<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def mask(mask_key, data):
 """ mask or unmask data. Just do xor for each byte mask_key: 4 byte string(byte). data: data to mask/unmask. """

        if data is None:
            data = ""

        if isinstance(mask_key, six.text_type):
            mask_key = six.b(mask_key)

        if isinstance(data, six.text_type):
            data = six.b(data)

        if numpy:
            origlen = len(data)
            _mask_key = mask_key[3] << 24 | mask_key[2] << 16 | mask_key[1] << 8 | mask_key[0]

            # We need data to be a multiple of four...
            data += bytes(" " * (4 - (len(data) % 4)), "us-ascii")
            a = numpy.frombuffer(data, dtype="uint32")
            masked = numpy.bitwise_xor(a, [_mask_key]).astype("uint32")
            if len(data) > origlen:
              return masked.tobytes()[:origlen]
            return masked.tobytes()
        else:
            _m = array.array("B", mask_key)
            _d = array.array("B", data)
            return _mask(_m, _d)