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.. _petsc_options:
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 <the_options_database>` for additional information.
In order to use command-line options in a petsc4py program, it is important to initialize the module as follows:
.. code-block:: python
# 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:
.. code-block:: console
$ 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.:
.. code-block:: console
$ 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:
.. code-block:: python
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
.. code-block:: console
$ 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:
.. code-block:: python
OptDB['draw_pause'] = 1
del OptDB['draw_pause']
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