Oddity with 'yield' as expression - parentheses demanded

[Terry Reedy]

On 8/1/2013 1:58 PM, Chris Angelico wrote:
> 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?

As a statement (which is the primary use of yield), yield is like 
return, except that the execution frame is not discarded. So I think it 
should bind like return, which is very loosely.


-- 
Terry Jan Reedy