[SOLVED] Function annotation for a list containing ABCMeta instances

Function annotation for a list containing ABCMeta instances

I'm trying to use the Python function annotations (PEP 3107) as type hints for PyCharm, but failed to do so. The problem is probably related to my use of ABCMeta:

import abc

class base(object, metaclass=abc.ABCMeta):
    @abc.abstractmethod
    def test(self):
        pass

class deriv1(base):
    def test(self):
        return "deriv1"

class deriv2(base):
    def test(self):
        return "deriv2"

my_list = []
def append_to_list(el: base) -> list(base):
# def append_to_list(el):
#     """
#     :param el: item to add
#     :type: base
#     :return: items so far
#     :rtype: list[base]
#     """
   my_list.append(el)
   return my_list

append_to_list(deriv1())
a = append_to_list(deriv2())
for o in a:
    print(o.test())

This code does not run. Instead, I get a TypeError: 'ABCMeta' object is not iterable on the def append_to_list line.

When I use the alternative function with docstring type hints (the commented lines in the code above), everything works great.

Is it possible to use annotations for this kind of type hinting?

Solution

It's not related to abc but because you told Python to literally evaluate

list(base)

which is impossible because base is not iterable. That's what the error message is telling you.

You need to change it to square brackets and wrap it in quotes (because the list type is not subscriptable):

def append_to_list(el: base) -> 'list[base]':

or use typing.List which is subscriptable:

from typing import List

def append_to_list(el: base) -> List[base]:

To indicate it's a list containing base objects.