2007-04-04 02:41:20 -04:00
|
|
|
|
PEP: 3117
|
|
|
|
|
|
Title: Postfix type declarations
|
2007-04-09 05:03:03 -04:00
|
|
|
|
Version: $Revision$
|
|
|
|
|
|
Last-Modified: $Date$
|
2007-05-01 16:39:17 -04:00
|
|
|
|
Author: Georg Brandl <georg@python.org>
|
2007-05-10 16:17:53 -04:00
|
|
|
|
Status: Rejected
|
2007-04-04 02:41:20 -04:00
|
|
|
|
Type: Standards Track
|
|
|
|
|
|
Content-Type: text/x-rst
|
|
|
|
|
|
Created: 01-Apr-2007
|
|
|
|
|
|
Python-Version: 3.0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Abstract
|
|
|
|
|
|
========
|
|
|
|
|
|
|
|
|
|
|
|
This PEP proposes the addition of a postfix type declaration syntax to
|
|
|
|
|
|
Python. It also specifies a new ``typedef`` statement which is used to create
|
|
|
|
|
|
new mappings between types and declarators.
|
|
|
|
|
|
|
|
|
|
|
|
Its acceptance will greatly enhance the Python user experience as well as
|
|
|
|
|
|
eliminate one of the warts that deter users of other programming languages from
|
|
|
|
|
|
switching to Python.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Rationale
|
|
|
|
|
|
=========
|
|
|
|
|
|
|
|
|
|
|
|
Python has long suffered from the lack of explicit type declarations. Being one
|
|
|
|
|
|
of the few aspects in which the language deviates from its Zen, this wart has
|
|
|
|
|
|
sparked many a discussion between Python heretics and members of the PSU (for
|
|
|
|
|
|
a few examples, see [EX1]_, [EX2]_ or [EX3]_), and it also made it a large-scale
|
|
|
|
|
|
enterprise success unlikely.
|
|
|
|
|
|
|
|
|
|
|
|
However, if one wants to put an end to this misery, a decent Pythonic syntax
|
|
|
|
|
|
must be found. In almost all languages that have them, type declarations lack
|
|
|
|
|
|
this quality: they are verbose, often needing *multiple words* for a single
|
|
|
|
|
|
type, or they are hard to comprehend (e.g., a certain language uses completely
|
|
|
|
|
|
unrelated [#]_ adjectives like ``dim`` for type declaration).
|
|
|
|
|
|
|
|
|
|
|
|
Therefore, this PEP combines the move to type declarations with another bold
|
|
|
|
|
|
move that will once again prove that Python is not only future-proof but
|
|
|
|
|
|
future-embracing: the introduction of Unicode characters as an integral
|
|
|
|
|
|
constituent of source code.
|
|
|
|
|
|
|
|
|
|
|
|
Unicode makes it possible to express much more with much less characters, which
|
|
|
|
|
|
is in accordance with the Zen ("Readability counts.") [ZEN]_. Additionally, it
|
|
|
|
|
|
eliminates the need for a separate type declaration statement, and last but not
|
|
|
|
|
|
least, it makes Python measure up to Perl 6, which already uses Unicode for its
|
|
|
|
|
|
operators. [#]_
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Specification
|
|
|
|
|
|
=============
|
|
|
|
|
|
|
|
|
|
|
|
When the type declaration mode is in operation, the grammar is changed so that
|
|
|
|
|
|
each ``NAME`` must consist of two parts: a name and a type declarator, which is
|
|
|
|
|
|
exactly one Unicode character.
|
|
|
|
|
|
|
|
|
|
|
|
The declarator uniquely specifies the type of the name, and if it occurs on the
|
|
|
|
|
|
left hand side of an expression, this type is enforced: an ``InquisitionError``
|
|
|
|
|
|
exception is raised if the returned type doesn't match the declared type. [#]_
|
|
|
|
|
|
|
|
|
|
|
|
Also, function call result types have to be specified. If the result of the call
|
|
|
|
|
|
does not have the declared type, an ``InquisitionError`` is raised. Caution: the
|
|
|
|
|
|
declarator for the result should not be confused with the declarator for the
|
|
|
|
|
|
function object (see the example below).
|
|
|
|
|
|
|
|
|
|
|
|
Type declarators after names that are only read, not assigned to, are not strictly
|
|
|
|
|
|
necessary but enforced anyway (see the Python Zen: "Explicit is better than
|
|
|
|
|
|
implicit.").
|
|
|
|
|
|
|
|
|
|
|
|
The mapping between types and declarators is not static. It can be completely
|
|
|
|
|
|
customized by the programmer, but for convenience there are some predefined
|
|
|
|
|
|
mappings for some built-in types:
|
|
|
|
|
|
|
|
|
|
|
|
========================= ===================================================
|
|
|
|
|
|
Type Declarator
|
|
|
|
|
|
========================= ===================================================
|
|
|
|
|
|
``object`` <20> (REPLACEMENT CHARACTER)
|
|
|
|
|
|
``int`` ℕ (DOUBLE-STRUCK CAPITAL N)
|
|
|
|
|
|
``float`` ℮ (ESTIMATED SYMBOL)
|
|
|
|
|
|
``bool`` ✓ (CHECK MARK)
|
|
|
|
|
|
``complex`` ℂ (DOUBLE-STRUCK CAPITAL C)
|
|
|
|
|
|
``str`` ✎ (LOWER RIGHT PENCIL)
|
|
|
|
|
|
``unicode`` ✒ (BLACK NIB)
|
|
|
|
|
|
``tuple`` ⒯ (PARENTHESIZED LATIN SMALL LETTER T)
|
|
|
|
|
|
``list`` ♨ (HOT SPRINGS)
|
|
|
|
|
|
``dict`` ⧟ (DOUBLE-ENDED MULTIMAP)
|
|
|
|
|
|
``set`` ∅ (EMPTY SET) (*Note:* this is also for full sets)
|
|
|
|
|
|
``frozenset`` ☃ (SNOWMAN)
|
|
|
|
|
|
``datetime`` ⌚ (WATCH)
|
|
|
|
|
|
``function`` ƛ (LATIN SMALL LETTER LAMBDA WITH STROKE)
|
|
|
|
|
|
``generator`` ⚛ (ATOM SYMBOL)
|
|
|
|
|
|
``Exception`` ⌁ (ELECTRIC ARROW)
|
|
|
|
|
|
========================= ===================================================
|
|
|
|
|
|
|
|
|
|
|
|
The declarator for the ``None`` type is a zero-width space.
|
|
|
|
|
|
|
|
|
|
|
|
These characters should be obvious and easy to remember and type for every
|
|
|
|
|
|
programmer.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Unicode replacement units
|
|
|
|
|
|
=========================
|
|
|
|
|
|
|
|
|
|
|
|
Since even in our modern, globalized world there are still some old-fashioned
|
|
|
|
|
|
rebels who can't or don't want to use Unicode in their source code, and since
|
|
|
|
|
|
Python is a forgiving language, a fallback is provided for those:
|
|
|
|
|
|
|
|
|
|
|
|
Instead of the single Unicode character, they can type ``name${UNICODE NAME OF
|
|
|
|
|
|
THE DECLARATOR}$``. For example, these two function definitions are equivalent::
|
|
|
|
|
|
|
|
|
|
|
|
def fooƛ(xℂ):
|
|
|
|
|
|
return None
|
|
|
|
|
|
|
|
|
|
|
|
and ::
|
|
|
|
|
|
|
|
|
|
|
|
def foo${LATIN SMALL LETTER LAMBDA WITH STROKE}$(x${DOUBLE-STRUCK CAPITAL C}$):
|
|
|
|
|
|
return None${ZERO WIDTH NO-BREAK SPACE}$
|
|
|
|
|
|
|
|
|
|
|
|
This is still easy to read and makes the full power of type-annotated Python
|
|
|
|
|
|
available to ASCII believers.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
The ``typedef`` statement
|
|
|
|
|
|
=========================
|
|
|
|
|
|
|
|
|
|
|
|
The mapping between types and declarators can be extended with this new statement.
|
|
|
|
|
|
|
|
|
|
|
|
The syntax is as follows::
|
|
|
|
|
|
|
|
|
|
|
|
typedef_stmt ::= "typedef" expr DECLARATOR
|
|
|
|
|
|
|
|
|
|
|
|
where ``expr`` resolves to a type object. For convenience, the ``typedef`` statement
|
|
|
|
|
|
can also be mixed with the ``class`` statement for new classes, like so::
|
|
|
|
|
|
|
|
|
|
|
|
typedef class Foo☺(object<63>):
|
|
|
|
|
|
pass
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Example
|
|
|
|
|
|
=======
|
|
|
|
|
|
|
|
|
|
|
|
This is the standard ``os.path.normpath`` function, converted to type declaration
|
|
|
|
|
|
syntax::
|
|
|
|
|
|
|
|
|
|
|
|
def normpathƛ(path✎)✎:
|
|
|
|
|
|
"""Normalize path, eliminating double slashes, etc."""
|
|
|
|
|
|
if path✎ == '':
|
|
|
|
|
|
return '.'
|
|
|
|
|
|
initial_slashes✓ = path✎.startswithƛ('/')✓
|
|
|
|
|
|
# POSIX allows one or two initial slashes, but treats three or more
|
|
|
|
|
|
# as single slash.
|
|
|
|
|
|
if (initial_slashes✓ and
|
|
|
|
|
|
path✎.startswithƛ('//')✓ and not path✎.startswithƛ('///')✓)✓:
|
|
|
|
|
|
initial_slashesℕ = 2
|
|
|
|
|
|
comps♨ = path✎.splitƛ('/')♨
|
|
|
|
|
|
new_comps♨ = []♨
|
|
|
|
|
|
for comp✎ in comps♨:
|
|
|
|
|
|
if comp✎ in ('', '.')⒯:
|
|
|
|
|
|
continue
|
|
|
|
|
|
if (comp✎ != '..' or (not initial_slashesℕ and not new_comps♨)✓ or
|
|
|
|
|
|
(new_comps♨ and new_comps♨[-1]✎ == '..')✓)✓:
|
|
|
|
|
|
new_comps♨.appendƛ(comp✎)
|
|
|
|
|
|
elif new_comps♨:
|
|
|
|
|
|
new_comps♨.popƛ()✎
|
|
|
|
|
|
comps♨ = new_comps♨
|
|
|
|
|
|
path✎ = '/'.join(comps♨)✎
|
|
|
|
|
|
if initial_slashesℕ:
|
|
|
|
|
|
path✎ = '/'*initial_slashesℕ + path✎
|
|
|
|
|
|
return path✎ or '.'
|
|
|
|
|
|
|
|
|
|
|
|
As you can clearly see, the type declarations add expressiveness, while at the
|
|
|
|
|
|
same time they make the code look much more professional.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Compatibility issues
|
|
|
|
|
|
====================
|
|
|
|
|
|
|
|
|
|
|
|
To enable type declaration mode, one has to write::
|
|
|
|
|
|
|
|
|
|
|
|
from __future__ import type_declarations
|
|
|
|
|
|
|
|
|
|
|
|
which enables Unicode parsing of the source [#]_, makes ``typedef`` a keyword
|
|
|
|
|
|
and enforces correct types for all assignments and function calls.
|
|
|
|
|
|
|
|
|
|
|
|
|
2007-05-10 16:17:53 -04:00
|
|
|
|
Rejection
|
|
|
|
|
|
=========
|
|
|
|
|
|
|
|
|
|
|
|
After careful considering, much soul-searching, gnashing of teeth and rending
|
|
|
|
|
|
of garments, it has been decided to reject this PEP.
|
|
|
|
|
|
|
|
|
|
|
|
|
2007-04-04 02:41:20 -04:00
|
|
|
|
References
|
|
|
|
|
|
==========
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
.. [EX1] http://mail.python.org/pipermail/python-list/2003-June/210588.html
|
|
|
|
|
|
|
|
|
|
|
|
.. [EX2] http://mail.python.org/pipermail/python-list/2000-May/034685.html
|
|
|
|
|
|
|
|
|
|
|
|
.. [EX3] http://groups.google.com/group/comp.lang.python/browse_frm/thread/6ae8c6add913635a/de40d4ffe9bd4304?lnk=gst&q=type+declarations&rnum=6
|
|
|
|
|
|
|
|
|
|
|
|
.. [#] Though, if you know the language in question, it may not be *that* unrelated.
|
|
|
|
|
|
|
|
|
|
|
|
.. [ZEN] http://www.python.org/dev/peps/pep-0020/
|
|
|
|
|
|
|
|
|
|
|
|
.. [#] Well, it would, if there was a Perl 6.
|
|
|
|
|
|
|
|
|
|
|
|
.. [#] Since the name ``TypeError`` is already in use, this name has been chosen
|
|
|
|
|
|
for obvious reasons.
|
|
|
|
|
|
|
|
|
|
|
|
.. [#] The encoding in which the code is written is read from a standard coding
|
|
|
|
|
|
cookie. There will also be an autodetection mechanism, invoked by ``from
|
|
|
|
|
|
__future__ import encoding_hell``.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Acknowledgements
|
|
|
|
|
|
================
|
|
|
|
|
|
|
|
|
|
|
|
Many thanks go to Armin Ronacher, Alexander Schremmer and Marek Kubica who helped
|
|
|
|
|
|
find the most suitable and mnemonic declarator for built-in types.
|
|
|
|
|
|
|
|
|
|
|
|
Thanks also to the Unicode Consortium for including all those useful characters
|
|
|
|
|
|
in the Unicode standard.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Copyright
|
|
|
|
|
|
=========
|
|
|
|
|
|
|
|
|
|
|
|
This document has been placed in the public domain.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
..
|
|
|
|
|
|
Local Variables:
|
|
|
|
|
|
coding: utf-8
|
|
|
|
|
|
mode: indented-text
|
|
|
|
|
|
indent-tabs-mode: nil
|
|
|
|
|
|
sentence-end-double-space: t
|
|
|
|
|
|
fill-column: 70
|
|
|
|
|
|
End:
|
