Implement few utility methods for metadata classes.

bob/io/stream/metadata.py

@@ -78,6 +78,8 @@ class MetaDatum:
             # type using the numpy item.
             elif isinstance(value, np.generic):
                 self.set(value.item())
+            elif value is None or self.value is None:
+                pass  # allow to set to None whichever the type of the Metadatum is.
             else:
                 raise TypeError(
                     "Metadatum "
@@ -212,6 +214,16 @@ class MetaData:
         """
         return self.__dict.__contains__(item)
 
+    def __bool__(self):
+        """Return True if the metadata collection is empty.
+
+        Returns
+        -------
+        bool
+            True if there are no metadatum in the metadata, False otherwise.
+        """
+        return bool(self.__dict)
+
     def keys(self):
         """Return an iterable on the collection's metadatum names."""
         return self.__dict.keys()
@@ -246,7 +258,7 @@ class MetaDataInfo:
 
     .. note::
         :class:`bob.io.stream.Stream` uses the property names `"herited"` and `"per_frame"` to determine how to access
-        the metadata.
+        the metadata. `"no_write"` and `"must_write"` are also used to override the default metadata writing behavior.
     """
 
     def __init__(self, info_dict=None):