[Python-ideas] PEP 484 (Type Hints) -- first draft round

[Guido van Rossum]

On Fri, Jan 16, 2015 at 12:05 PM, Philipp A. <flying-sheep at web.de> wrote:

> hi, nice work!
>
> I like most of the ideas, except the semantic comments and some specific
> abstract classes.
>
> so criticism first:
>
> 1. i don’t think comments should ever have an impact on the code.
>

They don't. They also change the errors emitted by the static checker
(which is not invoked when you run the code -- it is like a linter).


> i’d prefer “assert isinstance(...)” for type annotations on variables.
>

But that *would* affect runtime, by slowing down execution. Also for
generic types the type checking could be either very slow (if checking
List[int] would check every item of a list) or always fail (if a type
variable is involved).

See discussion at https://github.com/ambv/typehinting/issues/35


>
> 2. also i don’t see why Dict, (non-abstract) Set, and List exist as long
> as we have Mapping and so on. Tuple is different because of its “Multiple
> return value” status and status as “is treated as Union[Class1, Class2]
> when used with instanceof(), except and friends”.
>
> but checking for instanceof(..., list) is unpytonic! imho List and Dict
> really go against python’s data model and protocol-based nature.
>
> i’d be for consistency: Set and MutableSet, Sequence and MutableSequence,
> Mapping and MutableMapping. no “abstract” (because all of this is
> abstract), no “frozen” (because frozen sets are just immutable sets and
> immutable behavior is a subset of the corresponding mutable behavior).
>

But there is a lot of code that really needs the concrete types. E.g.
list+list returns a list, but sequence+sequence may fail (as list+tuple
exemplifies). Or list.sort. Etc.


> Questions:
>
> 1. will we be able to use Union[...] in places like an except clause,
> where tuples are used to mean the same right now? would make sense imho
>

No. Remember TOOWTDI. (Also the PEP intentionally stays far away from
exception handling.)


> 2. what are ItemsView and KeysView for? what makes them different from
> FrozenSet or Sequence?
>

They exist in collections.abc. Note that KeysView and ItemsView are
(abstract) Sets, but ValuesView is not (you can have multiple equal values).

-- 
--Guido van Rossum (python.org/~guido)
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