[Python] Proper use Generator typing

I'm attempting to add typing to a method that returns a generator. Whenever I run this program with the return type specified, a TypeError is raised.

Adding quotes or removing the typing fixes the error, but this seems like hack. Surely there is a correct way of doing this.

def inbox_files(self) -> "Generator[RecordsFile]":
...

# OR

def inbox_files(self):
...


from typing import Generator, List
from .records_file import RecordsFile

Class Marshaller:

...

def inbox_files(self) -> Generator[RecordsFile]:
return self._search_directory(self._inbox)

def _search_directory(self, directory: str) -> RecordsFile:
for item_name in listdir(directory):
item_path = path.join(item_name, directory)
if path.isdir(item_path):
yield from self._search_directory(item_path)
elif path.isfile(item_path):
yield RecordsFile(item_path)
else:
print(f"[WARN] Unknown item found: {item_path}")



The following stack trace is produced:

Traceback (most recent call last):
File "./bin/data_marshal", line 8, in <module>
from src.app import App
File "./src/app.py", line 9, in <module>
from .marshaller import Marshaller
File "./src/marshaller.py", line 9, in <module>
class Marshaller:
File "./src/marshaller.py", line 29, in Marshaller
def inbox_files(self) -> Generator[RecordsFile]:
File "/usr/local/Cellar/python/3.7.4/Frameworks/Python.framework/Versions/3.7/lib/python3.7/typing.py", line 254, in inner
return func(*args, **kwds)
File "/usr/local/Cellar/python/3.7.4/Frameworks/Python.framework/Versions/3.7/lib/python3.7/typing.py", line 630, in __getitem__
_check_generic(self, params)
File "/usr/local/Cellar/python/3.7.4/Frameworks/Python.framework/Versions/3.7/lib/python3.7/typing.py", line 208, in _check_generic
raise TypeError(f"Too {'many' if alen > elen else 'few'} parameters for {cls};"
TypeError: Too few parameters for typing.Generator; actual 1, expected 3


¯\_(ツ)_/¯

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