Though I'm not sure you necessarily can optimize this further in this case, a single call to filter() will be much more efficient.  I've done something similar elsewhere too:

https://github.com/pyfarm/pyfarm-master/commit/947f475790b3158a8c77a9fb351fc04352d84739#diff-cf9b4e72af5cc2e55ebeccc2b3953eafR375 (line 397)