In mpi4py, we define the reduction operation through the following statement:

comm.Reduce(sendbuf, recvbuf, rank_of_root_process, op = type_of_reduction_operation) 

We must note that the difference with the comm.gather statement resides in the op parameter, which is the operation that you wish to apply to your data, and the mpi4py module contains a set of reduction operations that can be used.