From 9f675ee5655520fe18d9b349043d024ff992feb5 Mon Sep 17 00:00:00 2001
From: Simon Hawkins <simonjayhawkins@gmail.com>
Date: Fri, 30 Apr 2021 13:00:16 +0100
Subject: [PATCH 1/2] [ArrowStringArray] isin using native pyarrow function if
 available

---
 asv_bench/benchmarks/algos/isin.py | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/asv_bench/benchmarks/algos/isin.py b/asv_bench/benchmarks/algos/isin.py
index a8b8a193dbcfc..44245295beafc 100644
--- a/asv_bench/benchmarks/algos/isin.py
+++ b/asv_bench/benchmarks/algos/isin.py
@@ -9,6 +9,8 @@
     date_range,
 )
 
+from ..pandas_vb_common import tm
+
 
 class IsIn:
 
@@ -22,6 +24,9 @@ class IsIn:
         "datetime64[ns]",
         "category[object]",
         "category[int]",
+        "str",
+        "string",
+        "arrow_string",
     ]
     param_names = ["dtype"]
 
@@ -57,6 +62,15 @@ def setup(self, dtype):
             self.values = np.random.choice(arr, sample_size)
             self.series = Series(arr).astype("category")
 
+        elif dtype in ["str", "string", "arrow_string"]:
+            from pandas.core.arrays.string_arrow import ArrowStringDtype  # noqa: F401
+
+            try:
+                self.series = Series(tm.makeStringIndex(N), dtype=dtype)
+            except ImportError:
+                raise NotImplementedError
+            self.values = list(self.series[:2])
+
         else:
             self.series = Series(np.random.randint(1, 10, N)).astype(dtype)
             self.values = [1, 2]

From 2e7e942b6d461e73643d0318b4df8574c62c2f60 Mon Sep 17 00:00:00 2001
From: Simon Hawkins <simonjayhawkins@gmail.com>
Date: Mon, 3 May 2021 13:33:11 +0100
Subject: [PATCH 2/2] add tests

---
 pandas/core/algorithms.py                  |  7 +++---
 pandas/core/arrays/string_arrow.py         | 28 ++++++++++++++++++++++
 pandas/tests/arrays/string_/test_string.py | 20 ++++++++++++++++
 3 files changed, 51 insertions(+), 4 deletions(-)

diff --git a/pandas/core/algorithms.py b/pandas/core/algorithms.py
index 2c4477056a112..c52105b77e4dc 100644
--- a/pandas/core/algorithms.py
+++ b/pandas/core/algorithms.py
@@ -468,10 +468,9 @@ def isin(comps: AnyArrayLike, values: AnyArrayLike) -> np.ndarray:
 
     comps = _ensure_arraylike(comps)
     comps = extract_array(comps, extract_numpy=True)
-    if is_extension_array_dtype(comps.dtype):
-        # error: Incompatible return value type (got "Series", expected "ndarray")
-        # error: Item "ndarray" of "Union[Any, ndarray]" has no attribute "isin"
-        return comps.isin(values)  # type: ignore[return-value,union-attr]
+    if not isinstance(comps, np.ndarray):
+        # i.e. Extension Array
+        return comps.isin(values)
 
     elif needs_i8_conversion(comps.dtype):
         # Dispatch to DatetimeLikeArrayMixin.isin
diff --git a/pandas/core/arrays/string_arrow.py b/pandas/core/arrays/string_arrow.py
index 72a2ab8a1b80a..01813cef97b8d 100644
--- a/pandas/core/arrays/string_arrow.py
+++ b/pandas/core/arrays/string_arrow.py
@@ -663,6 +663,34 @@ def take(
                 indices_array[indices_array < 0] += len(self._data)
             return type(self)(self._data.take(indices_array))
 
+    def isin(self, values):
+
+        # pyarrow.compute.is_in added in pyarrow 2.0.0
+        if not hasattr(pc, "is_in"):
+            return super().isin(values)
+
+        value_set = [
+            pa_scalar.as_py()
+            for pa_scalar in [pa.scalar(value, from_pandas=True) for value in values]
+            if pa_scalar.type in (pa.string(), pa.null())
+        ]
+
+        # for an empty value_set pyarrow 3.0.0 segfaults and pyarrow 2.0.0 returns True
+        # for null values, so we short-circuit to return all False array.
+        if not len(value_set):
+            return np.zeros(len(self), dtype=bool)
+
+        kwargs = {}
+        if LooseVersion(pa.__version__) < "3.0.0":
+            # in pyarrow 2.0.0 skip_null is ignored but is a required keyword and raises
+            # with unexpected keyword argument in pyarrow 3.0.0+
+            kwargs["skip_null"] = True
+
+        result = pc.is_in(self._data, value_set=pa.array(value_set), **kwargs)
+        # pyarrow 2.0.0 returned nulls, so we explicily specify dtype to convert nulls
+        # to False
+        return np.array(result, dtype=np.bool_)
+
     def value_counts(self, dropna: bool = True) -> Series:
         """
         Return a Series containing counts of each unique value.
diff --git a/pandas/tests/arrays/string_/test_string.py b/pandas/tests/arrays/string_/test_string.py
index e2d8e522abb35..43ba5667d4d93 100644
--- a/pandas/tests/arrays/string_/test_string.py
+++ b/pandas/tests/arrays/string_/test_string.py
@@ -566,3 +566,23 @@ def test_to_numpy_na_value(dtype, nulls_fixture):
     result = arr.to_numpy(na_value=na_value)
     expected = np.array(["a", na_value, "b"], dtype=object)
     tm.assert_numpy_array_equal(result, expected)
+
+
+def test_isin(dtype, request):
+    s = pd.Series(["a", "b", None], dtype=dtype)
+
+    result = s.isin(["a", "c"])
+    expected = pd.Series([True, False, False])
+    tm.assert_series_equal(result, expected)
+
+    result = s.isin(["a", pd.NA])
+    expected = pd.Series([True, False, True])
+    tm.assert_series_equal(result, expected)
+
+    result = s.isin([])
+    expected = pd.Series([False, False, False])
+    tm.assert_series_equal(result, expected)
+
+    result = s.isin(["a", pd.Timestamp.now()])
+    expected = pd.Series([True, False, False])
+    tm.assert_series_equal(result, expected)