Serializes given triplets of python and wire values and a descriptor.

def _SerializeEntries(entries):
  """Serializes given triplets of python and wire values and a descriptor."""

  output = []
  for python_format, wire_format, type_descriptor in entries:

    if wire_format is None or (python_format and
                               type_descriptor.IsDirty(python_format)):
      wire_format = type_descriptor.ConvertToWireFormat(python_format)

    precondition.AssertIterableType(wire_format, bytes)
    output.extend(wire_format)

  return b"".join(output)