reorder computation to avoid an extra scan through the data

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

@@ -1904,7 +1904,7 @@ def infer_config(args, constructor, trace_patch, layout_patch):
     return trace_specs, grouped_mappings, sizeref, show_colorbar
 
 
-def get_orderings(args, grouper, grouped, all_same_group):
+def get_orderings(args, grouper):
     """
     `orders` is the user-supplied ordering with the remaining data-frame-supplied
     ordering appended if the column is used for grouping. It includes anything the user
@@ -1916,47 +1916,42 @@ def get_orderings(args, grouper, grouped, all_same_group):
     of a single dimension-group
     """
     orders = {} if "category_orders" not in args else args["category_orders"].copy()
-    sorted_group_names = []
-
-    if all_same_group:
-        for col in grouper:
-            if col != one_group:
-                single_val = args["data_frame"][col].iloc[0]
-                sorted_group_names.append(single_val)
-                orders[col] = [single_val]
-            else:
-                sorted_group_names.append("")
-        return orders, [tuple(sorted_group_names)]
 
+    # figure out orders and what the single group name would be if there were one
+    single_group_name = []
     for col in grouper:
-        if col != one_group:
+        if col == one_group:
+            single_group_name.append("")
+        else:
             uniques = list(args["data_frame"][col].unique())
+            if len(uniques) == 1:
+                single_group_name.append(uniques[0])
             if col not in orders:
                 orders[col] = uniques
             else:
                 orders[col] = list(OrderedDict.fromkeys(list(orders[col]) + uniques))
 
-    for group_name in grouped.groups:
-        if len(grouper) == 1:
-            group_name = (group_name,)
-        sorted_group_names.append(group_name)
-
-    for i, col in reversed(list(enumerate(grouper))):
-        if col != one_group:
-            sorted_group_names = sorted(
-                sorted_group_names,
-                key=lambda g: orders[col].index(g[i]) if g[i] in orders[col] else -1,
-            )
-    return orders, sorted_group_names
-
+    if len(single_group_name) == len(grouper):
+        # we have a single group, so we can skip all group-by operations!
+        grouped = None
+        sorted_group_names = [tuple(single_group_name)]
+    else:
+        grouped = args["data_frame"].groupby(grouper, sort=False)
+        sorted_group_names = []
+        for group_name in grouped.groups:
+            if len(grouper) == 1:
+                group_name = (group_name,)
+            sorted_group_names.append(group_name)
 
-def _all_same_group(args, grouper):
-    for g in set(grouper):
-        if g != one_group:
-            arr = args["data_frame"][g].values
-            if not (arr[0] == arr).all(axis=0):
-                return False
-    return True
+        for i, col in reversed(list(enumerate(grouper))):
+            if col != one_group:
+                sorted_group_names = sorted(
+                    sorted_group_names,
+                    key=lambda g: orders[col].index(g[i])
+                    if g[i] in orders[col]
+                    else -1,
+                )
+    return grouped, orders, sorted_group_names
 
 
 def make_figure(args, constructor, trace_patch=None, layout_patch=None):
@@ -1975,12 +1970,7 @@ def make_figure(args, constructor, trace_patch=None, layout_patch=None):
         args, constructor, trace_patch, layout_patch
     )
     grouper = [x.grouper or one_group for x in grouped_mappings] or [one_group]
-    grouped = None
-    all_same_group = _all_same_group(args, grouper)
-    if not all_same_group:
-        grouped = args["data_frame"].groupby(grouper, sort=False)
-
-    orders, sorted_group_names = get_orderings(args, grouper, grouped, all_same_group)
+    grouped, orders, sorted_group_names = get_orderings(args, grouper)
 
     col_labels = []
     row_labels = []