'Check whether the field type is a fixed set lookup (IS_IN_SET)
@return: True if field type is a fixed set lookup, else False'

@property
def is_lookup(self):

    is_lookup = self._is_lookup
    if (is_lookup is None):
        is_lookup = False
        ftype = self.ftype
        field = self.field
        if field:
            requires = field.requires
            if requires:
                if (not isinstance(requires, (list, tuple))):
                    requires = [requires]
                requires = requires[0]
                if isinstance(requires, IS_EMPTY_OR):
                    requires = requires.other
                if isinstance(requires, IS_IN_SET):
                    is_lookup = True
            if (is_lookup and requires and (self.ftype == 'integer')):
                options = requires.options(zero=False)
                if all(((k == v) for (k, v) in options)):
                    is_lookup = False
        self._is_lookup = is_lookup
    return is_lookup
