mnist-reduce-download-proposal.txt

diff --git a/torchvision/datasets/mnist.py b/torchvision/datasets/mnist.py
index e87cd46e..bfd59914 100644
--- a/torchvision/datasets/mnist.py
+++ b/torchvision/datasets/mnist.py
@@ -131,6 +131,11 @@ class MNIST(VisionDataset):
     def class_to_idx(self) -> Dict[str, int]:
         return {_class: i for i, _class in enumerate(self.classes)}
 
+    def _check_raw_data_exists(self) -> bool:
+        return all([
+            os.path.exists(os.path.join(self.raw_folder, filename)) for (filename, _) in self.resources
+        ])
+
     def _check_exists(self) -> bool:
         return (os.path.exists(os.path.join(self.processed_folder,
                                             self.training_file)) and
@@ -140,34 +145,36 @@ class MNIST(VisionDataset):
     def download(self) -> None:
         """Download the MNIST data if it doesn't exist in processed_folder already."""
 
+        if not self._check_raw_data_exists():
+            print('Dowanloading raw MNIST...')
+            os.makedirs(self.raw_folder, exist_ok=True)
+            os.makedirs(self.processed_folder, exist_ok=True)
+
+            # download files
+            for filename, md5 in self.resources:
+                for mirror in self.mirrors:
+                    url = "{}{}".format(mirror, filename)
+                    try:
+                        print("Downloading {}".format(url))
+                        download_and_extract_archive(
+                            url, download_root=self.raw_folder,
+                            filename=filename,
+                            md5=md5
+                        )
+                    except URLError as error:
+                        print(
+                            "Failed to download (trying next):\n{}".format(error)
+                        )
+                        continue
+                    finally:
+                        print()
+                    break
+                else:
+                    raise RuntimeError("Error downloading {}".format(filename))
+
         if self._check_exists():
             return
 
-        os.makedirs(self.raw_folder, exist_ok=True)
-        os.makedirs(self.processed_folder, exist_ok=True)
-
-        # download files
-        for filename, md5 in self.resources:
-            for mirror in self.mirrors:
-                url = "{}{}".format(mirror, filename)
-                try:
-                    print("Downloading {}".format(url))
-                    download_and_extract_archive(
-                        url, download_root=self.raw_folder,
-                        filename=filename,
-                        md5=md5
-                    )
-                except URLError as error:
-                    print(
-                        "Failed to download (trying next):\n{}".format(error)
-                    )
-                    continue
-                finally:
-                    print()
-                break
-            else:
-                raise RuntimeError("Error downloading {}".format(filename))
-
         # process and save as torch files
         print('Processing...')