def set_condition(self, conkey, val):
        """Set condition conkey to value val. Convert val to str if not
        None.

        conkey: str
            A valid condition key.

        val: str, int, float, None
            Can always be None. Can be number or string depending on conkey.
        """

        if not any([conkey.startswith(c) for c in _COND_PREFIXES]):
            raise KeyError(conkey)

        if val in NONES:
            self.conditions[conkey] = None
        else:
            self.conditions[conkey] = str(val)
Set condition conkey to value val. Convert val to str if not
        None.

        conkey: str
            A valid condition key.

        val: str, int, float, None
            Can always be None. Can be number or string depending on conkey.