Typehints for Sized Iterable in Python

Question

I have a function that uses the len function on one of it's parameters and iterates over the parameter. Now I can choose whether to annotate the type with Iterable or with Sized, but both gives errors in mypy.

from typing import Sized, Iterable   def foo(some_thing: Iterable):     print(len(some_thing))     for part in some_thing:         print(part) 

Gives

error: Argument 1 to "len" has incompatible type "Iterable[Any]"; expected "Sized" 

While

def foo(some_thing: Sized): ... 

Gives

error: Iterable expected error: "Sized" has no attribute "__iter__" 

Since there is no Intersection as discussed in this issue I need to have some kind of mixed class.

from abc import ABCMeta from typing import Sized, Iterable   class SizedIterable(Sized, Iterable[str], metaclass=ABCMeta):     pass   def foo(some_thing: SizedIterable):     print(len(some_thing))     for part in some_thing:         print(part)   foo(['a', 'b', 'c']) 

This gives an error when using foo with a list.

error: Argument 1 to "foo" has incompatible type "List[str]"; expected "SizedIterable" 

This is not too surprising since:

>>> SizedIterable.__subclasscheck__(list) False 

So I defined a __subclasshook__ (see docs).

class SizedIterable(Sized, Iterable[str], metaclass=ABCMeta):      @classmethod     def __subclasshook__(cls, subclass):         return Sized.__subclasscheck__(subclass) and Iterable.__subclasscheck__(subclass) 

Then the subclass check works:

>>> SizedIterable.__subclasscheck__(list) True 

But mypy still complains about my list.

error: Argument 1 to "foo" has incompatible type "List[str]"; expected "SizedIterable" 

How can I use type hints when using both the len function and iterate over my parameter? I think casting foo(cast(SizedIterable, ['a', 'b', 'c'])) is not a good solution.

Answer 1

Starting from Python3.6 there's a new type called Collection. See here.

Answer 2

In the future Protocols will be introduced. They are already available through typing_extensions. See also PEP 544. Using Protocol the code above would be:

from typing_extensions import Protocol   class SizedIterable(Protocol):      def __len__(self):         pass      def __iter__(self):         pass   def foo(some_thing: SizedIterable):     print(len(some_thing))     for part in some_thing:         print(part)   foo(['a', 'b', 'c']) 

mypy takes that code without complaining. But PyCharm is saying

Expected type 'SizedIterable', got 'List[str]'

about the last line.