DEPR: Groupby obj, ax, indexer

Author: jbrockmendel

Diff for: doc/source/whatsnew/v2.0.0.rst

@@ -783,6 +783,9 @@ Deprecations
 - Deprecated calling ``float`` or ``int`` on a single element :class:`Series` to return a ``float`` or ``int`` respectively. Extract the element before calling ``float`` or ``int`` instead (:issue:`51101`)
 - Deprecated :meth:`Grouper.groups`, use :meth:`Groupby.groups` instead (:issue:`51182`)
 - Deprecated :meth:`Grouper.grouper`, use :meth:`Groupby.grouper` instead (:issue:`51182`)
+- Deprecated :meth:`Grouper.obj`, use :meth:`Groupby.obj` instead (:issue:`??`)
+- Deprecated :meth:`Grouper.indexer`, use :meth:`Resampler.indexer` instead (:issue:`??`)
+- Deprecated :meth:`Grouper.ax`, use :meth:`Resampler.ax` instead (:issue:`??`)
 -
 
 .. ---------------------------------------------------------------------------

Diff for: pandas/core/groupby/grouper.py

@@ -270,21 +270,13 @@ def __init__(
         self.dropna = dropna
 
         self._grouper_deprecated = None
+        self._indexer_deprecated = None
+        self._obj_deprecated = None
         self._gpr_index = None
-        self.obj = None
-        self.indexer = None
         self.binner = None
         self._grouper = None
         self._indexer = None
 
-    @final
-    @property
-    def ax(self) -> Index:
-        index = self._gpr_index
-        if index is None:
-            raise ValueError("_set_grouper must be called before ax is accessed")
-        return index
-
     def _get_grouper(
         self, obj: NDFrameT, validate: bool = True
     ) -> tuple[ops.BaseGrouper, NDFrameT]:
@@ -299,7 +291,7 @@ def _get_grouper(
         -------
         a tuple of grouper, obj (possibly sorted)
         """
-        obj, _ = self._set_grouper(obj)
+        obj, _, _ = self._set_grouper(obj)
         grouper, _, obj = get_grouper(
             obj,
             [self.key],
@@ -335,17 +327,20 @@ def _set_grouper(
         -------
         NDFrame
         Index
+        np.ndarray[np.intp] | None
         """
         assert obj is not None
 
+        indexer = None
+
         if self.key is not None and self.level is not None:
             raise ValueError("The Grouper cannot specify both a key and a level!")
 
         # Keep self._grouper value before overriding
         if self._grouper is None:
             # TODO: What are we assuming about subsequent calls?
             self._grouper = gpr_index
-            self._indexer = self.indexer
+            self._indexer = self._indexer_deprecated
 
         # the key must be a valid info item
         if self.key is not None:
@@ -387,17 +382,53 @@ def _set_grouper(
         if (self.sort or sort) and not ax.is_monotonic_increasing:
             # use stable sort to support first, last, nth
             # TODO: why does putting na_position="first" fix datetimelike cases?
-            indexer = self.indexer = ax.array.argsort(
+            indexer = self._indexer_deprecated = ax.array.argsort(
                 kind="mergesort", na_position="first"
             )
             ax = ax.take(indexer)
             obj = obj.take(indexer, axis=self.axis)
 
         # error: Incompatible types in assignment (expression has type
         # "NDFrameT", variable has type "None")
-        self.obj = obj  # type: ignore[assignment]
+        self._obj_deprecated = obj  # type: ignore[assignment]
         self._gpr_index = ax
-        return obj, ax
+        return obj, ax, indexer
+
+    @final
+    @property
+    def ax(self) -> Index:
+        warnings.warn(
+            f"{type(self).__name__}.ax is deprecated and will be removed in a "
+            "future version. Use Resampler.ax instead",
+            FutureWarning,
+            stacklevel=find_stack_level(),
+        )
+        index = self._gpr_index
+        if index is None:
+            raise ValueError("_set_grouper must be called before ax is accessed")
+        return index
+
+    @final
+    @property
+    def indexer(self):
+        warnings.warn(
+            f"{type(self).__name__}.indexer is deprecated and will be removed "
+            "in a future version. Use Resampler.indexer instead.",
+            FutureWarning,
+            stacklevel=find_stack_level(),
+        )
+        return self._indexer_deprecated
+
+    @final
+    @property
+    def obj(self):
+        warnings.warn(
+            f"{type(self).__name__}.obj is deprecated and will be removed "
+            "in a future version. Use GroupBy.indexer instead.",
+            FutureWarning,
+            stacklevel=find_stack_level(),
+        )
+        return self._obj_deprecated
 
     @final
     @property

Diff for: pandas/core/resample.py

@@ -133,7 +133,7 @@ class Resampler(BaseGroupBy, PandasObject):
     _timegrouper: TimeGrouper
     binner: DatetimeIndex | TimedeltaIndex | PeriodIndex  # depends on subclass
     exclusions: frozenset[Hashable] = frozenset()  # for SelectionMixin compat
-    _internal_names_set = set({"obj", "ax"})
+    _internal_names_set = set({"obj", "ax", "_indexer"})
 
     # to the groupby descriptor
     _attributes = [
@@ -168,7 +168,7 @@ def __init__(
         self.group_keys = group_keys
         self.as_index = True
 
-        self.obj, self.ax = self._timegrouper._set_grouper(
+        self.obj, self.ax, self._indexer = self._timegrouper._set_grouper(
             self._convert_obj(obj), sort=True, gpr_index=gpr_index
         )
         self.binner, self.grouper = self._get_binner()
@@ -235,7 +235,7 @@ def _get_binner(self):
         """
         binner, bins, binlabels = self._get_binner_for_time()
         assert len(bins) == len(binlabels)
-        bin_grouper = BinGrouper(bins, binlabels, indexer=self._timegrouper.indexer)
+        bin_grouper = BinGrouper(bins, binlabels, indexer=self._indexer)
         return binner, bin_grouper
 
     @Substitution(
@@ -1681,7 +1681,7 @@ def _get_resampler(self, obj: NDFrame, kind=None) -> Resampler:
         TypeError if incompatible axis
 
         """
-        _, ax = self._set_grouper(obj, gpr_index=None)
+        _, ax, indexer = self._set_grouper(obj, gpr_index=None)
 
         if isinstance(ax, DatetimeIndex):
             return DatetimeIndexResampler(

Diff for: pandas/tests/groupby/test_grouping.py

@@ -1066,3 +1066,16 @@ def test_grouper_groups():
     with tm.assert_produces_warning(FutureWarning, match=msg):
         res = grper.grouper
     assert res is gb.grouper
+
+    msg = "Grouper.obj is deprecated and will be removed"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        res = grper.obj
+    assert res is gb.obj
+
+    msg = "Use Resampler.ax instead"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        grper.ax
+
+    msg = "Grouper.indexer is deprecated"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        grper.indexer