[Python-Dev] Python 3000 PEP: Postfix type declarations

[Collin Winter]

On 4/1/07, Georg Brandl <g.brandl at gmx.net> wrote:
[snip several pages of excellent ideas]
>
> 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.
>
[snip]
>
> 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.

My only concern is that this doesn't go far enough. While knowing that
some object is a ⒯ is a good start, it would be so much more helpful
to know that it's a ⒯ of ✎s. I think something like ✎✎✎3⒯ to indicate
a 3-⒯ of ✎s would be nice. This would change the line in the above
from "if comp✎ in ('', '.')⒯:" to "if comp✎ in ('', '.')✎✎2⒯:", which
I think is a nice win in terms of readability, EIBTI and all that.

(Sidebar: I think the PEP should feature a section on how these new
type declarations will cut down on mailing list volume and
documentation size.)

In light of this PEP, PEP 3107's function annotations should be
rejected. All that hippie feel-good crap about "user-defined
annotations" and "open-ended semantics" and "no rules, man" was just
going to get us into trouble. This PEP's more modern conception of
type annotations give the language a power and expressiveness that my
PEP could never hope to match.

This is clearly a move in the right direction. +4 billion.

Collin Winter