Fix validation of empty frame names

arrow/arrow.py

@@ -1855,7 +1855,7 @@ def _get_frames(cls, name: _T_FRAMES) -> Tuple[str, str, int]:
         """
         if name in cls._ATTRS:
             return name, f"{name}s", 1
-        elif name[-1] == "s" and name[:-1] in cls._ATTRS:
+        elif name.endswith("s") and name[:-1] in cls._ATTRS:
             return name[:-1], name, 1
         elif name in ["week", "weeks"]:
             return "week", "weeks", 1

tests/test_arrow.py

@@ -1084,11 +1084,12 @@ def test_imaginary(self):
         assert len(pacific_range) == len(set(pacific_range))
         assert len(utc_range) == len(set(utc_range))
 
-    def test_unsupported(self):
-        with pytest.raises(ValueError):
+    @pytest.mark.parametrize("frame", ["abc", ""])
+    def test_unsupported(self, frame):
+        with pytest.raises(ValueError, match="not supported. Supported frames:"):
             next(
                 arrow.Arrow.range(
-                    "abc", datetime.now(timezone.utc), datetime.now(timezone.utc)
+                    frame, datetime.now(timezone.utc), datetime.now(timezone.utc)
                 )
             )
 
@@ -1631,9 +1632,10 @@ def test_exact(self):
 
 @pytest.mark.usefixtures("time_2013_02_15")
 class TestArrowSpan:
-    def test_span_attribute(self):
-        with pytest.raises(ValueError):
-            self.arrow.span("span")
+    @pytest.mark.parametrize("frame", ["span", ""])
+    def test_span_attribute(self, frame):
+        with pytest.raises(ValueError, match="not supported. Supported frames:"):
+            self.arrow.span(frame)
 
     def test_span_year(self):
         floor, ceil = self.arrow.span("year")