24. Changing CPython’s Grammar

24.1. Abstract

There’s more to changing Python’s grammar than editing 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.)

24.2. Checklist

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

  • Grammar/python.gram: The grammar, with actions that build AST nodes. After changing it, run make regen-pegen, to regenerate Parser/parser.c. (This runs Python’s parser generator, Tools/peg_generator).
  • Grammar/Tokens is a place for adding new token types. After changing it, run make regen-token to regenerate Include/token.h, Parser/token.c, Lib/token.py and Doc/library/token-list.inc. If you change both python.gram and Tokens, run make regen-token before make regen-pegen.
  • Parser/Python.asdl may need changes to match the grammar. Then run make regen-ast to regenerate Include/Python-ast.h and Python/Python-ast.c.
  • Parser/tokenizer.c contains the tokenization code. This is where you would add a new type of comment or string literal, for example.
  • Python/ast.c will need changes to validate AST objects involved with the grammar change.
  • 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 Design of CPython’s Compiler has its own page.
  • _Unparser in the 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 pyclbr.
  • Lib/tokenize.py needs changes to match changes to the tokenizer.
  • Lib/lib2to3/Grammar.txt may need changes to match the Grammar.
  • Documentation must be written! Specifically, one or more of the pages in Doc/reference/ will need to be updated.