b, c = ..' to even work. 'yield *[1, 2], 3' is parsed as 'yield (*[1, 2], 3)', not 'yield *([1, 2], 3)'. What you suggest (three separate values) requires 'yield *(*[1, 2], 3)'. >>> def f1(): ... yield (*[1, 2], 3) ... >>> list(f1()) [(1, 2, 3)] >>> def f2(): ... yield *([1, 2], 3) ... >>> list(f2()) [[1, 2], 3] >>> def f3(): ... yield *(*[1, 2], 3) ... >>> list(f3()) [1, 2, 3] I think this is inconsistent, since > > (1, 2, 3) > > and > > (*[1, 2], 3) > > are equivalent, and so are > > yield 1, 2, 3 > > and > > yield (1, 2, 3) Yes, they are indeed all equivalent :-) > > > I'm not sure how to solve this except by adopting a different syntax > for the multiple-yield. Perhaps > > *yield x > > instead of > > yield *x > > ??? If there really were an inconsistency here, I would certainly not suggest fixing it that way, yuuueghh. > > On Sat, Mar 15, 2008 at 4:58 PM, Terry Reedy <[EMAIL PROTECTED]> wrote: > > > > "Guido van Rossum" <[EMAIL PROTECTED]> wrote in message > > news:[EMAIL PROTECTED] > > > > | Thomas Wouters suggests some new syntax: > > > > I see this as two suggestions: > > 1. Complete the extension of the validity of *expression syntax from > > function call/definition to expression/assignment. > > 2. Give *expression a related meaning in yield statements. > > > > > > | > > | http://bugs.python.org/issue2292 > > | > > | >>> a, b, *c = range(5) > > | > > | >>> *a, b, c = a, b, *c > > | >>> a, b, c > > | ([0, 1, 2], 3, 4) > > | >>> [ *a, b, c ] > > | [0, 1, 2, 3, 4] > > > > I understand 'f(*a)' to mean 'execute the expression as if the items of > > iterable a has been written literally in the code (with 'as if' > > optimizations allowed for special cases). The above follow the same > rule. > > +1 > > A tutorial written for Py3 without regard to history should then > introduce > > *expressions in examples like the above first, and only later the usage > for > > functions, as if things had been this way all along. > > > > > > | >>> L = [ a, (3, 4), {5}, {6: None}, (i for i in range(7, 10)) ] > > | >>> [ *item for item in L ] > > | [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] > > > > Since above will become equivalent to list(gen_exp), I see this as > being > > tied to yield *exp. > > > > > > | Also, yielding everything from an iterator: > > | > > | >>> def flatten(iterables): > > | ... for it in iterables: > > | ... yield *it > > > > Following the general rule above for *exp, that would be the same as > yield > > tuple(it). But that is nearly useless, whereas the the implicit inner > for > > loop meaning is quite useful, with, perhaps, a speedup over an explicit > > inner loop. Since yield is already pretty magical,a bit more might not > > hurt. > > > > But, ... what do you do with > > yield *a,b,c # a,b,c as above? > > Yield a 5-tuple? That would clash badly with 'yield *a' not yielding a > > 3-tuple. > > Raise an exception? That establishes/requires a new grammatical > > category -- yield_expression. But I would see this as preferable to > the > > clash. > > > > Terry Jan Reedy > > > > > > > > > > _______________________________________________ > > Python-3000 mailing list > > Python-3000@python.org > > http://mail.python.org/mailman/listinfo/python-3000 > > Unsubscribe: > http://mail.python.org/mailman/options/python-3000/guido%40python.org > > > > > > -- > --Guido van Rossum (home page: > http://www.python.org/~guido/<http://www.python.org/%7Eguido/> > ) > _______________________________________________ > Python-3000 mailing list > Python-3000@python.org > http://mail.python.org/mailman/listinfo/python-3000 > Unsubscribe: > http://mail.python.org/mailman/options/python-3000/thomas%40python.org > -- Thomas Wouters <[EMAIL PROTECTED]> Hi! I'm a .signature virus! copy me into your .signature file to help me spread!
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