Fix update method on empty and uninitialized array property (#1092)

* Fix for GH1072. Empty array properties are replaced during update

* Don't error when a literal property is set to it's correct value.

This avoids errors when performing updates like figure.update(data=[go.Scatter(...), ...])

* Fix two failing validator tests

Author: jonmmease

_plotly_utils/basevalidators.py

@@ -1658,16 +1658,21 @@ class LiteralValidator(BaseValidator):
     """
     Validator for readonly literal values
     """
-    def __init__(self, plotly_name, parent_name, **kwargs):
-        super(LiteralValidator, self).__init__(plotly_name=plotly_name,
-                         parent_name=parent_name,
-                         **kwargs)
+    def __init__(self, plotly_name, parent_name, val, **kwargs):
+        super(LiteralValidator, self).__init__(
+            plotly_name=plotly_name,
+            parent_name=parent_name,
+            **kwargs)
+        self.val = val
 
     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
-        ))
+        if v != self.val:
+            raise ValueError("""\
+    The '{plotly_name}' property of {parent_name} is read-only""".format(
+                plotly_name=self.plotly_name, parent_name=self.parent_name
+            ))
+        else:
+            return v
 
 
 class ImageUriValidator(BaseValidator):

_plotly_utils/tests/validators/test_basetraces_validator.py

@@ -10,7 +10,8 @@ def validator():
                                         'bar': 'Bar',
                                         'box': 'Box'},
                              plotly_name='prop',
-                             parent_name='parent')
+                             parent_name='parent',
+                             set_uid=True)
 
 
 # Tests