There was a duplicated call to url_adapter.build(...)

try:
        rv = url_adapter.build(endpoint, values, method=method,
                               force_external=external)
    except BuildError, error:
        # We need to inject the values again so that the app callback can
        # deal with that sort of stuff.
        values['_external'] = external
        values['_anchor'] = anchor
        values['_method'] = method
        return appctx.app.handle_url_build_error(error, endpoint, values)

   rv = url_adapter.build(endpoint, values, method=method,
                          force_external=external)

If no exception was raised for url_adapter.build(...) then the same method call
would be made after the try...except block. This is unnecessary.

flask/helpers.py

@@ -295,8 +295,6 @@ def url_for(endpoint, **values):
         values['_method'] = method
         return appctx.app.handle_url_build_error(error, endpoint, values)
 
-    rv = url_adapter.build(endpoint, values, method=method,
-                           force_external=external)
     if anchor is not None:
         rv += '#' + url_quote(anchor)
     return rv