Working with PETSc options

A very powerful feature of PETSc is that objects can be configured via command-line options. In this way, one can choose the method to be used or set different parameters without changing the source code. See the PETSc manual for additional information.

In order to use command-line options in a petsc4py program, it is important to initialize the module as follows:

# We first import petsc4py and sys to initialize PETSc
import sys, petsc4py
petsc4py.init(sys.argv)

# Import the PETSc module
from petsc4py import PETSc

Then one can provide command-line options when running a script:

$ python foo.py -ksp_type gmres -ksp_gmres_restart 100 -ksp_view

When the above initialization method is not possible, PETSc options can be also specified via environment variables or configuration files, e.g.:

$ PETSC_OPTIONS='-ksp_type gmres -ksp_gmres_restart 100 -ksp_view' python foo.py

Command-line options can be read via an instance of the Options class. For instance:

OptDB = PETSc.Options()
n     = OptDB.getInt('n', 16)
eta   = OptDB.getReal('eta', 0.014)
alpha = OptDB.getScalar('alpha', -12.3)

In this way, if the script is run with

$ python foo.py -n 50 -alpha 8.8

the options, n and alpha will get the values 50 and 8.8, respectively, while eta will be assigned the value specified as default, 0.014.

The options database is accessible also as a Python dictionary, so that one can for instance override, insert or delete an option:

OptDB['draw_pause'] = 1
del OptDB['draw_pause']