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

[Terry Reedy]

On 1/18/2015 10:40 PM, Guido van Rossum wrote:
> On Sun, Jan 18, 2015 at 12:36 PM, Stefan Behnel

>     does anyway. If it means "exactly list and no subtypes of list",
>     Cython can
>     use it to avoid generating generic fallback code.
>
>
> Can you explain how this works? How does Cython manage to use the
> knowledge that the argument's __class__ is equal to builtins.list to
> generate more efficient code?

The obvious part is that Cython will directly access the C array of 
pointers .. and *assume* that each pointer points to a member of the 
sequence, in order.  The less obvious part is how a subclass can break 
that assumption.

Suppose, for instance, one wants a list of pairs, without the space 
overhead of a concrete tuple object for each pair.  Here is the 
beginning of a list subclass solution (not necessarily the best 
approach, but a possible one).

class PairList(list):
     def __getitem__(self, i):
         g = super().__getitem__
         return g(2*i), g(2*i+1)

pl = PairList([1,2,3,4])
print(pl[0], pl[1])
 >>>
(1, 2) (3, 4)

For Cython to execute this correctly, it would need the subclass 
fallback code that would go through the custom .__getitem__.

Storing a triangular matrix in a list, with __getitem__ calculating the 
linear index from an input row, col pair, is another possible example.

 > Very few people subclass builtins.list ...
 > So perhaps Cython could just assume that typing.List means
 > builtins.list and (dynamically) reject calls that pass a subclass
 > of builtins.list?

Identifying a triangular matrix as a List might not be too useful in any 
case.
...

 >    would be nice to at least be able to explicitly express somehow
 >    that subtypes should be disallowed.

 > Can you come up with a specific proposal? Maybe we're not as far
 > apart as we think. :-)

Is the use of concrete types like list disallowed?

---
My memory of the pre-3.0 discussion is that we did not add something 
like typing then because we did not know how to write it.  You suggested 
(something like) "let people experiment and we can adopt and adapt the 
best result."  Now is the anticipated future.  I agree that mypy seems 
to be the best starting point of something both usable and useful that 
we are likely to get. It would be nice to have adapted version in an 
early alpha.  I presume you plan to label typing 'experimental', as I 
believe asyncio was.

-- 
Terry Jan Reedy