BUG: GroupBy.cummin altering nullable arrays inplace

doc/source/whatsnew/v1.5.0.rst

@@ -429,8 +429,9 @@ Groupby/resample/rolling
 - Bug in :meth:`.ExponentialMovingWindow.mean` with ``axis=1`` and ``engine='numba'`` when the :class:`DataFrame` has more columns than rows (:issue:`46086`)
 - Bug when using ``engine="numba"`` would return the same jitted function when modifying ``engine_kwargs`` (:issue:`46086`)
 - Bug in :meth:`.DataFrameGroupby.transform` fails when ``axis=1`` and ``func`` is ``"first"`` or ``"last"`` (:issue:`45986`)
-- Bug in :meth:`DataFrameGroupby.cumsum` with ``skipna=False`` giving incorrect results (:issue:`??`)
-- Bug in :meth:`.GroupBy.cumsum` with ``timedelta64[ns]`` dtype failing to recognize ``NaT`` as a null value (:issue:`??`)
+- Bug in :meth:`DataFrameGroupby.cumsum` with ``skipna=False`` giving incorrect results (:issue:`46216`)
+- Bug in :meth:`.GroupBy.cumsum` with ``timedelta64[ns]`` dtype failing to recognize ``NaT`` as a null value (:issue:`46216`)
+- Bug in :meth:`GroupBy.cummin` and :meth:`GroupBy.cummax` with nullable dtypes incorrectly altering the original data in place (:issue:`46220`)
 -
 
 Reshaping

pandas/_libs/groupby.pyx

@@ -1477,7 +1477,8 @@ def group_min(
 cdef group_cummin_max(
     iu_64_floating_t[:, ::1] out,
     ndarray[iu_64_floating_t, ndim=2] values,
-    uint8_t[:, ::1] mask,
+    const uint8_t[:, ::1] mask,
+    uint8_t[:, ::1] result_mask,
     const intp_t[::1] labels,
     int ngroups,
     bint is_datetimelike,
@@ -1496,6 +1497,9 @@ cdef group_cummin_max(
     mask : np.ndarray[bool] or None
         If not None, indices represent missing values,
         otherwise the mask will not be used
+    result_mask : ndarray[bool, ndim=2], optional
+        If not None, these specify locations in the output that are NA.
+        Modified in-place.
     labels : np.ndarray[np.intp]
         Labels to group by.
     ngroups : int
@@ -1558,7 +1562,7 @@ cdef group_cummin_max(
 
                 if not skipna and na_possible and seen_na[lab, j]:
                     if uses_mask:
-                        mask[i, j] = 1   # FIXME: shouldn't alter inplace
+                        result_mask[i, j] = 1
                         # Set to 0 ensures that we are deterministic and can
                         #  downcast if appropriate
                         out[i, j] = 0
@@ -1595,19 +1599,21 @@ def group_cummin(
     const intp_t[::1] labels,
     int ngroups,
     bint is_datetimelike,
-    uint8_t[:, ::1] mask=None,
+    const uint8_t[:, ::1] mask=None,
+    uint8_t[:, ::1] result_mask=None,
     bint skipna=True,
 ) -> None:
     """See group_cummin_max.__doc__"""
     group_cummin_max(
-        out,
-        values,
-        mask,
-        labels,
-        ngroups,
-        is_datetimelike,
-        skipna,
-        compute_max=False
+        out=out,
+        values=values,
+        mask=mask,
+        result_mask=result_mask,
+        labels=labels,
+        ngroups=ngroups,
+        is_datetimelike=is_datetimelike,
+        skipna=skipna,
+        compute_max=False,
     )
 
 
@@ -1619,17 +1625,19 @@ def group_cummax(
     const intp_t[::1] labels,
     int ngroups,
     bint is_datetimelike,
-    uint8_t[:, ::1] mask=None,
+    const uint8_t[:, ::1] mask=None,
+    uint8_t[:, ::1] result_mask=None,
     bint skipna=True,
 ) -> None:
     """See group_cummin_max.__doc__"""
     group_cummin_max(
-        out,
-        values,
-        mask,
-        labels,
-        ngroups,
-        is_datetimelike,
-        skipna,
-        compute_max=True
+        out=out,
+        values=values,
+        mask=mask,
+        result_mask=result_mask,
+        labels=labels,
+        ngroups=ngroups,
+        is_datetimelike=is_datetimelike,
+        skipna=skipna,
+        compute_max=True,
     )

pandas/core/groupby/ops.py

@@ -418,9 +418,13 @@ def _masked_ea_wrap_cython_operation(
         """
         orig_values = values
 
-        # Copy to ensure input and result masks don't end up shared
-        mask = values._mask.copy()
-        result_mask = np.zeros(ngroups, dtype=bool)
+        # libgroupby functions are responsible for NOT altering mask
+        mask = values._mask
+        if self.kind != "aggregate":
+            result_mask = mask.copy()
+        else:
+            result_mask = np.zeros(ngroups, dtype=bool)
+
         arr = values._data
 
         res_values = self._cython_op_ndim_compat(
@@ -438,12 +442,7 @@ def _masked_ea_wrap_cython_operation(
         #  dtype; last attempt ran into trouble on 32bit linux build
         res_values = res_values.astype(dtype.type, copy=False)
 
-        if self.kind != "aggregate":
-            out_mask = mask
-        else:
-            out_mask = result_mask
-
-        return orig_values._maybe_mask_result(res_values, out_mask)
+        return orig_values._maybe_mask_result(res_values, result_mask)
 
     @final
     def _cython_op_ndim_compat(
@@ -579,6 +578,7 @@ def _call_cython_op(
                     ngroups=ngroups,
                     is_datetimelike=is_datetimelike,
                     mask=mask,
+                    result_mask=result_mask,
                     **kwargs,
                 )
             else:

pandas/tests/groupby/test_function.py

@@ -843,11 +843,15 @@ def test_cummax(dtypes_for_minmax):
 def test_cummin_max_skipna(method, dtype, groups, expected_data):
     # GH-34047
     df = DataFrame({"a": Series([1, None, 2], dtype=dtype)})
+    orig = df.copy()
     gb = df.groupby(groups)["a"]
 
     result = getattr(gb, method)(skipna=False)
     expected = Series(expected_data, dtype=dtype, name="a")
 
+    # check we didn't accidentally alter df
+    tm.assert_frame_equal(df, orig)
+
     tm.assert_series_equal(result, expected)