Fixing MNIST dataloader

src/lava/utils/dataloader/mnist.py

@@ -7,14 +7,25 @@
 
 
 class MnistDataset:
+    mirrors = [
+        "http://yann.lecun.com/exdb/mnist/",
+        "https://ossci-datasets.s3.amazonaws.com/mnist/",
+        "https://storage.googleapis.com/cvdf-datasets/mnist/",
+    ]
+
+    files = [
+        "train-images-idx3-ubyte.gz", "train-labels-idx1-ubyte.gz",
+        "t10k-images-idx3-ubyte.gz", "t10k-labels-idx1-ubyte.gz",
+    ]
+
     def __init__(self, data_path=os.path.join(os.path.dirname(__file__),
                                               'mnist.npy')):
         """data_path (str): Path to mnist.npy file containing the MNIST
         dataset"""
         if not os.path.exists(data_path):
             # Download MNIST from internet and convert it to .npy
             os.makedirs(os.path.join(os.path.dirname(__file__), 'temp'),
-                        exist_ok=False)
+                        exist_ok=True)
             MnistDataset. \
                 download_mnist(path=os.path.join(
                     os.path.dirname(__file__),
@@ -24,16 +35,35 @@ def __init__(self, data_path=os.path.join(os.path.dirname(__file__),
             # GUnzip, Parse and save MNIST data as .npy
             MnistDataset.decompress_convert_save(
                 download_path=os.path.join(os.path.dirname(__file__), 'temp'),
-                save_path=os.path.dirname(data_path))
+                save_path=data_path)
         self.data = np.load(data_path, allow_pickle=True)
 
-    # ToDo: Populate this method with a proper wget download from MNIST website
     @staticmethod
     def download_mnist(path=os.path.join(os.path.dirname(__file__), 'temp')):
-        pass
+        import urllib.request
+        import urllib.error
+
+        for file in MnistDataset.files:
+            err = None
+            for mirror in MnistDataset.mirrors:
+                try:
+                    url = f"{mirror}{file}"
+                    if url.lower().startswith("http"):
+                        # Disabling security linter and using hardcoded
+                        # URLs specified above
+                        res = urllib.request.urlopen(url)  # nosec
+                        with open(os.path.join(path, file), "wb") as f:
+                            f.write(res.read())
+                        break
+                    else:
+                        raise "Url does not start with http"
+                except urllib.error.URLError as exception:
+                    err = exception
+                    continue
+            else:
+                print("Failed to download mnist dataset")
+                raise err
 
-    # ToDo: Populate this method with proper code to decompress, parse,
-    #  and save MNIST as mnist.npy
     @staticmethod
     def decompress_convert_save(
             download_path=os.path.join(os.path.dirname(__file__), 'temp'),
@@ -42,16 +72,31 @@ def decompress_convert_save(
         download_path (str): path of downloaded raw MNIST dataset in IDX
         format
         save_path (str): path at which processed npy file will be saved
+
+        After loading data = np.load(), data is a np.array of np.arrays.
+        train_imgs = data[0][0]; shape = 60000 x 28 x 28
+        test_imgs = data[1][0]; shape = 10000 x 28 x 28
+        train_labels = data[0][1]; shape = 60000 x 1
+        test_labels = data[1][1]; shape = 10000 x 1
         """
-        # Gunzip, parse, and save as .npy
-        # Format of .npy:
-        # After loading data = np.load(), data is a np.array of np.arrays.
-        # train_imgs = data[0][0]; shape = 60000 x 28 x 28
-        # test_imgs = data[1][0]; shape = 10000 x 28 x 28
-        # train_labels = data[0][1]; shape = 60000 x 1
-        # test_labels = data[1][1]; shape = 10000 x 1
-        # save as 'mnist.npy' in save_path
-        pass
+
+        import gzip
+        arrays = []
+        for file in MnistDataset.files:
+            with gzip.open(os.path.join(download_path, file), "rb") as f:
+                if "images" in file:
+                    arr = np.frombuffer(f.read(), np.uint8, offset=16)
+                    arr = arr.reshape(-1, 28, 28)
+                else:
+                    arr = np.frombuffer(f.read(), np.uint8, offset=8)
+                arrays.append(arr)
+
+        np.save(
+            save_path,
+            np.array(
+                [[arrays[0], arrays[1]], [arrays[2], arrays[3]]],
+                dtype="object"),
+        )
 
     @property
     def train_images(self):