Update load_MNIST on learning.ipynb (aimacode#339)

Make load_MNIST function easier to read

Author: lucasmoura

learning.ipynb

@@ -374,27 +374,21 @@
    "source": [
     "def load_MNIST(path=\"aima-data/MNIST\"):\n",
     "    \"helper function to load MNIST data\"\n",
-    "    train_img_file = open(os.path.join(path, \"train-images-idx3-ubyte\"), \"rb\")\n",
-    "    train_lbl_file = open(os.path.join(path, \"train-labels-idx1-ubyte\"), \"rb\")\n",
-    "    test_img_file = open(os.path.join(path, \"t10k-images-idx3-ubyte\"), \"rb\")\n",
-    "    test_lbl_file = open(os.path.join(path, 't10k-labels-idx1-ubyte'), \"rb\")\n",
+    "    with open(os.path.join(path, \"train-images-idx3-ubyte\"), \"rb\") as train_img_file:\n",
+    "        magic_nr, tr_size, tr_rows, tr_cols = struct.unpack(\">IIII\", train_img_file.read(16))\n",
+    "        tr_img = array.array(\"B\", train_img_file.read())\n",
     "    \n",
-    "    magic_nr, tr_size, tr_rows, tr_cols = struct.unpack(\">IIII\", train_img_file.read(16))\n",
-    "    tr_img = array.array(\"B\", train_img_file.read())\n",
-    "    train_img_file.close()    \n",
-    "    magic_nr, tr_size = struct.unpack(\">II\", train_lbl_file.read(8))\n",
-    "    tr_lbl = array.array(\"b\", train_lbl_file.read())\n",
-    "    train_lbl_file.close()\n",
+    "    with open(os.path.join(path, \"train-labels-idx1-ubyte\"), \"rb\") as train_lbl_file:\n",
+    "        magic_nr, tr_size = struct.unpack(\">II\", train_lbl_file.read(8))\n",
+    "        tr_lbl = array.array(\"b\", train_lbl_file.read())\n",
     "    \n",
-    "    magic_nr, te_size, te_rows, te_cols = struct.unpack(\">IIII\", test_img_file.read(16))\n",
-    "    te_img = array.array(\"B\", test_img_file.read())\n",
-    "    test_img_file.close()\n",
-    "    magic_nr, te_size = struct.unpack(\">II\", test_lbl_file.read(8))\n",
-    "    te_lbl = array.array(\"b\", test_lbl_file.read())\n",
-    "    test_lbl_file.close()\n",
-    "\n",
-    "#     print(len(tr_img), len(tr_lbl), tr_size)\n",
-    "#     print(len(te_img), len(te_lbl), te_size)\n",
+    "    with open(os.path.join(path, \"t10k-images-idx3-ubyte\"), \"rb\") as test_img_file:\n",
+    "        magic_nr, te_size, te_rows, te_cols = struct.unpack(\">IIII\", test_img_file.read(16))\n",
+    "        te_img = array.array(\"B\", test_img_file.read())\n",
+    "    \n",
+    "    with open(os.path.join(path, \"t10k-labels-idx1-ubyte\"), \"rb\") as test_lbl_file:\n",
+    "        magic_nr, te_size = struct.unpack(\">II\", test_lbl_file.read(8))\n",
+    "        te_lbl = array.array(\"b\", test_lbl_file.read())\n",
     "    \n",
     "    train_img = np.zeros((tr_size, tr_rows*tr_cols), dtype=np.int16)\n",
     "    train_lbl = np.zeros((tr_size,), dtype=np.int8)\n",