The grammar in lib2to3 is out of date and can't parse `:=` nor `f(**not x)` from running on real code.  I've done a cursory `diff -uw Grammar/Grammar Lib/lib2to3/grammar.txt`, and would like to fix lib2to3 so we can merge into both fissix and blib2to3, to avoid further divergence of the forks.

I'm unsure if I need a separate bug per pull request, but need at least one to get started.

jreese reminded me of pep570, which will make more grammar changes.  I'm open to the idea of replacing the grammar with the live one, plus porting the 2isms forward like print, eval, except with comma.

My sincere hope is that everyone that depends on this structure will have tests (mine and lib2to3 do); the only big user I'm aware of is probably libfuturize.  Definitely worth a changelog entry if this is the way forward.

Here's approximately what it would look like to do the big change now: https://github.com/python/cpython/compare/master...thatch:lib2to3-update-grammar (one test failing, and some helpers may need more test coverage)

I agree we should get lib2to3 up to date.

Looks like for *args and **kwargs there is issue33348 (this has a PR) and issue32496 (no PR) and related closed issue24791 and issue24176. 

Adding `:=` seems straighforward to me, as for the big change maybe @benjamin.peterson would be interested in commenting?

For the changes of PEP570, please wait until I merge the implementation to do the grammar changes in lib2to3 for that.

My strong preference would be getting the lib2to3 grammar to be the python grammar + additions, to make future changes easier to merge.  The strongest argument against doing that is the backwards-incompatibility of patterns -- some won't compile, while others will compile but do something unexpected).

It's good to hear (or at least infer) that parsing modern code is also a goal of lib2to3.

Re: breakage due to changes in structure (https://bugs.python.org/issue36541#msg339669) ... this has already happened in the past (e.g., type annotations and async). 

It's probably a good idea to add some documentation that structure changes can be expected with each release of Python.

Also the Grammar.txt diffs look about the same size as I've seen with other upgrades to lib2to3 when the Python grammar changed.

New changeset 96c5f5a3a3fabf43e8114d0dbc30bed409da1ba6 by Tim Hatch in branch '3.7':
[3.7] bpo-36541: lib2to3: Support named assignment expressions (GH-12702) (GH-19317)
https://github.com/python/cpython/commit/96c5f5a3a3fabf43e8114d0dbc30bed409da1ba6

master/3.9 changeset:
https://github.com/python/cpython/commit/3c3aa4516c70753de06bb142b6793d01330fcf0f

3.8 changeset: https://github.com/python/cpython/commit/1098671e4e5ec1513247f05598158eaa3428c5be

Support for `:=` is in, are we still lacking `f(**not x)` support?

Parsing support for `f(**mapping)` support is indeed still missing.

as lib2to3 is pending deprecation at this point, i'm not going to work on this.  anyone is welcome to pick it up.  modifying the lib2to3 grammar, and any related code, and adding a test is what's needed to parse that syntax.