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>
Status: Active
Type: Informational
Content-Type: text/plain
Created: 29-Jan-2003
Post-History: 30-Jan-2003


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[1] 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.
       Use Parser/asdl_c.py to regenerate Include/Python-ast.h

    __ Python/Python-ast.c may 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

    __ 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 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 parsermodule.c until it passes.

    __ The compiler package.  A good test is to compile the standard
       library and test suite with the compiler package and then check
       it runs.  You did add some of your new syntax to the test
       suite, didn't you?

    __ 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] 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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