Implements no_write and must_write for metadata writing

bob/io/stream/stream.py

@@ -116,8 +116,8 @@ class Stream:
         self.reset()
 
         # initliaze metadata in set_metadata
-        self._metadata = None
-        self._metadata_info = None
+        self._metadata = MetaData()
+        self._metadata_info = MetaDataInfo()
         self.set_metadata(metadata, metadata_info)
 
     def reset(self):
@@ -235,7 +235,7 @@ class Stream:
         if key is None:
             return self._metadata
 
-        if self._metadata is None:
+        if not self._metadata:
             raise ValueError("Metadata are not set!")
 
         # metadata exists
@@ -265,6 +265,9 @@ class Stream:
     def write_metadata(self):
         """Write this stream global (not per-frame) and not herited metadata to the output streamfile.
 
+        Special entries in the metadata_info as "must_write" and "no_write" change the behavior of this function
+        to always write, respectively never write, a metadatum, independently of the "herited" or "per_frame" values.
+
         .. note::
             The per-frame metadata should be written to file use the :meth:`~bob.io.stream.Stream.put` method.
 
@@ -277,13 +280,14 @@ class Stream:
         if self.sink is None:
             raise ValueError("This stream is not part of a chain that ends writing in a streamfile.")
 
-        # breakpoint()
-        if self._metadata is not None:
+        if self._metadata:
             for metadatum in self._metadata.values():
                 # if not herited and not per-frame
-                if not self._metadata_info.has_info_value(
-                    "herited", True, metadatum.name
-                ) and not self._metadata_info.has_info_value("per_frame", True, metadatum.name):
+                if (
+                    not self._metadata_info.has_info_value("herited", True, metadatum.name)
+                    and not self._metadata_info.has_info_value("per_frame", True, metadatum.name)
+                    and not self._metadata_info.has_info_value("no_write", True, metadatum.name)
+                ) or self._metadata_info.has_info_value("must_write", True, metadatum.name):
                     self.sink.write_stream_metadata(self.name, metadatum)
 
         if self.child is not None: