There's more to changing Python's grammar than editing :file:`Grammar/python.gram`. Here's a checklist.

NOTE: These instructions are for Python 3.9 and beyond. Earlier versions use a different parser technology. You probably shouldn't try to change the grammar of earlier Python versions, but if you really want to, use GitHub to track down the earlier version of this file in the devguide. (Python 3.9 itself actually supports both parsers; the old parser can be invoked by passing -X oldparser.)

Note: sometimes things mysteriously don't work. Before giving up, try make clean.

• :file:`Grammar/python.gram`: The grammar, with actions that build AST nodes. After changing it, run make regen-pegen, to regenerate :file:`Parser/parser.c`. (This runs Python's parser generator, Tools/peg_generator).
• :file:`Grammar/Tokens` is a place for adding new token types. After changing it, run make regen-token to regenerate :file:`Include/token.h`, :file:`Parser/token.c`, :file:`Lib/token.py` and :file:`Doc/library/token-list.inc`. If you change both python.gram and Tokens, run make regen-token before make regen-pegen.
• :file:`Parser/Python.asdl` may need changes to match the grammar. Then run make regen-ast to regenerate :file:`Include/Python-ast.h` and :file:`Python/Python-ast.c`.
• :file:`Parser/tokenizer.c` contains the tokenization code. This is where you would add a new type of comment or string literal, for example.
• :file:`Python/ast.c` will need changes to validate AST objects involved with the grammar change.
• :file:`Python/ast_unparse.c` will need changes to unparse AST objects involved with the grammar change ("unparsing" is used to turn annotations into strings per PEP 563).
• The :doc:`compiler` has its own page.
• _Unparser in the :file:`Lib/ast.py` file may need changes to accommodate any modifications in the AST nodes.
• Add some usage of your new syntax to test_grammar.py.
• Certain changes may require tweaks to the library module :mod:`pyclbr`.
• :file:`Lib/tokenize.py` needs changes to match changes to the tokenizer.
• :file:`Lib/lib2to3/Grammar.txt` may need changes to match the Grammar.
• Documentation must be written! Specifically, one or more of the pages in :file:`Doc/reference/` will need to be updated.