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

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.

Answer 3

You should go with Sequence from typing if you plan to use only list or tuple and access its elements by index, like x[0]. Sequence is both Sized and Iterable, see here.

Answer 4

Maybe you need a class like this?

class MyArray:
  _array: list

  def as_tuple(self):
    return tuple(self._array)

  def as_list(self):
    return self._array
    
    # More Feature to implement