[Python-ideas] Proposal: Use mypy syntax for function annotations

[Andrew Barnert]

On Aug 14, 2014, at 7:37, Ryan Gonzalez <rymg19 at gmail.com> wrote:

>> On 8/13/2014 3:44 PM, Guido van Rossum wrote:

>> Now consider an extended version (after Lucas).
>> 
>> def fib(n, a, b):
>>     i = 0
>>     while i <= n:
>>         print(i,a)
>>         i += 1
>>         a, b = b, a+b
>> 
>> The only requirement of a, b is that they be addable. Any numbers should be allowed, as in fib(10, 1, 1+1j), but so should fib(5, '0', '1'). Addable would be approximated from below by Union(Number, str).
> 
> Unless MyPy added some sort of type classes...

By "type classes", do you mean this in the Haskell sense, or do you mean classes used just for typing--whether more granular ABCs (like an Addable which both Number and AnyStr and probably inherit) or typing.py type specifiers (like an Addable defined as Union(Number, AnyStr)?

It's also worth noting that the idea that this function should take a Number or a str seems way off. It's questionable whether it should accept str, but if it does, shouldn't it also accept bytes, bytearray, and other string-like types? What about sequences? And meanwhile, whether or not it accepts str, it should probably accept np.ndarray and other types of element-wise adding types. If you create an Addable type, it has to define, globally, which of those counts as addable, but different functions will have different definitions that make sense.

In fact, look at the other discussion going on. People want to ensure that sum only works on numbers or number-like types (and does that include NumPy arrays or not?), while others want to change it to work on all sequences, or only mutable sequences with += plus str because it effectively has magical += handling under the covers, etc. If we can't even decide what Addable means for one specific function that everyone has experience with...

On the other hand, if sum could have been annotated to tell us the author's intention (or, rather, the consensus of the dev list), then all of these recurring arguments about summing str would go away. Until someone defined a number-like class (maybe even one that meets the ABC, but by calling register on it) and sum won't work for him.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20140814/90648cbe/attachment-0001.html>