REF: Share Block/ArrayManager set_axis (#41348)

Author: jbrockmendel

pandas/core/groupby/generic.py

@@ -1625,14 +1625,14 @@ def _wrap_transformed_output(
     def _wrap_agged_manager(self, mgr: Manager2D) -> DataFrame:
         if not self.as_index:
             index = np.arange(mgr.shape[1])
-            mgr.set_axis(1, ibase.Index(index), verify_integrity=False)
+            mgr.set_axis(1, ibase.Index(index))
             result = self.obj._constructor(mgr)
 
             self._insert_inaxis_grouper_inplace(result)
             result = result._consolidate()
         else:
             index = self.grouper.result_index
-            mgr.set_axis(1, index, verify_integrity=False)
+            mgr.set_axis(1, index)
             result = self.obj._constructor(mgr)
 
         if self.axis == 1:

pandas/core/internals/array_manager.py

@@ -160,21 +160,10 @@ def _normalize_axis(axis: int) -> int:
         axis = 1 if axis == 0 else 0
         return axis
 
-    def set_axis(
-        self, axis: int, new_labels: Index, verify_integrity: bool = True
-    ) -> None:
+    def set_axis(self, axis: int, new_labels: Index) -> None:
         # Caller is responsible for ensuring we have an Index object.
+        self._validate_set_axis(axis, new_labels)
         axis = self._normalize_axis(axis)
-        if verify_integrity:
-            old_len = len(self._axes[axis])
-            new_len = len(new_labels)
-
-            if new_len != old_len:
-                raise ValueError(
-                    f"Length mismatch: Expected axis has {old_len} elements, new "
-                    f"values have {new_len} elements"
-                )
-
         self._axes[axis] = new_labels
 
     def consolidate(self: T) -> T:

pandas/core/internals/base.py

@@ -44,6 +44,23 @@ def ndim(self) -> int:
     def shape(self) -> Shape:
         return tuple(len(ax) for ax in self.axes)
 
+    @final
+    def _validate_set_axis(self, axis: int, new_labels: Index) -> None:
+        # Caller is responsible for ensuring we have an Index object.
+        old_len = len(self.axes[axis])
+        new_len = len(new_labels)
+
+        if axis == 1 and len(self.items) == 0:
+            # If we are setting the index on a DataFrame with no columns,
+            #  it is OK to change the length.
+            pass
+
+        elif new_len != old_len:
+            raise ValueError(
+                f"Length mismatch: Expected axis has {old_len} elements, new "
+                f"values have {new_len} elements"
+            )
+
     def reindex_indexer(
         self: T,
         new_axis,

pandas/core/internals/managers.py

@@ -211,20 +211,9 @@ def _normalize_axis(self, axis: int) -> int:
             axis = 1 if axis == 0 else 0
         return axis
 
-    def set_axis(
-        self, axis: int, new_labels: Index, verify_integrity: bool = True
-    ) -> None:
+    def set_axis(self, axis: int, new_labels: Index) -> None:
         # Caller is responsible for ensuring we have an Index object.
-        if verify_integrity:
-            old_len = len(self.axes[axis])
-            new_len = len(new_labels)
-
-            if new_len != old_len:
-                raise ValueError(
-                    f"Length mismatch: Expected axis has {old_len} elements, new "
-                    f"values have {new_len} elements"
-                )
-
+        self._validate_set_axis(axis, new_labels)
         self.axes[axis] = new_labels
 
     @property