+def urljoin(base, path):
+    if not isinstance(path, compat_str) or not path:
+        return None
+    if re.match(r'^(?:https?:)?//', path):
+        return path
+    if not isinstance(base, compat_str) or not re.match(r'^(?:https?:)?//', base):
+        return None
+    return compat_urlparse.urljoin(base, path)
+
+