Every piece of code that uses either lib2to3 or a parser derived from it (including parso and LibCST) will eventually not be able to upgrade the parser because PEG can handle grammars that LL(k) can't. That's why I proposed adding some functionality to ast.parse, to make the whitespace and token information easily available - this seems to be what @BTaskaya says is "easy" (maybe they mean it's easy using LibCST? It seems to be fiddly using ast.parse). The alternative is that all these projects (black, LibCST, yapf, etc.) will have to roll their own solutions, which doesn't seem a very productive use of people's time and makes version upgrades slow.

If people are interested in using ast.parse extensions as a replacement for lib2to3, I suggest discussing at https://mail.python.org/archives/list/python-ideas@python.org/thread/X2HJ6I6XLIGRZDB27HRHIVQC3RXNZAY4/