Syntactic sugar for assignment statements: one value to multiple targets?

gc gc1223 at gmail.com
Wed Aug 17 12:33:58 CEST 2011


On Aug 17, 5:45 am, Chris Angelico <ros... at gmail.com> wrote:
(snip)
> > Right. Call the proposed syntax the "instantiate separately for each
> > target" operator.
>
(snip)
> It might just
> as easily be some other function call; for instance:
>
> head1,head2,head3=file.readline()

Hm--that's interesting! OK, call it the "evaluate separately for each
target" operator.

Same benefits in this use case; if I realize that the file only has
two header lines, I can just change

head1, head2, head3 = *file.readline()

to

head1, head2 = *file.readline()

without needing to keep a RHS like = [file.readline() for _ in
range(3)] in lockstep with the number of variables I'm assigning.

Presumably this syntax should be disallowed, as a grammatical matter,
when there's a starred target (per PEP 3132/language reference 6.2).
That is,

head, *body, tail = *file.readline()

is disallowed, since it is (by definition) simply sugar for

head = file.readline()
*body = file.readline()
tail = file.readline()

and

*body = file.readline() is disallowed (see PEP 3132). (Here, of
course, you'd just want head, *body, tail = file.readlines(), which is
perfectly good code.)

PS.

Off-topic, but the *target syntax already gets into similar territory,
since

a, *b, c = itertools.count()

crashes with a MemoryError--but what else could it do? Ruling out such
infinite-assignment statements on a grammatical basis would require
solving the halting problem through static analysis, which might be a
bit inefficient :P






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