Template updates

 - Add 'none' template that applies no defaults.
 - Update plotly express to fall back to v3-style defaults when there
 are no template colorway/colorscales specified.
 - Update the plotly express Plotly colorway with new v4 theme ordering

Author: jonmmease

packages/python/plotly/plotly/colors/qualitative.py

@@ -18,12 +18,12 @@ def swatches():
     "#EF553B",
     "#00cc96",
     "#ab63fa",
+    "#FFA15A",
     "#19d3f3",
-    "#e763fa",
-    "#fecb52",
-    "#ffa15a",
-    "#ff6692",
-    "#b6e880",
+    "#FF6692",
+    "#B6E880",
+    "#FF97FF",
+    "#FECB52",
 ]
 
 D3 = [

packages/python/plotly/plotly/colors/sequential.py

@@ -105,6 +105,7 @@ def swatches():
     PuBuGn,
     PuRd,
     Purples,
+    RdBu,
     RdPu,
     Reds,
     YlGn,

packages/python/plotly/plotly/express/_core.py

@@ -659,19 +659,19 @@ def apply_default_cascade(args):
                 args["color_continuous_scale"] = [
                     x[1] for x in template.layout.colorscale.sequential
                 ]
-            except AttributeError:
+            except (AttributeError, TypeError):
                 pass
         if args["color_continuous_scale"] is None:
-            args["color_continuous_scale"] = sequential.Plasma
+            args["color_continuous_scale"] = sequential.Viridis
 
     if "color_discrete_sequence" in args:
         if args["color_discrete_sequence"] is None:
             try:
                 args["color_discrete_sequence"] = template.layout.colorway
-            except AttributeError:
+            except (AttributeError, TypeError):
                 pass
         if args["color_discrete_sequence"] is None:
-            args["color_discrete_sequence"] = qualitative.Plotly
+            args["color_discrete_sequence"] = qualitative.D3
 
     # If both marginals and faceting are specified, faceting wins
     if args.get("facet_col", None) and args.get("marginal_y", None):

packages/python/plotly/plotly/io/_templates.py

@@ -40,6 +40,7 @@ def __init__(self):
             "plotly_dark",
             "presentation",
             "xgridoff",
+            "none",
         ]
 
         for template_name in default_templates:
@@ -64,13 +65,17 @@ def __getitem__(self, item):
         if template is Lazy:
             from plotly.graph_objs.layout import Template
 
-            # Load template from package data
-            path = os.path.join("package_data", "templates", item + ".json")
-            template_str = pkgutil.get_data("plotly", path).decode("utf-8")
-            template_dict = json.loads(template_str)
-            template = Template(template_dict)
-
-            self._templates[item] = template
+            if item == "none":
+                # "none" is a special built-in named template that applied no defaults
+                self._templates[item] = Template()
+            else:
+                # Load template from package data
+                path = os.path.join("package_data", "templates", item + ".json")
+                template_str = pkgutil.get_data("plotly", path).decode("utf-8")
+                template_dict = json.loads(template_str)
+                template = Template(template_dict)
+
+                self._templates[item] = template
 
         return template