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Python Enhancement Proposals

PEP 306 – How to Change Python’s Grammar

Michael Hudson <mwh at>, Jack Diederich <jackdied at>, Alyssa Coghlan <ncoghlan at>, Benjamin Peterson <benjamin at>

Table of Contents


This PEP has been moved to the Python dev guide [1].


There’s more to changing Python’s grammar than editing Grammar/Grammar and Python/compile.c. This PEP aims to be a checklist of places that must also be fixed.

It is probably incomplete. If you see omissions, just add them if you can – you are not going to offend the author’s sense of ownership. Otherwise submit a bug or patch and assign it to mwh.

This PEP is not intended to be an instruction manual on Python grammar hacking, for several reasons.


People are getting this wrong all the time; it took well over a year before someone noticed [2] that adding the floor division operator (//) broke the parser module.


  • Grammar/Grammar: OK, you’d probably worked this one out :)
  • Parser/Python.asdl may need changes to match the Grammar. Run make to regenerate Include/Python-ast.h and Python/Python-ast.c.
  • Python/ast.c will need changes to create the AST objects involved with the Grammar change. Lib/compiler/ will need matching changes to the pure-python AST objects.
  • Parser/pgen needs to be rerun to regenerate Include/graminit.h and Python/graminit.c. (make should handle this for you.)
  • Python/symbtable.c: This handles the symbol collection pass that happens immediately before the compilation pass.
  • Python/compile.c: You will need to create or modify the compiler_* functions to generate opcodes for your productions.
  • You may need to regenerate Lib/ and/or Lib/ and/or Lib/
  • The parser module. Add some of your new syntax to test_parser, bang on Modules/parsermodule.c until it passes.
  • Add some usage of your new syntax to
  • The compiler package. A good test is to compile the standard library and test suite with the compiler package and then check it runs. Note that this only needs to be done in Python 2.x.
  • If you’ve gone so far as to change the token structure of Python, then the Lib/ library module will need to be changed.
  • Certain changes may require tweaks to the library module pyclbr.
  • Documentation must be written!
  • After everything’s been checked in, you’re likely to see a new change to Python/Python-ast.c. This is because this (generated) file contains the SVN version of the source from which it was generated. There’s no way to avoid this; you just have to submit this file separately.



Last modified: 2023-10-11 12:05:51 GMT