PERF: ffill/bfill with non-numpy dtypes

doc/source/whatsnew/v2.1.0.rst

@@ -342,6 +342,7 @@ Performance improvements
 - Performance improvement in :meth:`DataFrame.loc` when selecting rows and columns (:issue:`53014`)
 - Performance improvement in :meth:`Series.add` for pyarrow string and binary dtypes (:issue:`53150`)
 - Performance improvement in :meth:`Series.corr` and :meth:`Series.cov` for extension dtypes (:issue:`52502`)
+- Performance improvement in :meth:`Series.ffill`, :meth:`Series.bfill`, :meth:`DataFrame.ffill`, :meth:`DataFrame.bfill` with pyarrow dtypes (:issue:`53950`)
 - Performance improvement in :meth:`Series.str.get_dummies` for pyarrow-backed strings (:issue:`53655`)
 - Performance improvement in :meth:`Series.str.get` for pyarrow-backed strings (:issue:`53152`)
 - Performance improvement in :meth:`Series.str.split` with ``expand=True`` for pyarrow-backed strings (:issue:`53585`)

pandas/_libs/algos.pyi

@@ -60,6 +60,10 @@ def nancorr_spearman(
 # ----------------------------------------------------------------------
 
 def validate_limit(nobs: int | None, limit=...) -> int: ...
+def get_fill_indexer(
+    mask: npt.NDArray[np.bool_],
+    limit: int | None = None,
+) -> npt.NDArray[np.intp]: ...
 def pad(
     old: np.ndarray,  # ndarray[numeric_object_t]
     new: np.ndarray,  # ndarray[numeric_object_t]

pandas/_libs/algos.pyx

@@ -525,6 +525,42 @@ def validate_limit(nobs: int | None, limit=None) -> int:
     return lim
 
 
+# TODO: overlap with libgroupby.group_fillna_indexer?
+@cython.boundscheck(False)
+@cython.wraparound(False)
+def get_fill_indexer(const uint8_t[:] mask, limit=None):
+    """
+    Find an indexer to use for ffill to `take` on the array being filled.
+    """
+    cdef:
+        ndarray[intp_t, ndim=1] indexer
+        Py_ssize_t i, N = len(mask), last_valid
+        int lim
+
+        # fill_count is the number of consecutive NAs we have seen.
+        #  If it exceeds the given limit, we stop padding.
+        int fill_count = 0
+
+    lim = validate_limit(N, limit)
+    indexer = np.empty(N, dtype=np.intp)
+
+    last_valid = -1  # haven't yet seen anything non-NA
+
+    for i in range(N):
+        if not mask[i]:
+            indexer[i] = i
+            last_valid = i
+            fill_count = 0
+        else:
+            if fill_count < lim:
+                indexer[i] = last_valid
+            else:
+                indexer[i] = -1
+            fill_count += 1
+
+    return indexer
+
+
 @cython.boundscheck(False)
 @cython.wraparound(False)
 def pad(

pandas/core/arrays/arrow/array.py

@@ -67,8 +67,6 @@
 
     from pandas.core.dtypes.dtypes import ArrowDtype
 
-    from pandas.core.arrays.arrow._arrow_utils import fallback_performancewarning
-
     ARROW_CMP_FUNCS = {
         "eq": pc.equal,
         "ne": pc.not_equal,
@@ -918,7 +916,6 @@ def fillna(
             return super().fillna(value=value, method=method, limit=limit)
 
         if method is not None:
-            fallback_performancewarning()
             return super().fillna(value=value, method=method, limit=limit)
 
         if isinstance(value, (np.ndarray, ExtensionArray)):

pandas/core/arrays/base.py

@@ -23,7 +23,10 @@
 
 import numpy as np
 
-from pandas._libs import lib
+from pandas._libs import (
+    algos as libalgos,
+    lib,
+)
 from pandas.compat import set_function_name
 from pandas.compat.numpy import function as nv
 from pandas.errors import AbstractMethodError
@@ -824,10 +827,16 @@ def fillna(
 
         if mask.any():
             if method is not None:
-                func = missing.get_fill_func(method)
-                npvalues = self.astype(object)
-                func(npvalues, limit=limit, mask=mask)
-                new_values = self._from_sequence(npvalues, dtype=self.dtype)
+                meth = missing.clean_fill_method(method)
+
+                npmask = np.asarray(mask)
+                if meth == "pad":
+                    indexer = libalgos.get_fill_indexer(npmask, limit=limit)
+                    return self.take(indexer, allow_fill=True)
+                else:
+                    # i.e. meth == "backfill"
+                    indexer = libalgos.get_fill_indexer(npmask[::-1], limit=limit)[::-1]
+                    return self[::-1].take(indexer, allow_fill=True)
             else:
                 # fill with value
                 new_values = self.copy()

pandas/tests/extension/test_arrow.py

@@ -38,7 +38,6 @@
     pa_version_under9p0,
     pa_version_under11p0,
 )
-from pandas.errors import PerformanceWarning
 
 from pandas.core.dtypes.dtypes import (
     ArrowDtype,
@@ -698,12 +697,6 @@ def test_fillna_no_op_returns_copy(self, data):
         assert result is not data
         self.assert_extension_array_equal(result, data)
 
-    def test_fillna_series_method(self, data_missing, fillna_method):
-        with tm.maybe_produces_warning(
-            PerformanceWarning, fillna_method is not None, check_stacklevel=False
-        ):
-            super().test_fillna_series_method(data_missing, fillna_method)
-
 
 class TestBasePrinting(base.BasePrintingTests):
     pass

pandas/tests/extension/test_string.py

@@ -18,10 +18,7 @@
 import numpy as np
 import pytest
 
-from pandas.errors import PerformanceWarning
-
 import pandas as pd
-import pandas._testing as tm
 from pandas.api.types import is_string_dtype
 from pandas.core.arrays import ArrowStringArray
 from pandas.core.arrays.string_ import StringDtype
@@ -169,14 +166,6 @@ def test_fillna_no_op_returns_copy(self, data):
         assert result is not data
         self.assert_extension_array_equal(result, data)
 
-    def test_fillna_series_method(self, data_missing, fillna_method):
-        with tm.maybe_produces_warning(
-            PerformanceWarning,
-            fillna_method is not None and data_missing.dtype.storage == "pyarrow",
-            check_stacklevel=False,
-        ):
-            super().test_fillna_series_method(data_missing, fillna_method)
-
 
 class TestNoReduce(base.BaseNoReduceTests):
     @pytest.mark.parametrize("skipna", [True, False])