Added validator for literal properties (e.g. the 'type' prop of traces)

This fixes issue where scatter['type'] and 'type' in scatter would fail

Author: Jon M. Mease

_plotly_utils/basevalidators.py

@@ -1553,6 +1553,22 @@ def present(self, v):
         return tuple(v)
 
 
+class LiteralValidator(BaseValidator):
+    """
+    Validator for readonly literal values
+    """
+    def __init__(self, plotly_name, parent_name, **kwargs):
+        super().__init__(plotly_name=plotly_name,
+                         parent_name=parent_name,
+                         **kwargs)
+
+    def validate_coerce(self, v):
+        raise ValueError("""\
+The '{plotly_name}' property of {parent_name} is read-only""".format(
+            plotly_name=self.plotly_name, parent_name=self.parent_name
+        ))
+
+
 class ImageUriValidator(BaseValidator):
     _PIL = None
 

codegen/datatypes.py

@@ -214,18 +214,20 @@ def __init__(self""")
         self.{subtype_node.name_property} = {subtype_node.name_property}""")
 
     # ### Literals ###
-    literal_nodes = [n for n in node.child_literals if
-                     n.plotly_name in ['type']]
     if literal_nodes:
         buffer.write(f"""
 
         # Read-only literals
-        # ------------------""")
+        # ------------------
+        from _plotly_utils.basevalidators import LiteralValidator""")
         for literal_node in literal_nodes:
             lit_name = literal_node.name_property
+            lit_parent = literal_node.parent_path_str
             lit_val = literal_node.node_data
             buffer.write(f"""
-        self._props['{lit_name}'] = '{lit_val}'""")
+        self._props['{lit_name}'] = '{lit_val}'
+        self._validators['{lit_name}'] =\
+LiteralValidator(plotly_name='{lit_name}', parent_name='{lit_parent}')""")
 
     buffer.write(f"""