Change media widgets to use memory views.

This can be more efficient with fewer copies in some cases, such as uploading a file and setting the value from the file content.

Author: jasongrout

ipywidgets/widgets/trait_types.py

@@ -146,6 +146,32 @@ def date_from_json(js, manager):
     'to_json': date_to_json
 }
 
+class ByteMemoryView(traitlets.TraitType):
+    """A trait for memory views of bytes."""
+
+    default_value = memoryview(b'')
+    info_text = 'a memory view object'
+
+    def validate(self, obj, value):
+        if isinstance(value, memoryview) and value.format == 'B':
+            return value
+        self.error(obj, value)
+
+
+class CByteMemoryView(ByteMemoryView):
+    """A casting version of the byte memory view trait."""
+
+    def validate(self, obj, value):
+        if isinstance(value, memoryview) and value.format == 'B':
+            return value
+
+        try:
+            mv = memoryview(value)
+            if mv.format != 'B':
+                mv = mv.cast('B')
+            return mv
+        except Exception:
+            self.error(obj, value)
 
 class InstanceDict(traitlets.Instance):
     """An instance trait which coerces a dict to an instance.

ipywidgets/widgets/widget_media.py

@@ -7,8 +7,8 @@
 from .domwidget import DOMWidget
 from .valuewidget import ValueWidget
 from .widget import register
-from traitlets import Unicode, CUnicode, Bytes, Bool
-from .trait_types import bytes_serialization
+from traitlets import Unicode, CUnicode, Bool
+from .trait_types import CByteMemoryView
 
 
 @register
@@ -23,7 +23,7 @@ class _Media(DOMWidget, ValueWidget, CoreWidget):
     """
 
     # Define the custom state properties to sync with the front-end
-    value = Bytes(help="The media data as a byte string.").tag(sync=True, **bytes_serialization)
+    value = CByteMemoryView(help="The media data as a memory view of bytes.").tag(sync=True)
 
     @classmethod
     def _from_file(cls, tag, filename, **kwargs):
@@ -116,12 +116,13 @@ def _get_repr(self, cls):
 
         # Return value first like a ValueWidget
         signature = []
-        sig_value = repr(self.value)
-        prefix, rest = sig_value.split("'", 1)
-        content = rest[:-1]
-        if len(content) > 100:
-            sig_value = "{}'{}...'".format(prefix, content[0:100])
-        signature.append('{}={}'.format('value', sig_value))
+
+        # strip off the starting b' and ending ' so we can truncate if needed
+        sig_value = repr(self.value[:100].tobytes())[2:-1]
+
+        if self.value.nbytes > 100:
+            sig_value += '...'
+        signature.append("{}=b'{}'".format('value', sig_value))
 
         for key in super(cls, self)._repr_keys():
             if key == 'value':