Why is this unpacking expression not allowed in python3.10?

Question

I used to unpack a long iterable expression like this:

In python 3.8.7:

>>> _, a, (*_), c = [1,2,3,4,5,6]
>>> a
2
>>> c
6

In python 3.10.7:

>>> _, a, (*_), c = [1,2,3,4,5,6]
  File "<stdin>", line 1
    _, a, (*_), c = [1,2,3,4,5,6]
           ^^
SyntaxError: cannot use starred expression here

I'm not sure which version of python between 3.8.7 and 3.10.7 introduced this backwards breaking behavior. What's the justification for this?

Answer 1

There's an official discussion here. The most relevant quote I can find is:

Also the current behavior allows (*x), y = 1 assignment. If (*x) is to be totally disallowed, (*x), y = 1 should also be rejected.

I agree.

The final "I agree" is from Guido van Rossum.

The rationale for rejecting (*x) was:

Honestly this seems like a bug in 3.8 to me (if it indeed behaves like this):

>>> (*x), y (1, 2, 3)

Every time I mistakenly tried (*x) I really meant (*x,), so it's surprising that (*x), y would be interpreted as (*x, y) rather than flagging (*x) as an error.

Please don't "fix" this even if it is a regression.

Also by Guido van Rossum. So it seems like (*x) was rejected because it looks too similar to unpacking into a singlet tuple.