Python tuple unpacking in return statement

Question

The Python language (especially 3.x) allows very general unpacking of iterables, a simple example of which is

a, *rest = 1, 2, 3

Over the years, this unpacking has been gradually generalized (see e.g. PEP 3132 and PEP 448), allowing it to be used in more and more circumstances. As so, I was surprised to discover that the following is invalid syntax in Python 3.6 (and remains so in Python 3.7):

def f():
    rest = [2, 3]
    return 1, *rest  # Invalid

I can make it work by encapsulating the returned tuple in parentheses like so:

def f():
    rest = [2, 3]
    return (1, *rest)  # Valid

The fact that I use this in a return statement seems to be important, as

t = 1, *rest

is indeed legal and results in the same with and without parentheses.

Have this case simply been forgotten by the Python developers, or are there any reason why this case is invalid syntax?

Why I care

This breaks an important contract I thought I had with the Python language. Consider the following (also valid) solution:

def f():
    rest = [2, 3]
    t = 1, *rest
    return t

Normally when I have code like this, I consider t to be a temporary name, which I ought to be able to get rid of simply be replacing t in the bottom line with its definition. In this case though, this leads to the invalid code

def f():
    rest = [2, 3]
    return 1, *rest

It's of course no big deal to have to place parentheses around the return value, but usually additional parentheses are only needed to distinguish between several possible outcomes (grouping). Here this is not the case, as leaving out the parentheses does not produce some other unwanted behavior, but rather no behavior at all.

Update

Since Python 3.8 (see item 7 on this list), the generalized syntax discussed above is now valid.

This is really more a consequence of the grammar syntax than anything else. — cs95
@lapisdecor Yea, but that's consistent with the fact that t = *rest is invalid. Also, return *rest and t = *rest does not represent any actual unpacking, so I don't find it a problem that this is not allowed. If it were allowed, *rest on its own would then only be a confusing syntax for tuple(rest). — jmd_dk
This happens with more than just return. Unpackings are also forbidden in a yield argument, a subscript, the RHS of an augmented assignment (but not a regular assignment), and on the right of the in in a for statement, despite unparenthesized tuples being allowed in all those positions, because the syntax for those things uses expression_list instead of starred_expression. — user2357112 supports Monica
Note the difference between t = *rest and t = *rest,. The latter is valid. — senderle

David Cuthbert David Cuthbert · Accepted Answer · 2017-11-16T10:10:48

I suspect this is an accident, based on the comments from this commit for Python 3.2.

That commit enabled the assignment expression to take a testlist_star_expr production (what allows the unparenthesized unpacking), but left the return statement taking a testlist production. I suspect the commit just missed this (and possibly other locations, but I'm focusing on the return_stmt production for now).

I went ahead and modified the Python Grammar/Grammar file to allow this. All of the tests continue to pass, including those in the test_grammar.py file (but this doesn't seem terribly exhaustive).

If you're curious, this is the change I made. Feel free to clone or download my fork.

UPDATE: I've submitted a bpo issue and a pull request for the return (and yield) unpacking.

Python tuple unpacking in return statement

Why I care

Update

1 Answers