#!/usr/local/bin/python3.7
from typing import Dict
foobar: Dict[str] = {"a": 1, "b": 2}


result:

~% ./test.py
Traceback (most recent call last):
  File "./test.py", line 3, in <module>
    foobar: Dict[str] = {"a": 1, "b": 2}
  File "/usr/local/lib/python3.7/typing.py", line 251, in inner
    return func(*args, **kwds)
  File "/usr/local/lib/python3.7/typing.py", line 627, in __getitem__
    _check_generic(self, params)
  File "/usr/local/lib/python3.7/typing.py", line 205, in _check_generic
    raise TypeError(f"Too {'many' if alen > elen else 'few'} parameters for {cls};"
TypeError: Too few parameters for typing.Dict; actual 1, expected 2