PEP: 3000 Title: Python 3.0 Plans Version: $Revision$ Last-Modified: $Date$ Author: A.M. Kuchling , Brett Cannon 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.) (Do we keep iter*() methods or remove them? I vote remove. -- nn) * Replace ``print`` by a function [16]_ * 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]_ * ``None``, ``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]_ * floats will not be acceptable as arguments in place of ints for operations where floats are inadvertantly accepted (PyArg_ParseTuple() i & l formats) * Imports will be absolute by default. Relative imports must be explicitly specified [19]_ * __init__.py will be optional in sub-packages. __init__.py will still be required for top-level packages. * Cleanup the Py_InitModule() variants {,3,4} (also import and parser APIs) * Cleanup the APIs exported in pythonrun, etc. * Fix (or remove) {}.setdefault() [21]_ * Some expressions will require parethenses that didn't in 2.x: - List comprehensions will require parentheses around the iterables. This will make list comprehensions more similar to generator comprehensions. [x for x in 1, 2] will need to be: [x for x in (1, 2)] - Lambdas will have to be parenthesized [23]_ * Builtin module init function names (PyMODINIT_FUNC) will be prefixed with _Py (or Py). Currently they aren't namespace safe since the names start with init. To be removed: * String exceptions: use instances of an Exception class [2]_ * ``raise Exception, "message"``: use ``raise Exception("message")`` [14]_ * ```x```: use ``repr(x)`` [2]_ * The ``<>`` operator: use ``!=`` instead [3]_ * Unbound methods [7]_ * METH_OLDARGS, WITH_CYCLE_GC * __getslice__, __setslice__, __delslice__ [17]_ * Remove slice opcodes and use slice objects * C APIs (see code): PyFloat_AsString, PyFloat_AsReprString, PyFloat_AsStringEx, PySequence_In, PyEval_EvalFrame, PyEval_CallObject, _PyObject_Del, _PyObject_GC_Del, _PyObject_GC_Track, _PyObject_GC_UnTrack PyString_AsEncodedString, PyString_AsDecodedString PyArg_NoArgs, PyArg_GetInt typedefs: intargfunc, intintargfunc Built-in Namespace ================== * 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]_ * Exception hierarchy changes [20]_ 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]_ Atomic Types ============ To be removed: * ``basestring.find()`` and ``basestring.rfind()``; use ``basestring.index()`` or ``basestring.rindex()`` in a try/except block [15]_ * ``file.xreadlines()`` method [17]_ * ``dict.setdefault()`` [22]_ Standard library ================ * Reorganize the standard library to not be as shallow * Move test code to where it belongs, there will be no more test() functions in the standard library To be removed: * Deprecated modules, methods, parameters, attributes, etc. [1]_ [17]_ [18]_ There may be other modules, the most common are listed below. stdlib modules to be removed (see docstrings and comments in the source): * ``macfs``, ``new``, ``reconvert``, ``stringold``, ``xmllib`` * ``pcre``, ``pypcre``, ``strop`` stdlib modules to be removed (see PEP 4): [18]_ * ``posixfile``, ``pre``, ``regsub``, ``rfc822``, * ``statcache``, ``string``, ``TERMIOS`` * ``mimetools``, ``MimeWriter``, ``mimify``, * ``mpz``, ``rgbimage`` * Everything in lib-old: [18]_ * Para.py, addpack.py, cmp.py, cmpcache.py, codehack.py, * dircmp.py, dump.py, find.py, fmt.py, grep.py, lockfile.py, * newdir.py, ni.py, packmail.py, poly.py, rand.py, statcache.py, * tb.py, tzparse.py, util.py, whatsound.py, whrandom.py, zmod.py * ``sys.exitfunc``: use atexit module instead [17]_ * ``sys.exc_type``, ``sys.exc_values``, ``sys.exc_traceback``: not thread-safe; use ``sys.exc_info()`` or an attribute of the exception [2]_ [13]_ [17]_ * ``array.read``, ``array.write`` [17]_ * ``operator.isCallable``, ``operator.sequenceIncludes`` [17]_ 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 .. [14] python-dev email ("PEP 8: exception style") http://mail.python.org/pipermail/python-dev/2005-August/055190.html .. [15] python-dev email (Remove str.find in 3.0?) http://mail.python.org/pipermail/python-dev/2005-August/055705.html .. [16] python-dev email (Replacement for print in Python 3.0) http://mail.python.org/pipermail/python-dev/2005-September/056154.html .. [17] Python docs http://docs.python.org/ref/sequence-methods.html http://docs.python.org/lib/module-sys.html http://docs.python.org/lib/module-operator.html http://docs.python.org/lib/module-array.html http://docs.python.org/lib/bltin-file-objects.html .. [18] PEP 4 ("Deprecation of Standard Modules") http://www.python.org/peps/pep-0004.html .. [19] PEP 328 ("Imports: Multi-Line and Absolute/Relative") http://www.python.org/peps/pep-0328.html .. [20] PEP 352 ("Required Superclass for Exceptions") http://www.python.org/peps/pep-0352.html .. [21] python-dev email http://mail.python.org/pipermail/python-dev/2006-February/061169.html .. [22] python-dev email ("defaultdict") http://mail.python.org/pipermail/python-dev/2006-February/061261.html .. [23] PEP 308 ("Conditional Expressions") http://www.python.org/peps/pep-0308.html Copyright ========= This document has been placed in the public domain. .. 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