def func(x, a, b):
    """
    Model function for non linear regression fit.

    Function used by scipy.optimize.curve_fit method to calculate a and b
    coefficients minimizing squared error given independent and dependent
    variables.

    Parameters
    ----------
    x : float
        Independent variable of non linear regression fit.
    a : float
        First coefficient of non linear regression fit.
    b : float
        Second coefficient of non linear regression fit.

    Returns
    -------
    float
        Value of a * b ^ x.

    See Also
    --------
    scipy.optimize.curve_fit : Use non-linear least squares to fit a function
        to data.

    """
    return a * pow(b, x)