BUG: raise when wrong level name is passed to "unstack"

doc/source/whatsnew/v0.25.1.rst

@@ -128,7 +128,7 @@ Groupby/resample/rolling
 Reshaping
 ^^^^^^^^^
 
--
+- A ``KeyError`` is now raised if ``.unstack()`` is called on a :class:`Series` or :class:`DataFrame` with a flat :class:`Index` passing a name which is not the correct one (:issue:`18303`)
 -
 -
 

pandas/core/indexes/base.py

@@ -1524,7 +1524,11 @@ def _validate_index_level(self, level):
                     "Too many levels:" " Index has only 1 level, not %d" % (level + 1)
                 )
         elif level != self.name:
-            raise KeyError("Level %s must be same as name (%s)" % (level, self.name))
+            raise KeyError(
+                "Requested level ({}) does not match index name ({})".format(
+                    level, self.name
+                )
+            )
 
     def _get_level_number(self, level):
         self._validate_index_level(level)

pandas/core/reshape/reshape.py

@@ -12,6 +12,7 @@
     ensure_platform_int,
     is_bool_dtype,
     is_extension_array_dtype,
+    is_integer,
     is_integer_dtype,
     is_list_like,
     is_object_dtype,
@@ -402,6 +403,10 @@ def unstack(obj, level, fill_value=None):
         else:
             level = level[0]
 
+    # Prioritize integer interpretation (GH #21677):
+    if not is_integer(level) and not level == "__placeholder__":
+        level = obj.index._get_level_number(level)
+
     if isinstance(obj, DataFrame):
         if isinstance(obj.index, MultiIndex):
             return _unstack_frame(obj, level, fill_value=fill_value)

pandas/tests/frame/test_alter_axes.py

@@ -1083,7 +1083,7 @@ def test_reset_index_level(self):
 
         # Missing levels - for both MultiIndex and single-level Index:
         for idx_lev in ["A", "B"], ["A"]:
-            with pytest.raises(KeyError, match="Level E "):
+            with pytest.raises(KeyError, match=r"(L|l)evel \(?E\)?"):
                 df.set_index(idx_lev).reset_index(level=["A", "E"])
             with pytest.raises(IndexError, match="Too many levels"):
                 df.set_index(idx_lev).reset_index(level=[0, 1, 2])

pandas/tests/indexes/test_base.py

@@ -2004,7 +2004,7 @@ def test_isin_level_kwarg_bad_label_raises(self, label, indices):
             msg = "'Level {} not found'"
         else:
             index = index.rename("foo")
-            msg = r"'Level {} must be same as name \(foo\)'"
+            msg = r"Requested level \({}\) does not match index name \(foo\)"
         with pytest.raises(KeyError, match=msg.format(label)):
             index.isin([], level=label)
 

pandas/tests/indexes/test_common.py

@@ -35,7 +35,8 @@ def test_droplevel(self, indices):
 
         for level in "wrong", ["wrong"]:
             with pytest.raises(
-                KeyError, match=re.escape("'Level wrong must be same as name (None)'")
+                KeyError,
+                match=r"'Requested level \(wrong\) does not match index name \(None\)'",
             ):
                 indices.droplevel(level)
 
@@ -200,7 +201,7 @@ def test_unique(self, indices):
         with pytest.raises(IndexError, match=msg):
             indices.unique(level=3)
 
-        msg = r"Level wrong must be same as name \({}\)".format(
+        msg = r"Requested level \(wrong\) does not match index name \({}\)".format(
             re.escape(indices.name.__repr__())
         )
         with pytest.raises(KeyError, match=msg):

pandas/tests/series/test_alter_axes.py

@@ -319,9 +319,9 @@ def test_reset_index_drop_errors(self):
 
         # KeyError raised for series index when passed level name is missing
         s = Series(range(4))
-        with pytest.raises(KeyError, match="must be same as name"):
+        with pytest.raises(KeyError, match="does not match index name"):
             s.reset_index("wrong", drop=True)
-        with pytest.raises(KeyError, match="must be same as name"):
+        with pytest.raises(KeyError, match="does not match index name"):
             s.reset_index("wrong")
 
         # KeyError raised for series when level to be dropped is missing

pandas/tests/test_multilevel.py

@@ -524,6 +524,22 @@ def test_stack_unstack_preserve_names(self):
         restacked = unstacked.stack()
         assert restacked.index.names == self.frame.index.names
 
+    @pytest.mark.parametrize("method", ["stack", "unstack"])
+    def test_stack_unstack_wrong_level_name(self, method):
+        # GH 18303 - wrong level name should raise
+
+        # A DataFrame with flat axes:
+        df = self.frame.loc["foo"]
+
+        with pytest.raises(KeyError, match="does not match index name"):
+            getattr(df, method)("mistake")
+
+        if method == "unstack":
+            # Same on a Series:
+            s = df.iloc[:, 0]
+            with pytest.raises(KeyError, match="does not match index name"):
+                getattr(s, method)("mistake")
+
     def test_unstack_level_name(self):
         result = self.frame.unstack("second")
         expected = self.frame.unstack(level=1)