python-peps/peps/pep-0306.rst

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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>, Alyssa 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Status: Withdrawn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
https://devguide.python.org/grammar/
.. [2] SF Bug #676521, parser module validation failure
https://bugs.python.org/issue676521
Copyright
=========
This document has been placed in the public domain.
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