cpython/InternalDocs/changing_grammar.md

Changing CPython's grammar

There's more to changing Python's grammar than editing Grammar/python.gram. Below is a checklist of things that may need to change.

[!NOTE]

Many of these changes require re-generating some of the derived files. If things mysteriously don't work, it may help to run make clean.

Checklist

• Grammar/python.gram: The grammar definition, with actions that build AST nodes. After changing it, run make regen-pegen (or build.bat --regen on Windows), 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/internal/pycore_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. On Windows, build.bat --regen will regenerate both at the same time.

• Parser/Python.asdl may need changes to match the grammar. Then run make regen-ast to regenerate Include/internal/pycore_ast.h and Python/Python-ast.c.

• Parser/lexer/ 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 involved with the grammar change ("unparsing" is used to turn annotations into strings per PEP 563.

• The compiler may need to change when there are changes to the AST.

• _Unparser in the Lib/ast.py file may need changes to accommodate any modifications in the AST nodes.

• Doc/library/ast.rst may need to be updated to reflect changes to 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.

• Documentation must be written! Specifically, one or more of the pages in Doc/reference/ will need to be updated.