DOC: Improve documentation of XmlSerializable class, update fallback …

…return type

nibabel/xmlutils.py

@@ -15,17 +15,24 @@
 
 
 class XmlSerializable:
-    """Basic interface for serializing an object to xml"""
+    """Basic interface for serializing an object to XML"""
 
-    def _to_xml_element(self):
+    def _to_xml_element(self) -> Element:
         """Output should be a xml.etree.ElementTree.Element"""
-        raise NotImplementedError()
+        raise NotImplementedError
+
+    def to_xml(self, enc='utf-8', **kwargs) -> bytes:
+        r"""Generate an XML bytestring with a given encoding.
 
-    def to_xml(self, enc='utf-8', **kwargs):
-        """Output should be an xml string with the given encoding.
-        (default: utf-8)"""
+        Parameters
+        ----------
+        enc : :class:`string`
+            Encoding to use for the generated bytestring. Default: 'utf-8'
+        \*\*kwargs : :class:`dict`
+            Additional keyword arguments to :func:`xml.etree.ElementTree.tostring`.
+        """
         ele = self._to_xml_element()
-        return '' if ele is None else tostring(ele, enc, **kwargs)
+        return b'' if ele is None else tostring(ele, enc, **kwargs)
 
 
 class XmlBasedHeader(FileBasedHeader, XmlSerializable):