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[Python-ideas] Improving the expressivity of function annotations

Carl M. Johnson cmjohnson.mailinglist at gmail.com
Tue Apr 5 02:44:16 CEST 2011 
On Mon, Apr 4, 2011 at 9:03 AM, Masklinn <masklinn at masklinn.net> wrote:

> Again, the issue is function annotations, which was my context. The example was just to give an idea of the operation performed. In function annotations, unless you went with type parameters and ``tuple[A, B, C]``, ``(A, B, C)`` would likely be interpreted as "a triple of instances of types A, B and C".

Yes, but since function annotation don't do anything at present (and
maybe never will by default), your function will have to use a
decorator to turn on the type checking. At that point, it's just a
matter of API design how best to do it. For example, let's say I have
a sum function, and I want it to take an iterator containing some type
and then return something of the same type. In semi-C-ish style, we'd
call this list[int] -> int or list[Generic number] -> Generic number.
But since we're making up our own API for type checking, in this case
we can make it work however we want. We don't need to have a
particular grammar baked into the language. Try something like:

from type_checking_annotations import *

g = Generic()

@typechecked
def sum(iterable: Container(g)) -> g:
    iterator = iter(iterable)
    total = next(iterator)
    for item in iterator:
        total += item
    return total

sum([0, 0.0]) #Raise TypeError: First item in container was type int,
subsequent item was type float
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