moving implementation details into util func

requests/sessions.py

@@ -29,7 +29,7 @@ from .adapters import HTTPAdapter
 
 from .utils import (
     requote_uri, get_environ_proxies, get_netrc_auth, should_bypass_proxies,
-    get_auth_from_url
+    get_auth_from_url, is_valid_location
 )
 
 from .status_codes import codes
@@ -99,7 +99,7 @@ class SessionRedirectMixin(object):
         request = response.request
 
         while response.is_redirect:
-            if len(response.raw.headers.getlist('location')) > 1:
+            if not is_valid_location(response):
                 raise InvalidHeader('Response contains multiple Location headers. '
                                     'Unable to perform redirect.')
 

requests/utils.py

@@ -665,6 +665,17 @@ def parse_header_links(value):
 
     return links
 
+def is_valid_location(response):
+    """Verify that multiple Location headers weren't
+    returned from the last response.
+    """
+    headers = getattr(response.raw, 'headers', None)
+    if headers is not None:
+        getlist = getattr(headers, 'getlist', None)
+        if getlist is not None:
+            return len(getlist('location')) <= 1
+    # If response.raw isn't urllib3-like we can't reliably check this
+    return True  
 
 # Null bytes; no need to recreate these on each call to guess_json_utf
 _null = '\x00'.encode('ascii')  # encoding to ASCII for Python 3