python-peps/pep-3000.txt

PEP: 3000
Title: Python 3.0 Plans
Version: $Revision$
Last-Modified: $Date$
Author: A.M. Kuchling <amk@amk.ca>,
        Brett Cannon <drifty@alum.berkeley.edu>
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: 20-Aug-2004
Post-History:


Abstract
========

This PEP describes the changes currently envisioned in Python 3.0
(also called Python 3000), a hypothetical future release of Python that
can break backwards compatibility with the existing body of Python code.

The list of features included in this document is subject to change
and isn't binding on the Python development community; features may be
added, removed, and modified at any time.  The purpose of this list is
to focus our language development effort on changes that are steps to
3.0, and to encourage people to invent ways to smooth the transition.

This document is not a wish-list that anyone can extend.  While there
are two authors of this PEP, we're just supplying the text; the
decisions for which changes are listed in this document are made by
Guido van Rossum, who has chosen them as goals for Python 3.0.


General goals
=============

A general goal is to reduce feature duplication by removing old ways
of doing things.  A general principle of the design will be that one
obvious way of doing something is enough. [1]_


Core language
=============

* Remove distinction between int and long types [1]_
* True division becomes default behavior [10]_
* Make all strings be Unicode, and have a separate bytes() type [1]_
* ``exec`` as a statement is not worth it -- make it a function
* Add optional declarations for static typing [11]_
* Support only new-style classes; classic classes will be gone [1]_
* Return iterators instead of lists where appropriate for atomic type methods
  (e.g. ``dict.keys()``, ``dict.values()``, ``dict.items()``, etc.)
* Replace ``print`` by a function: ``write(x,y,z)``,
  ``writeln(x,y,z)`` [2]_
* Do something so you can catch multiple exceptions using ``except E1,
  E2, E3:``. Maybe use ``except E1, E2, E3 as err:`` if you want the
  error variable? [3]_
* ``True`` and ``False`` become keywords [4]_
* ``as`` becomes a keyword [5]_
* Have list comprehensions be syntactic sugar for passing an
  equivalent generator expression to ``list()``; as a consequence the
  loop variable will no longer be exposed [12]_
* Comparisons other than ``==`` and ``!=`` between disparate types
  will raise an exception unless explicitly supported by the type [6]_
* Exceptions will grow an attribute to store the traceback [13]_

To be removed:

* The ``lambda`` statement: use nested or named functions [1]_, [9]_
* String exceptions: use instances of an Exception class [2]_
* ```x```: use ``repr(x)`` [2]_
* The ``<>`` operator: use ``!=`` instead [3]_
* Unbound methods [7]_


Built-ins
=========

* Make built-ins return an iterator where appropriate (e.g. ``range()``,
  ``zip()``, etc.)
* Relevant functions should consume iterators (e.g. ``min()``,
  ``max()``)
* Introduce ``trunc()``, which would call the ``__trunc__()`` method on its
  argument; suggested use is for objects like float where calling ``__int__()``
  has data loss, but an integral representation is still desired [8]_

To be removed:

* ``apply()``: use ``f(*args, **kw)`` instead [2]_
* ``buffer()``: must die (use a bytes() type instead) [2]_
* ``callable()``: just call the object and catch the exception [2]_
* ``compile()``: put in ``sys`` (or perhaps in a module of its own) [2]_
* ``coerce()``: no longer needed [2]_
* ``execfile()``, ``reload()``: use ``exec()`` [2]_
* ``input()``: use ``eval(sys.stdin.readline())`` [2]_
* ``intern()``, ``id()``: put in ``sys`` [2]_
* ``map()``, ``filter()``: use list comprehensions instead [1]_, [9]_
* ``reduce()``: write a loop instead [2]_, [9]_
* ``raw_input()``: use ``sys.stdin.readline()`` [2]_
* ``xrange()``: use ``range()`` instead [1]_


Standard library
================

* Reorganize the standard library to not be as shallow

To be removed:

* ``string`` and other deprecated modules [1]_
* ``sys.exc_type``: not thread-safe; use ``sys.exc_info`` [2]_


References
==========

.. [1] PyCon 2003 State of the Union:
   http://www.python.org/doc/essays/ppt/pycon2003/pycon2003.ppt

.. [2] Python Regrets:
   http://www.python.org/doc/essays/ppt/regrets/PythonRegrets.pdf

.. [3] Python Wiki:
   http://www.python.org/moin/Python3.0

.. [4] python-dev email ("Constancy of None")
   http://mail.python.org/pipermail/python-dev/2004-July/046294.html

.. [5] python-dev email (' "as" to be a keyword?')
   http://mail.python.org/pipermail/python-dev/2004-July/046316.html

.. [6] python-dev email ("Comparing heterogeneous types")
   http://mail.python.org/pipermail/python-dev/2004-June/045111.html

.. [7] python-dev email ("Let's get rid of unbound methods")
   http://mail.python.org/pipermail/python-dev/2005-January/050625.html

.. [8] python-dev email ("Fixing _PyEval_SliceIndex so that integer-like
   objects can be used")
   http://mail.python.org/pipermail/python-dev/2005-February/051674.html

.. [9] Guido's blog ("The fate of reduce() in Python 3000")
   http://www.artima.com/weblogs/viewpost.jsp?thread=98196

.. [10] PEP 238 ("Changing the Division Operator")
   http://www.python.org/peps/pep-0238.html

.. [11] Guido's blog ("Python Optional Typechecking Redux")
   http://www.artima.com/weblogs/viewpost.jsp?thread=89161

.. [12] PEP 289 ("Generator Expressions")
   http://www.python.org/peps/pep-0289.html

.. [13] python-dev email ("anonymous blocks")
   http://mail.python.org/pipermail/python-dev/2005-April/053060.html


Copyright
=========

This document has been placed in the public domain.



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