def _padding_to_conv_op_padding(padding):
  """Whether to use SAME or VALID for the underlying convolution op.

  Args:
    padding: A tuple of members of ALLOWED_PADDINGS, e.g. as returned from
      `_fill_and_verify_padding`.

  Returns:
    One of CONV_OP_ALLOWED_PADDINGS, the padding method to use for the
    underlying convolution op.

  Raises:
    ValueError: If padding is not a tuple.
  """
  if not isinstance(padding, tuple):
    raise ValueError("padding should be a tuple.")
  if all(p == SAME for p in padding):
    # If we want SAME padding for all dimensions then we can use SAME for the
    # conv and avoid doing any extra padding.
    return SAME
  else:
    # Otherwise we prefer to use VALID, since we can implement all the other
    # padding types just by adding some extra padding before doing a VALID conv.
    # (We could use SAME but then we'd also have to crop outputs in some cases).
    return VALID