Oddity with 'yield' as expression - parentheses demanded

[Chris Angelico]

On Thu, Aug 1, 2013 at 6:35 PM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> yield was a statement before it became an expression, and the syntax
> "yield x, y, z" was (and still is) perfectly legal, with all three
> expressions (technically a single tuple expression) being governed by
> the yield.  That is to say, "yield x, y, z" and "yield (x, y, z)" are
> semantically equivalent.  When it became an expression, in order to
> preserve this equivalence, that meant that the yield expression needed
> to bind even less tightly than the comma.  In terms of the grammar,
> yield needed to take an expression_list, not just an expression.
>
> There are only three places in the grammar where expression_lists are
> used without enclosing them in brackets:  expression statements (in
> this case analogous to the yield statement), the return statement (not
> normally used to return a value in a generator), and the assignment
> statements.  So for consistency and clarity the rules for
> parenthesizing yield statements are basically adopted from the
> existing rules for parenthesizing expression_lists.

Ahh, right. That makes good sense.

If this were being created anew now, would yield be made to bind more
tightly than the comma? That would mean that

yield x, y, z    # Yield a tuple

would need to be written as:

yield (x, y, z)

which wouldn't, in my opinion, be a bad thing. (It'd probably catch a
few people out, but a linter could notice that a tuple is being
created and ignored.) So this is a bit of a wart, but it's for
backward compatibility. Right?

ChrisA