PETSc is a suite of data structures and routines for the scalable (parallel) solution of scientific applications modeled by partial differential equations. It employs the MPI standard for all message-passing communication.
petsc4py are Python bindings for PETSc, the Portable, Extensible Toolkit for Scientific Computation.
A good friend of petsc4py is:
mpi4py: Python bindings for MPI, the Message Passing Interface. Other two projects depend on petsc4py:
slepc4py: Python bindings for SLEPc, the Scalable Library for Eigenvalue Problem Computations. tao4py: Python bindings for TAO, the Toolkit for Advanced Optimization.