'Parameters
str_func : string
String to be parsed.'

def __init__(self, str_func):

    if (not isinstance(str_func, six.string_types)):
        raise ValueError((u"'%s'    must    be    a    string." % str_func))
    self._str_func = six.text_type(str_func)
    (self._key, self._params) = self._get_key_params()
    self._func = self._parse_func()
