Fix direct mapping to sequences with one item

fixes #733

plotnine/guides/guide_legend.py

@@ -91,7 +91,12 @@ def train(self, scale, aesthetic=None):
         # create a hash of the important information in the guide
         labels = " ".join(str(x) for x in self.key["label"])
         info = "\n".join(
-            [self.title, labels, str(self.direction), self.__class__.__name__]
+            [
+                str(self.title),
+                labels,
+                str(self.direction),
+                self.__class__.__name__,
+            ]
         )
         self.hash = hashlib.sha256(info.encode("utf-8")).hexdigest()
         return self

plotnine/mapping/aes.py

@@ -2,7 +2,7 @@
 
 import re
 import typing
-from collections.abc import Iterable
+from collections.abc import Iterable, Sequence
 from contextlib import suppress
 from copy import deepcopy
 from dataclasses import fields
@@ -14,7 +14,7 @@
 from .evaluation import after_stat, stage
 
 if typing.TYPE_CHECKING:
-    from typing import Sequence, TypeVar
+    from typing import TypeVar
 
     THasAesNames = TypeVar("THasAesNames", bound=list[str] | dict[str, Any])
 
@@ -503,10 +503,14 @@ def make_labels(mapping: dict[str, Any] | aes) -> labels_view:
     """
 
     def _nice_label(value: Any) -> str | None:
-        if isinstance(value, pd.Series):
+        if isinstance(value, str):
+            return value
+        elif isinstance(value, pd.Series):
             return value.name  # pyright: ignore
-        elif not isinstance(value, Iterable) or isinstance(value, str):
+        elif not isinstance(value, Iterable):
             return str(value)
+        elif isinstance(value, Sequence) and len(value) == 1:
+            return str(value[0])
         else:
             return None
 
@@ -519,7 +523,6 @@ def _make_label(ae: str, value: Any) -> str | None:
             elif value.after_scale is not None:
                 return value.after_scale
             else:
-                # return ''
                 raise ValueError("Unknown mapping")
         else:
             if value.after_stat is not None:

tests/test_aes.py

@@ -15,6 +15,7 @@
     stat_ecdf,
     stat_function,
 )
+from plotnine.mapping.aes import make_labels
 
 data = pd.DataFrame(
     {
@@ -93,3 +94,13 @@ def test_geom_crossbar(self):
             + scale_y_log10()
         )
         assert p == "geom_crossbar"
+
+
+def test_make_labels():
+    mapping = {"y": "y", "color": ["Treatment"]}
+    labels = make_labels(mapping)
+    assert labels.color == "Treatment"
+
+    mapping = {"y": "y", "color": ["Treatment", "Control"]}
+    labels = make_labels(mapping)
+    assert labels.color is None