pep-3117.txt (original) (raw)

georg.brandl python-checkins at python.org
Wed Apr 4 08:41:20 CEST 2007

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Author: georg.brandl Date: Wed Apr 4 08:41:20 2007 New Revision: 54680

Added: peps/trunk/pep-3117.txt Modified: peps/trunk/pep-0000.txt Log: Checkin PEP 3117.

Modified: peps/trunk/pep-0000.txt

--- peps/trunk/pep-0000.txt (original) +++ peps/trunk/pep-0000.txt Wed Apr 4 08:41:20 2007 @@ -115,6 +115,7 @@ S 3108 Standard Library Reorganization Cannon S 3114 Renaming iterator.next() to iterator.next() Yee S 3116 New I/O Stutzbach, Verdone, GvR

S 3117 Postfix Type Declarations Brandl

Finished PEPs (done, implemented in Subversion)

@@ -468,6 +469,7 @@ S 3114 Renaming iterator.next() to iterator.next() Yee SA 3115 Metaclasses in Python 3000 Talin S 3116 New I/O Stutzbach, Verdone, GvR

S 3117 Postfix Type Declarations Brandl

Key

Added: peps/trunk/pep-3117.txt

--- (empty file) +++ peps/trunk/pep-3117.txt Wed Apr 4 08:41:20 2007 @@ -0,0 +1,236 @@ +PEP: 3117 +Title: Postfix type declarations +Version: Revision:Revision: Revision: +Last-Modified: Date:Date: Date: +Author: Georg Brandl <g.brandl at gmx.net> +Status: Draft +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 � (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�):
```
   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.
+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:

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