SENSITIVITY

=SENSITIVITY(model, variables, parameters, [variable_names], [parameter_names])

=SENSITIVITY("x1^2 + a1*x1", {2}, {3}, {"x1"}, {"a1"})

=SENSITIVITY("ca.exp(a*x) + b*y^2", {1,2}, {0.5,2}, {"x","y"}, {"a","b"})

=SENSITIVITY("a*x^3 + b*x^2 + c*x", {2}, {1,2,3}, {"x"}, {"a","b","c"})

=SENSITIVITY("x^2 + a*ca.sin(x) + b", {1}, {2,5}, {"x"}, {"a","b"})

import re
import casadi as ca
 
 
def sensitivity(model, variables, parameters, variable_names=None, parameter_names=None):
    """Compute the sensitivity of a scalar model with respect to its parameters using CasADi.
 
    Wraps CasADi automatic differentiation to return the row vector of partial derivatives
    of a scalar expression f(x, a) with respect to parameters `a` evaluated at the provided
    point.
 
    Args:
        model: String expression representing the model output. The expression may use
            CasADi functions via the `ca` namespace (for example `ca.exp`, `ca.sin`).
        variables: 2D list or scalar containing the evaluation point for each independent variable.
        parameters: 2D list or scalar containing the parameter values at which the sensitivity is evaluated.
        variable_names: Optional 2D list of strings specifying the variable names in order.
        parameter_names: Optional 2D list of strings specifying the parameter names in order.
 
    Returns:
        A 2D list containing a single row of partial derivatives (floats) with respect to each parameter,
        or an error message string if validation fails or CasADi encounters an error.
 
    Notes:
        This function exposes a minimal wrapper around CasADi. See https://web.casadi.org/ for
        CasADi documentation. This example function is provided as-is without any representation of accuracy.
    """
 
    # Helper to normalize values that may be passed as scalars by Excel for single-cell ranges.
    def to2d(x):
        return [[x]] if not isinstance(x, list) else x
 
    if not isinstance(model, str) or model.strip() == "":
        return "Invalid input: model must be a non-empty string."
 
    # Normalize inputs that may be passed as scalars.
    variables = to2d(variables)
    parameters = to2d(parameters)
 
    try:
        variable_values = [float(v) for v in variables[0]]
    except Exception:
        return "Invalid input: variables must contain numeric values."
 
    try:
        parameter_values = [float(p) for p in parameters[0]]
    except Exception:
        return "Invalid input: parameters must contain numeric values."
 
    if len(variable_values) == 0:
        return "Invalid input: variables must include at least one value."
    if len(parameter_values) == 0:
        return "Invalid input: parameters must include at least one value."
 
    # Validate and normalize names
    if variable_names is not None:
        variable_names = to2d(variable_names)
        variable_names_list = variable_names[0]
        if len(variable_names_list) != len(variable_values):
            return "Invalid input: number of variable names must match number of variable values."
        if not all(isinstance(n, str) and n.strip() for n in variable_names_list):
            return "Invalid input: variable_names must contain non-empty strings."
    else:
        variable_names_list = [f"x{i+1}" for i in range(len(variable_values))]
 
    if parameter_names is not None:
        parameter_names = to2d(parameter_names)
        parameter_names_list = parameter_names[0]
        if len(parameter_names_list) != len(parameter_values):
            return "Invalid input: number of parameter names must match number of parameter values."
        if not all(isinstance(n, str) and n.strip() for n in parameter_names_list):
            return "Invalid input: parameter_names must contain non-empty strings."
    else:
        parameter_names_list = [f"a{i+1}" for i in range(len(parameter_values))]
 
    # Create CasADi symbols
    variable_symbols = [ca.MX.sym(n) for n in variable_names_list]
    parameter_symbols = [ca.MX.sym(n) for n in parameter_names_list]
 
    # Build evaluation context for eval() so user can reference variable and parameter names
    evaluation_context = {name: variable_symbols[idx] for idx, name in enumerate(variable_names_list)}
    evaluation_context.update({name: parameter_symbols[idx] for idx, name in enumerate(parameter_names_list)})
    evaluation_context["ca"] = ca
 
    # Convert Excel exponentiation (^) to Python exponentiation (**)
    model = re.sub(r'\^', '**', model)
 
    try:
        expression = eval(model, evaluation_context)
    except Exception as exc:
        return f"Invalid model expression: {exc}"
 
    try:
        jac = ca.jacobian(expression, ca.vertcat(*parameter_symbols))
        sensitivity_fn = ca.Function("sensitivity_fn", variable_symbols + parameter_symbols, [jac])
        casadi_result = sensitivity_fn(*(variable_values + parameter_values))
    except Exception as exc:
        return f"Error during CasADi calculation: {exc}"
 
    # casadi_result is typically a DM; return as nested Python lists
    try:
        if isinstance(casadi_result, ca.DM):
            return casadi_result.full().tolist()
        # Some CasADi versions return a list/tuple with one element
        if isinstance(casadi_result, (list, tuple)) and len(casadi_result) == 1 and isinstance(casadi_result[0], ca.DM):
            return casadi_result[0].full().tolist()
    except Exception:
        return "Error during CasADi calculation: Unexpected result type."
 
    return "Error during CasADi calculation: Unexpected result type."