On 11/25/2017 1:55 AM, David Cuthbert wrote:

First time contributing back -- if I should be filing a PEP or something like that for this, please let me know.

I don't think a PEP is needed.

Coming from https://bugs.python.org/issue32117, unparenthesized tuple unpacking is allowed in assignments:

     rest = (4, 5, 6)
     a = 1, 2, 3, *rest

Because except for (), it is ',', not '()' that makes a tuple a tuple.

but not in yield or return statements (these result in SyntaxErrors):

     return 1, 2, 3, *rest
     yield 1, 2, 3, *rest

To be crystal clear, a parenthesized tuple with unpacking *is* valid.

      return (1, 2, 3, *rest)
      yield (1, 2, 3, *rest)

So is an un-parenthesized tuple without unpacking.

Since return and yield are often the first half of a cross-namespace assignment, requiring the () is a bit surprising.  Perhaps someone else has a good reason for the difference.  Otherwise, +1 on the change.

The unpacking in assignments was enabled by a pre-3.2 commit that I haven't yet been able to track back to a discussion, but I suspect this asymmetry is unintentional. Here's the original commit:
https://github.com/python/cpython/commit/4905e80c3d2f6abb613d212f0313d1dfe09475dc

I've submitted a patch (CLA is signed and submitted, not yet processed), and Serihy said that since it changes the grammar I should have it reviewed here and have signoff by the BDFL.

While I haven't had a need for this myself, it was brought up by a user on StackOverflow (https://stackoverflow.com/questions/47272460/python-tuple-unpacking-in-return-statement/47326859).

Thanks!
Dave

--
Terry Jan Reedy

_______________________________________________
Python-Dev mailing list
Python-Dev@python.org
https://mail.python.org/mailman/listinfo/python-dev
Unsubscribe: https://mail.python.org/mailman/options/python-dev/guido%40python.org