Working with Variables

In OpenMDAO, a variable is an attribute that can be seen or manipulated by other entities in the framework. Any data that is passed between components in a model must use variables to declare the inputs and outputs for each component.

You can create a variable for a component in two ways. The first is to declare it in the component’s class definition. A simple component that takes a floating point number as an input and provides a floating point number as an output would look like this:

from openmdao.main.api import Component
from openmdao.main.datatypes.api import Float

class Simple(Component):
    """ A simple multiplication """

    # set up interface to the framework
    x = Float(1.0, iotype='in', desc='The input x')
    y = Float(iotype='out', desc='The output y')

    def execute(self):
        """ y = 3*x """

        self.y = 3.0*self.x

Built-in Variable Types

Summary of Variable Types

Name Callable Signature
Array Array( [default_value = None, shape = None, value = None, dtype = None, units = None, iotype = None, desc = None, units = None] )
Bool Bool( [value = None, desc = None, iotype = None] )
Complex Complex( [value = None, desc = None, iotype = None] )
Enum Enum( [default_value, values = (), desc = None, iotype = None, aliases = ()] )
File File( [default_value = None, iotype = None, desc = None, low = None, high = None, path = None, content_type = None, binary = False, local_path = None] )
Float Float( [default_value = None, iotype = None, desc = None, low = None, high = None, exclude_low = False, exclude_high = False, units = None] )
Int Int( [default_value = None, iotype = None, desc = None, low = None, high = None, exclude_low = False, exclude_high = False] )
Slot Slot( [klass = None, desc = None, iotype = None, factory = None, args = None, kw = None, allow_none = True, adapt = None, required = False] )
Str Str( [value = None, desc = None, iotype = None] )

When a variable is declared, it gets passed a number of arguments, many of which are optional.

But the iotype attribute is required for all variables regardless of type. Its function is to tell the framework whether the variable should be treated as an input or an output.

Summary of iotypes

iotype Description
iotype='in' Component input
iotype='out' Component output

The desc attribute is a concise description of the variable – one or two sentences should be fine. While nothing in the framework requires this description, it would be wise to include one for every input and output of your components.

It is also possible to create new types of variables to use in your models. For an example of a user-created variable, see Building a Variable Plugin.


You can use an array as a variable by using the Array trait. The value for an Array can be expressed as either a Python array or a NumPy array. NumPy arrays are very useful because of NumPy’s built-in mathematical capabilities. Either array can be n-dimensional and potentially of any type.

Constructing an Array variable requires a couple of additional parameters that are illustrated in the following example:

>>> from openmdao.lib.datatypes.api import Array
>>> from numpy import array
>>> from numpy import float as numpy_float
>>> z = Array(array([[1.0,2.0],[3.0,5.0]]), dtype=numpy_float, shape=(2,2), iotype='in')
>>> z.default_value
array([[ 1.,  2.],
       [ 3.,  5.]])
>>> z.default_value[0][1]

Here, we import the Array variable and the NumPy array, which is a general-purpose n-dimensional array class. A 2-dimensional array is assigned as the default value for the variable named z.

The dtype parameter defines the type of variable that is in the array. For example, using a string (str) for a dtype would give an array of strings. Any of Python’s standard types and NumPy’s additional types should be valid for the dtype parameter. The alternate typecode specification is also supported for non-NumPy arrays (e.g., typecode='I' for unsigned integers.)

The shape parameter is not a required attribute; the Array will default to the dimensions of the array that are given as the value. However, it is often useful to specify the size explicitly, so an exception is generated if an array of a different size or shape is passed into it. If the size of an array is not determined until runtime (e.g., a driver that takes an array of constraint equations as an input), then the shape should be left blank.

An array can also have a single unit defined with the units parameter. This unit applies to every element in the array, and it enables unit checking and conversion when connecting an array output to an array input.

Below is an example of a simple component that takes two Arrays as inputs and calculates their dot product as an output.

from numpy import array, sum, float

from openmdao.main.api import Component
from openmdao.lib.datatypes.api import Array, Float

class Dot(Component):
    """ A component that outputs a dot product of two arrays"""

    # set up interface to the framework
    x1 = Array(array([1.0,2.0]), dtype=float, desc = "Input 1",
    x2 = Array(array([7.0,8.0]), dtype=float, desc = "Input 2",

    y = Float(0.0, iotype='out', desc = "Dot Product")

    def execute(self):
        """ calculate dot product """

        if len(self.x1) != len(self.x2):
            self.raise_exception('Input vectors must be of equal length',

        # Note: array multiplication is element by element
        self.y = sum(self.x1*self.x2)

        # print the first element of x1
        print x1[0]

Multiplication of a NumPy array is element by element, so sum is used to complete the calculation of the dot product. Individual elements of the array can also be accessed using brackets. An OpenMDAO Array behaves like a NumPy array, so it can be used as an argument in a NumPy function like sum.

Note that this is a horrible way to do a dot product. Numpy has a dot function which is much faster than sum.


It is possible to use an Enum (enumeration) type as a variable in OpenMDAO. This is useful for cases where an input has certain fixed values that are possible. For example, consider a variable that can be one of three colors:

from openmdao.lib.datatypes.api import Enum
from openmdao.main.api import Component

class TrafficLight(Component):
    color2 = Enum('Red', ('Red', 'Yellow', 'Green'), iotype='in')

Then we can interact like this:

>>> test = TrafficLight()
>>> test.color2
>>> test.color2="Purple"
Traceback (most recent call last):
ValueError: : Variable 'color2' must be in ('Red', 'Yellow', 'Green'), but a value of Purple <type 'str'> was specified.
>>> test.color2="Green"
>>> test.color2

However, if the Enum is being used to select the input for an old code, then you will most likely need to feed it integers, not strings. To make this more convenient, the Enum includes an optional parameter alias that can be used to provide descriptive strings to go along with the numbers the code expects.

from openmdao.lib.datatypes.api import Enum
from openmdao.main.api import Component

class TrafficLight(Component):
    color = Enum(0, (0, 1, 2), iotype='in', aliases=("Red", "Yellow", "Green"))

Let’s create an instance of this component and try setting the Enum.

>>> test = TrafficLight()
>>> test.color=2
>>> test.color

If we set to an invalid value, an exception is raised.

>>> test.color=4
Traceback (most recent call last):
ValueError: : Variable 'color' must be in (0, 1, 2), but a value of 4 <type 'int'> was specified.`

We can also access the list of indices and the list of aliases directly from the trait.

>>> color_trait = test.get_trait('color')
>>> color_trait.aliases
('Red', 'Yellow', 'Green')
>>> color_trait.values
(0, 1, 2)
>>> color_trait.aliases[test.color]

If the default value is not given, then the first value of the list is taken as the default.

color2 = Enum(('Red', 'Yellow', 'Green'), iotype='in')

This is the simplest form of the Enum constructor.

It is also possible to produce a simple array that behaves like an Enum, where each element of the array can contain only a value that is in the Enum. This kind of variable can be defined by creating a List of Enums.

from openmdao.lib.datatypes.api import Enum, List
from openmdao.main.api import Component

class Dice(Component):
    roll = List( Enum(1, (1, 2, 3, 4, 5, 6)), iotype='in')

This example defines a variable named roll that can contain the values for any number of dice. Instead of giving a List as the default value, we’ve given it the definition for an Enum variable that has a default value of 1 and a set of valid values spanning the integers 1 to 6. Note that the Enum doesn’t need an iotype, but the List does.

>>> my_dice = Dice()
>>> # Valid
>>> my_dice.roll = [1, 6, 3, 2, 2]
>>> # Invalid
>>> my_dice.roll = [1, 6, 3, 2, 7]
Traceback (most recent call last):
ValueError: : Variable 'roll' must be in (1, 2, 3, 4, 5, 6), but a value of 7 <type 'int'> was specified.

File Variables

The File variable contains a reference to an input or output file on disk. It is more than just a text string that contains a path and filename; it is a FileReference that can be passed into other functions expecting such an object. FileReferences have methods for copying the reference and opening the referenced file for reading. The available flags are defined by FileMetadata, which supports arbitrary user metadata.

from openmdao.lib.datatypes.api import File

text_file = File(path='source.txt', iotype='out', content_type='txt')
binary_file = File(path='source.bin', iotype='out', binary=True,
                        extra_stuff='Hello world!')

The path must be a descendant of the parent component’s path. The binary flag can be used to mark a file as binary.


Provide some examples to demonstrate the options.

Slot Variables

A Slot is a variable that requires any value assigned to it to be either an instance of a specific class or an implementation of a specific Interface. The class or Interface to be matched is the first argument to the constructor. Failure to match the specified class or Interface will result in an exception being raised. Slot traits are typically used to implement placeholders for plugins within a component, but they may also be used to implement Variables by setting their iotype metadata attribute to 'in' or 'out'. In this case, it is important to also set the copy metadata attribute so the framework knows how to copy the data to connected components. Allowable values for copy are 'deep' (the default), 'shallow', and None. A copy value of None indicates that the data will be passed by reference and no copy will be made.

from openmdao.main.api import Component
from openmdao.lib.datatypes.api import Slot
from openmdao.main.interfaces import ICaseRecorder, ICaseIterator

class Fred(Component):
    """ A component that takes a class as an input """

    recorder = Slot(ICaseRecorder, desc='Something to append() to.',
    caseiter = Slot(ICaseIterator, desc='set of cases to run.',

In this example, we have one Slot and one input that is a Slot. The input called caseiter requires data objects that implement the ICaseIterator interface. The Slot called recorder is required to implement the ICaseRecorder interface.

The attribute required is used to indicate whether the object that plugs into a Slot is required. If required is True, then an exception will be raised if the object is not present.

Unit Conversions with Float and Array

OpenMDAO also supports variables with explicitly defined units using the Float and Array variable types, which are included as part of the Standard Library. Both types provide the following useful effects when utilized in the framework.

  • Automatically convert a value passed from an output to an input with compatible units (e.g., 'inch' and 'm')
  • Raise an exception when attempting to pass a value from an output to an input having incompatible units (e.g., 'kg' and 'm')
  • Allow values to be passed between unitless variables and variables with units; no unit conversion occurs

A complete list of the available units is given in the Appendix: Summary of Units. The unit conversion code and the base set of units come from the PhysicalQuantities package found in Scientific Python. It was necessary to add a few units to the existing ones in PhysicalQuantities (in particular, a currency unit), so a new Units package was derived and is included in OpenMDAO as openmdao.units. This package has the same basic function as that of PhysicalQuantities, but to make it more extensible, the unit definitions were moved from the internal dictionary into an externally readable text file called unitLibdefault.ini. For more information on the OpenMDAO units package, including how to add units, see the source documentation.  As an example, consider a component that calculates a pressure (in Pascals) given a known force (in Newtons) applied to a known area (in square meters). Such a component would look like this:

from openmdao.main.api import Component
from openmdao.lib.datatypes.api import Float

class Pressure(Component):
    """Simple component to calculate pressure given force and area"""

    # set up interface to the framework
    force = Float(1.0, iotype='in', desc='force', units='N')
    area = Float(1.0, iotype='in', low=0.0, exclude_low=True, desc='m*m')

    pressure = Float(1.0, iotype='out', desc='Pa')

    def execute(self):
        """calculate pressure"""

        self.pressure = self.force/self.area

The low and exclude_low parameters are used in the declaration of area to prevent a value of zero from being assigned, resulting in a division error. Of course, you could still get very large values for pressure if area is near machine zero.

This units library can also be used to convert internal variables by importing the function convert_units from openmdao.main.api.

>>> from openmdao.main.api import convert_units
>>> convert_units(12.0,'inch','ft')

Coercion and Casting

OpenMDAO variables have a certain pre-defined behavior when a value from a variable of a different type is assigned. Generally, they do not try to coerce the given value into the type that they expect. This means that most mis-assignments in variable connections (e.g., a float connected to a string) will generate an exception. However, certain widening coercions are permitted (e.g., Int->Float, Bool->Int, Bool->Float). No coercion from Str or to Str is allowed. If you need to apply different coercion behavior, just create a new class inherited from Variable and perform the coercion in the validate function.

More details can be found in the Traits 3 User Manual.

Variable Trees

For components with many variables, it is often useful to compartmentalize them into a hierarchy of containers to enhance readability and “findability.”

Variables in OpenMDAO can be compartmentalized by creating a container from the VariableTree base class. This container merely contains variables or other VariableTrees.

Normally a variable is accessed in the data hierarchy as:


but when it is in a VariableTree, it can be accessed as:


Consider an example of an aircraft simulation that requires values for three variables that define two flight conditions:

from openmdao.main.api import Component, VariableTree, Slot
from openmdao.lib.datatypes.api import Float

class FlightCondition(VariableTree):
    """Container of variables"""

    airspeed = Float(120.0, units='nmi/h')
    angle_of_attack = Float(0.0, units='deg')
    sideslip_angle = Float(0.0, units='deg')

class AircraftSim(Component):
    """This component contains variables in a VariableTree"""

    # create Slots to handle updates to our FlightCondition attributes
    fcc1 = Slot(FlightCondition, iotype='in')
    fcc2 = Slot(FlightCondition, iotype='out')

    weight = Float(5400.0, iotype='in', units='kg')
    # etc.

    def __init__(self):
        """Instantiate variable containers here"""

        super(AircraftSim, self).__init__()

        # Instantiate and add our variable containers.
        self.add('fcc1', FlightCondition())
        self.add('fcc2', FlightCondition())

    def execute(self):
        """Do something."""

        print "FCC1 angle of attack = ", self.fcc1.angle_of_attack
        print "FCC2 angle of attack = ", self.fcc2.angle_of_attack


It’s important to create a Slot variable for each VariableTree object contained in your component if you intend to connect it to variables in other components. Also make sure to set the iotype attribute in the Slot. If you don’t, changes to variables within the VariableTree object won’t properly notify the component. If you have a nested VariableTree, it’s only necessary to create a Slot in the component that contains it. Adding Slots for VariableTrees inside of another VariableTree is not necessary.

Here, we defined the class FlightCondition, containing three variables. The component AircraftSim is also defined with a variable weight, the input FlightCondition fcc1 and the output FlightCondition fcc2. We can access weight through self.weight; likewise, we can access the airspeed of the output flight condition through self.fcc2.airspeed. In this example we had only one level of nesting in our FlightCondition class, but a VariableTree can be added to another VariableTree, so any level of nesting is possible. For example:

class MyNestedVars(VariableTree):
    """A nested container of variables"""

    f1 = Float(120.0)
    f2 = Float(0.0)

    def __init__(self):
        super(MyNestedVars, self).__init__()
        self.add('sub_vartree', FlightCondition())

An interesting thing about this example is that we’ve implemented a data structure with this VariableTree and used it to create multiple copies of a set of variables. This can prove useful for blocks of variables that are repeated in a component. At the framework level, connections can be made either to individual variables within a VariableTree or to entire VariableTrees. It is also possible to create custom data objects and validators to use when connecting components. This is explained in Building a Variable Plugin.