# Declaring Partial Derivatives¶

If you know additional information about the structure of partial derivatives in your component (for example, if an output does not depend on a particular input), you can use the declare_partials() method to inform the framework. This will allow the framework to be more efficient in terms of memory and computation (especially if using a sparse AssembledJacobian). This information should be declared in the setup method of your component.

Component.declare_partials(of, wrt, dependent=True, rows=None, cols=None, val=None, method='exact', step=None, form=None, step_calc=None)[source]

Parameters: of : str or list of str The name of the residual(s) that derivatives are being computed for. May also contain a glob pattern. wrt : str or list of str The name of the variables that derivatives are taken with respect to. This can contain the name of any input or output variable. May also contain a glob pattern. dependent : bool(True) If False, specifies no dependence between the output(s) and the input(s). This is only necessary in the case of a sparse global jacobian, because if ‘dependent=False’ is not specified and declare_partials is not called for a given pair, then a dense matrix of zeros will be allocated in the sparse global jacobian for that pair. In the case of a dense global jacobian it doesn’t matter because the space for a dense subjac will always be allocated for every pair. rows : ndarray of int or None Row indices for each nonzero entry. For sparse subjacobians only. cols : ndarray of int or None Column indices for each nonzero entry. For sparse subjacobians only. val : float or ndarray of float or scipy.sparse Value of subjacobian. If rows and cols are not None, this will contain the values found at each (row, col) location in the subjac. method : str The type of approximation that should be used. Valid options include: ‘fd’: Finite Difference, ‘cs’: Complex Step, ‘exact’: use the component defined analytic derivatives. Default is ‘exact’. step : float Step size for approximation. Defaults to None, in which case the approximation method provides its default value. form : string Form for finite difference, can be ‘forward’, ‘backward’, or ‘central’. Defaults to None, in which case the approximation method provides its default value. step_calc : string Step type for finite difference, can be ‘abs’ for absolute’, or ‘rel’ for relative. Defaults to None, in which case the approximation method provides its default value.

## Usage¶

1. Specifying that a variable does not depend on another. Note that this is not typically required, because by default OpenMDAO assumes that all variables are independent. However, in some cases it might be needed if a previous glob pattern matched a large set of variables and some sub-set of that needs to be marked as independent.

def setup(self):

self.declare_partials('f', 'y1', dependent=False)
self.declare_partials('f', 'y2', dependent=False)
self.declare_partials('f', 'y3', dependent=False)
self.declare_partials('g', 'z', dependent=False)

1. Declaring multiple derivatives using glob patterns (see https://docs.python.org/3.6/library/fnmatch.html).
def setup(self):

# This matches y1, y2, and y3.
self.declare_partials('f', 'y*', dependent=False)

# This matches y1 and y3.
self.declare_partials('g', 'y[13]', val=[[1, 0], [1, 0], [0, 1], [0, 1]])


3. Using the val argument to set a constant partial derivative. Note that this is intended for cases when the derivative value is constant, and hence the derivatives do not ever need to be recomputed in compute_partials. Here are several examples of how you can specify derivative values for differently-shaped partial derivative sub-Jacobians.

• Scalar [see $$\displaystyle\frac{\partial f}{\partial x}$$]
• Dense Array [see $$\displaystyle\frac{\partial f}{\partial z}$$]
• Nested List [see $$\displaystyle\frac{\partial g}{\partial y_1}$$ and $$\displaystyle\frac{\partial g}{\partial y_3}$$]
• Sparse Matrix (see Sparse Partial Derivatives doc for more details) [see $$\displaystyle\frac{\partial g}{\partial y_2}$$ and $$\displaystyle\frac{\partial g}{\partial x}$$]
class SimpleCompConst(ExplicitComponent):
def setup(self):

# Declare derivatives

self.declare_partials('f', ['y1', 'y2', 'y3'], dependent=False)
self.declare_partials('g', 'z', dependent=False)

self.declare_partials('f', 'x', val=1.)
self.declare_partials('f', 'z', val=np.ones((1, 4)))
# y[13] is a glob pattern for ['y1', 'y3']
self.declare_partials('g', 'y[13]', val=[[1, 0], [1, 0], [0, 1], [0, 1]])
self.declare_partials('g', 'y2', val=[1., 1., 1., 1.], cols=[0, 0, 1, 1], rows=[0, 2, 1, 3])
self.declare_partials('g', 'x', val=sp.sparse.coo_matrix(((1., 1.), ((0, 3), (0, 0)))))

def compute(self, inputs, outputs):
outputs['f'] = np.sum(inputs['z']) + inputs['x']
outputs['g'] = np.outer(inputs['y1'] + inputs['y3'], inputs['y2']) + inputs['x'] * np.eye(2)

def compute_partials(self, inputs, partials):
# note: all the partial derivatives are constant, so no calculations happen here.
pass