class StringFunction_v3:
    def __init__(self, expression, independent_variable='x',
                 set_parameters=''):
        self._f_compiled = compile(expression, 
                                   '<string>', 'eval')
        self._var = independent_variable  # 'x', 't' etc.
        self._code = set_parameters

    def set_parameters(self, code):
        self._code = code

    def __call__(self, x):
        exec '%s = %g' % (self._var, x)  # assign indep. var.
        if self._code:  exec(self._code) # parameters?
        return eval(self._f_compiled)