python-peps/pep-0306.txt

PEP: 306
Title: How to Change Python's Grammar
Version: $Revision$
Last-Modified: $Date$
Author: Michael Hudson <mwh@python.net>, Jack Diederich <jackdied@gmail.com>, Nick Coghlan <ncoghlan@gmail.com>, Benjamin Peterson <benjamin@python.org>
Status: Withdrawn
Type: Informational
Content-Type: text/x-rst
Created: 29-Jan-2003
Post-History: 30-Jan-2003


Note
====

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


Abstract
========

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.


Rationale
=========

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.


Checklist
=========

- 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/ast.py 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/symbol.py and/or Lib/token.py
  and/or Lib/keyword.py.

- 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 test_grammar.py

- 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/tokenizer.py 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.


References
==========

.. [1] CPython Developer's Guide: Changing CPython's Grammar
       http://cpython-devguide.readthedocs.io/en/latest/grammar.html

.. [2] SF Bug #676521, parser module validation failure
       http://www.python.org/sf/676521


Copyright
=========

This document has been placed in the public domain.



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