Fix logic error in check_integrity (#871)

* fixed check integrity

* stylistic changes

* flake8

torchvision/datasets/utils.py

@@ -17,20 +17,25 @@ def gen_bar_updater():
     return bar_update
 
 
+def calculate_md5(fpath, chunk_size=1024 * 1024):
+    md5 = hashlib.md5()
+    with open(fpath, 'rb') as f:
+        for chunk in iter(lambda: f.read(chunk_size), b''):
+            md5.update(chunk)
+    return md5.hexdigest()
+
+
+def check_md5(fpath, md5, **kwargs):
+    return md5 == calculate_md5(fpath, **kwargs)
+
+
 def check_integrity(fpath, md5=None):
-    if md5 is None:
-        return True
     if not os.path.isfile(fpath):
         return False
-    md5o = hashlib.md5()
-    with open(fpath, 'rb') as f:
-        # read in 1MB chunks
-        for chunk in iter(lambda: f.read(1024 * 1024), b''):
-            md5o.update(chunk)
-    md5c = md5o.hexdigest()
-    if md5c != md5:
-        return False
+    if md5 is None:
         return True
+    else:
+        return check_md5(fpath, md5)
 
 
 def makedir_exist_ok(dirpath):