Building a Component Plugin

For this example we’ll build a plugin for a simple component. This component merely computes the value of its single output by adding its two inputs.

These instructions apply to any plugin component distribution that is pure Python, i.e., not containing any Python extensions. They will also work with file wrapped components as long as the distribution includes only the wrapper component and not the application being file wrapped.

Getting Started Quickly

To create the initial directory structure for our plugin distribution, we use the plugin quickstart command.

plugin quickstart dist_name [-v version [-c class_name [-g plugin_group [-d dest_directory]]]]

The name of the distribution we want to create.


Version id of the distribution. Defaults to 0.1.


Name of the plugin class. By default the plugin class has the same name as the distribution except the first letter is capitalized.


Plugin group. Defaults to openmdao.component. Other possible values are: openmdao.driver and openmdao.variable.


Directory where the plugin subdirectory will be created. Defaults to the current directory.

The first thing we’ll do is to use plugin quickstart to build a directory structure in our current directory for a distribution named simpleadder containing a component plugin class named SimpleAdder and having a version id of 0.9.

plugin quickstart simpleadder -c SimpleAdder -v 0.9

After running plugin_quickstart, the current directory will contain a directory named simpleadder. The simpleadder directory structure will look something like this:

|-- docs
|   |--
|   |-- index.rst
|   |-- pkgdocs.rst
|   |-- srcdocs.rst
|   `-- usage.rst
|-- README.txt
|-- setup.cfg
`-- src
    `-- simpleadder
        `-- test

Customizing our Plugin

The plugin quickstart automatically generates skeleton files for our plugin class, distribution metadata, and documentation, but we’ll want to tailor these specifically to our plugin by editing the following files found in our distribution directory:

A Python source file containing our plugin class.
A config file that specifies metadata related to the plugin. This is where we would specify license information, contact emails, etc.
If we have additional files to include in our distribution beyond the standard set of Python source files and setup files, we can specify them here. For more info about, look here.
A reStructuredText file containing any docs that we want to add to those that are generated automatically.
A simple text file with miscellaneous instructions about the plugin.
A Python source file containing a unit test for our plugin class. It actually doesn’t run any tests by default, but there is a skeletal version of a unittest.TestCase defined here to make it as easy as possible to add some unit tests for our plugin.

The following sections describe how to edit these files in more detail.

Editing our Plugin Class

The most important file to edit is the Python file that defines our plugin class. The plugin class definition is found in:


or in our case:


When we’re done modifying the skeleton plugin class in the file, it should look like this:

from openmdao.lib.datatypes.api import Float

from openmdao.main.api import Component

class SimpleAdder(Component):
    """A simple component whose output *c* is the sum of
    its inputs *a* and *b*.
    a = Float(0.0, iotype='in', desc='an input added to *b* to make *c*')
    b = Float(0.0, iotype='in', desc='an input added to *a* to make *c*')
    c = Float(0.0, iotype='out', desc='the sum of *a* and *b*')

    def execute(self):
         """Calculate c as the sum of a and b."""
         self.c = self.a + self.b

The code defines the class SimpleAdder, which inherits from the Component class defined in openmdao.main.api, so we have to import it from there. The function in our component that performs a computation is called execute(), and there we define c as the sum of a and b. The self object that is passed as an argument to execute() represents an instance of our SimpleAdder class.

SimpleAdder has three variables of type Float with the names a, b, and c. All three variables have a default value of 0.0. Attributes a and b are inputs, so we specify that they have an iotype of 'in'. Attribute c is an output, so it has an iotype of 'out'.

The Float variable is defined in the package openmdao.lib.datatypes.api, so we have to import it from there before we can use it. This package defines a wide variety of variables, including basic types like Int, Str, and Bool; containers like List and Dict; and others.

Developing a plugin is often an iterative process, so it’s convenient to have a way to install the plugin and hack on it, test it, etc., without having to reinstall it each time we change it. Luckily this is easy to do by just installing our plugin as a develop egg. To do this, we just run the plugin install command from the top directory of our plugin distribution. After that, our plugin can be imported and used in the OpenMDAO environment just like any other installed plugin. For example, we could import our plugin class like this:

from <package_name>.<module_name> import <plugin_class>

or, in this specific case:

from simpleadder.simpleadder import SimpleAdder

Adding Documentation

Now that our plugin class is fully defined, we should write up some documentation about how to use it. The packaging command that we’ll run later, plugin makedist, will automatically generate source documentation for our plugin, but we can add to that by editing the docs/usage.rst file, perhaps providing some detailed usage instructions and maybe a few examples. The format of the usage.rst file is reStructuredText and we use Sphinx to generate our documentation, so any reST or Sphinx directives may be used there.


Make sure to give your plugin class a doc string, because otherwise the automatic source documentation for your input and output variables won’t be generated.

The other documentation file that you may want to edit is the README.txt file. A small amount of information is put there automatically but you may want to add more.

Sometimes you want to add a static document like a PDF file to a plugin package. For example, if you are file wrapping a code and want to include the documentation for the code, you need to put the file into one of the directories listed in the static_path variable inside the setup.cfg file.

Setting Metadata

The final step in preparing to package our plugin is to define metadata for our distribution. You specify that metadata by editing the setup.cfg file. The skeleton version of setup.cfg generated by plugin quickstart in our case looks like this:

name = simpleadder
version = 0.9
summary =
description-file = README.txt
keywords = openmdao
home-page =
download-url =
author =
author-email =
maintainer =
maintainer-email =
license =
classifier = Intended Audience :: Science/Research
    Topic :: Scientific/Engineering

requires-dist = openmdao.main
provides-dist =
obsoletes-dist =
requires-python =
requires-externals =
project-url =

copyright =
static_path = [ '_static' ]

You should set whatever of these values you feel are applicable to your plugin. The name and version values are the only ones that are mandatory, but you should fill in as many as possible to better inform potential users about your plugin.


Distributions tend to evolve over time, so providing a version id for a package is extremely important. It is assumed that once a distribution is created from a particular version of a package, that distribution will never change. People may build things that depend on a particular version of your distribution, so changing that version could break their code. If, however, you update your distribution’s version id, then users have the option of either using the updated distribution and modifying their own code to make it work or sticking with an older version that already works with their code.

More descriptions of the various metadata values can be found here.

The values in the metadata section are specified by PEP 345 and apply to any Python distribution. We’ve added an openmdao section to the file to provide a place to put metadata that isn’t mentioned in PEP 345, for example, the copyright notice for the documentation.

Additional Customization

In some cases, you may want to add multiple plugin classes to your distribution, either in the <dist_name>.py file or in separate Python source files that you add to the src directory, possibly as part of a nested package directory structure. The plugin makedist script knows how to handle this sort of a situation and will generate the appropriate source documentation and metadata for whatever plugins you define under the src tree.

If you plan to use plugin makedist to create your distribution, you should not modify any of the files listed below because they will be overwritten by the script.

  • docs/
  • docs/pkgdocs.rst
  • docs/srcdocs.rst

If for some reason you must modify any of the files above, then you must build your distribution using the standard Python packaging procedure, for example:

python sdist

That will create a source distribution of your plugin; but keep in mind that in this case you will have to specify entry point metadata in the file manually for each of your plugins. To specify entry points manually, you must add an entry_points keyword argument to the setup call inside of the file.

Entry points are divided into groups, and each type of OpenMDAO plugin has a particular group. For example, Component plugins are found in the openmdao.component group. Each entry point is specified by its name followed by an equals (=) sign; followed by dotted module path (dotted path you would use to import the module in Python); followed by a colon (:) and the name of the plugin class. The value of entry_points should be a string in INI file format or a dictionary.

For example:

simpleadder.simpleadder.SimpleAdder = simpleadder.simpleadder:SimpleAdder

mydriver.mydriver.MyDriver = mydriver.mydriver:MyDriver


{ 'openmdao.component': ['simpleadder.simpleadder.SimpleAdder = simpleadder.simpleadder:SimpleAdder'],
  'openmdao.driver': ['mydriver.mydriver.MyDriver = mydriver.mydriver:MyDriver']

Building Just the Plugin Docs

Sometimes when we’re iterating on the plugin documentation it’s convenient to regenerate just the docs instead of creating a new distribution every time. We can do this using the plugin build_docs command.

plugin build_docs [dist_directory]

Top level directory of the distribution. Defaults to the current directory.

We can view the docs for a plugin using the openmdao docs command. Note that this only works for installed plugin distributions.

openmdao docs plugin_dist_name

The name of the plugin distribution.

If we install our plugin as a develop egg by running plugin install from the top level directory of our plugin distribution, we can then edit and view our docs efficiently by repeating the following sequence:

... hack, hack, hack
plugin build_docs
openmdao docs <plugin_dist_name>

Creating Our Plugin Distribution

Eventually our plugin will be ready to package as a distribution. Doing this makes it easier to share our plugin with others in the OpenMDAO community. To create our distribution, we issue the command:

plugin makedist <dist_dir>

where dist_dir is the name of the directory containing our distribution. It defaults to the current directory. The script will automatically detect plugins within the distribution src directory and generate any necessary entry points for them in the file. It will also generate the Sphinx documentation and place the Sphinx-generated files and all other necessary files in a source distribution that will be named as follows: