add typehints for torchvision.datasets.omniglot (#2533)

torchvision/datasets/omniglot.py

 from PIL import Image
 from os.path import join
 import os
+from typing import Any, Callable, List, Optional, Tuple
 from .vision import VisionDataset
 from .utils import download_and_extract_archive, check_integrity, list_dir, list_files
 
@@ -27,8 +28,14 @@ class Omniglot(VisionDataset):
         'images_evaluation': '6b91aef0f799c5bb55b94e3f2daec811'
     }
 
-    def __init__(self, root, background=True, transform=None, target_transform=None,
-                 download=False):
+    def __init__(
+            self,
+            root: str,
+            background: bool = True,
+            transform: Optional[Callable] = None,
+            target_transform: Optional[Callable] = None,
+            download: bool = False,
+    ) -> None:
         super(Omniglot, self).__init__(join(root, self.folder), transform=transform,
                                        target_transform=target_transform)
         self.background = background
@@ -42,16 +49,16 @@ class Omniglot(VisionDataset):
 
         self.target_folder = join(self.root, self._get_target_folder())
         self._alphabets = list_dir(self.target_folder)
-        self._characters = sum([[join(a, c) for c in list_dir(join(self.target_folder, a))]
-                                for a in self._alphabets], [])
+        self._characters: List[str] = sum([[join(a, c) for c in list_dir(join(self.target_folder, a))]
+                                           for a in self._alphabets], [])
         self._character_images = [[(image, idx) for image in list_files(join(self.target_folder, character), '.png')]
                                   for idx, character in enumerate(self._characters)]
-        self._flat_character_images = sum(self._character_images, [])
+        self._flat_character_images: List[Tuple[str, int]] = sum(self._character_images, [])
 
-    def __len__(self):
+    def __len__(self) -> int:
         return len(self._flat_character_images)
 
-    def __getitem__(self, index):
+    def __getitem__(self, index: int) -> Tuple[Any, Any]:
         """
         Args:
             index (int): Index
@@ -71,13 +78,13 @@ class Omniglot(VisionDataset):
 
         return image, character_class
 
-    def _check_integrity(self):
+    def _check_integrity(self) -> bool:
         zip_filename = self._get_target_folder()
         if not check_integrity(join(self.root, zip_filename + '.zip'), self.zips_md5[zip_filename]):
             return False
         return True
 
-    def download(self):
+    def download(self) -> None:
         if self._check_integrity():
             print('Files already downloaded and verified')
             return
@@ -87,5 +94,5 @@ class Omniglot(VisionDataset):
         url = self.download_url_prefix + '/' + zip_filename
         download_and_extract_archive(url, self.root, filename=zip_filename, md5=self.zips_md5[filename])
 
-    def _get_target_folder(self):
+    def _get_target_folder(self) -> str:
         return 'images_background' if self.background else 'images_evaluation'