avoid reading file to ram in ox.net.save_url

ox/net.py

@@ -8,6 +8,11 @@ import os
 import re
 import struct
 
+try:
+    import requests
+    USE_REQUESTS = True
+except:
+    USE_REQUESTS = False
 from six import BytesIO, PY2
 from six.moves import urllib
 from chardet.universaldetector import UniversalDetector
@@ -117,9 +122,17 @@ def save_url(url, filename, overwrite=False):
         dirname = os.path.dirname(filename)
         if dirname and not os.path.exists(dirname):
             os.makedirs(dirname)
-        data = read_url(url)
-        with open(filename, 'wb') as f:
-            f.write(data)
+        headers = DEFAULT_HEADERS.copy()
+        if USE_REQUESTS:
+            r = requests.get(url, headers=headers, stream=True)
+            with open(filename, 'wb') as f:
+                for chunk in r.iter_content(chunk_size=1024):
+                    if chunk:  # filter out keep-alive new chunks
+                        f.write(chunk)
+        else:
+            data = read_url(url)
+            with open(filename, 'wb') as f:
+                f.write(data)
 
 def _get_size(url):
     req = urllib.request.Request(url, headers=DEFAULT_HEADERS.copy())