What is the difference between mpich2 and mpi4py? I just installed MPICH2 on my raspbian cluster. Do I need mpi4py as well?
mpich2 is the underlying library, mpi4py are the Python bindings to it. mpi4py will depend on mpich2 in the same way PySide depends on Qt5.
The pypi page for mpi4py has a description and other information you might want to read: