import math
def eval_str(formula, varname, value):
    """
    Evaluate the formula for a given value.
    The only allowed functions to evaluate are contained in
    the python math module.

    Arguments:
        formula : string to be evaluated via eval(formula)
        varname : argument to be substituted
        value : value to substitute with varname
    Returns:
        float, evaluated formula
    """

    #Obtain list of allowable math functions
    allowable_func = {}
    for fn in dir(math):
        if "_" not in fn:
            allowable_func[fn] = getattr(math, fn)

    return eval(formula, allowable_func, {varname: value})