Repository: pandas-dev/pandas (python)

Issue:
ENH: provide nunique method on Rolling object
GroupBy objects expose a `nunique` method of Series/DataFrame (and also for `resample` it works), but Rolling does not:

```
In [89]: values = np.random.choice(np.arange(10), 100)                                                                                                                                                             

In [90]: s = pd.Series(values, index=pd.date_range("2019-01-01", freq='H', periods=100))                                                                                                                           

In [92]: s.groupby(pd.Grouper(freq='D')).nunique()                                                                                                                                                                 
Out[92]: 
2019-01-01    10
2019-01-02     8
2019-01-03    10
2019-01-04    10
2019-01-05     4
Freq: D, dtype: int64
```
vs

```
In [93]: s.rolling('D').apply(pd.Series.nunique, raw=False)                                                                                                                                                        
Out[93]: 
2019-01-01 00:00:00     1.0
2019-01-01 01:00:00     2.0
2019-01-01 02:00:00     3.0
...

# or 
In [95]: s.rolling('D').apply(lambda x: len(pd.unique(x)), raw=True)                                                                                                                                               
Out[95]: 
2019-01-01 00:00:00     1.0
2019-01-01 01:00:00     2.0
2019-01-01 02:00:00     3.0
...
```

I think it would be nice to also have this method available on `rolling` objects. 



Pull request description:
ENH: Add Rolling.nunique()
- [ ] closes #26958 
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/v3.0.0.rst` file if fixing a bug or adding a new feature.

Hash-map (C++ `unordered_map` via Cython) based implementation of `Rolling.nunique()` with `O(N)` average time complexity.

Patch:
diff --git a/asv_bench/benchmarks/rolling.py b/asv_bench/benchmarks/rolling.py
index bd4da00bfd..f9a5f38c2e 100644
--- a/asv_bench/benchmarks/rolling.py
+++ b/asv_bench/benchmarks/rolling.py
@@ -10,7 +10,19 @@ class Methods:
         ["DataFrame", "Series"],
         [("rolling", {"window": 10}), ("rolling", {"window": 1000}), ("expanding", {})],
         ["int", "float"],
-        ["median", "mean", "max", "min", "std", "count", "skew", "kurt", "sum", "sem"],
+        [
+            "median",
+            "mean",
+            "max",
+            "min",
+            "std",
+            "count",
+            "skew",
+            "kurt",
+            "sum",
+            "sem",
+            "nunique",
+        ],
     )
     param_names = ["constructor", "window_kwargs", "dtype", "method"]
 
diff --git a/doc/source/reference/window.rst b/doc/source/reference/window.rst
index 2aeb57faac..2bd63f02fa 100644
--- a/doc/source/reference/window.rst
+++ b/doc/source/reference/window.rst
@@ -42,6 +42,7 @@ Rolling window functions
    Rolling.quantile
    Rolling.sem
    Rolling.rank
+   Rolling.nunique
 
 .. _api.functions_window:
 
@@ -86,6 +87,7 @@ Expanding window functions
    Expanding.quantile
    Expanding.sem
    Expanding.rank
+   Expanding.nunique
 
 .. _api.functions_ewm:
 
diff --git a/doc/source/whatsnew/v2.3.0.rst b/doc/source/whatsnew/v2.3.0.rst
index 0913476397..9352063ad3 100644
--- a/doc/source/whatsnew/v2.3.0.rst
+++ b/doc/source/whatsnew/v2.3.0.rst
@@ -118,6 +118,7 @@ Conversion
 
 Strings
 ^^^^^^^
+- Bug in :meth:`DataFrame.sum` with ``axis=1``, :meth:`.DataFrameGroupBy.sum` or :meth:`.SeriesGroupBy.sum` with ``skipna=True``, and :meth:`.Resampler.sum` on :class:`StringDtype` with all NA values resulted in ``0`` and is now the empty string ``""`` (:issue:`60229`)
 - Bug in :meth:`Series.__pos__` and :meth:`DataFrame.__pos__` did not raise for :class:`StringDtype` with ``storage="pyarrow"`` (:issue:`60710`)
 - Bug in :meth:`Series.rank` for :class:`StringDtype` with ``storage="pyarrow"`` incorrectly returning integer results in case of ``method="average"`` and raising an error if it would truncate results (:issue:`59768`)
 - Bug in :meth:`Series.replace` with :class:`StringDtype` when replacing with a non-string value was not upcasting to ``object`` dtype (:issue:`60282`)
diff --git a/doc/source/whatsnew/v3.0.0.rst b/doc/source/whatsnew/v3.0.0.rst
index ad086c4d63..25c26253e6 100644
--- a/doc/source/whatsnew/v3.0.0.rst
+++ b/doc/source/whatsnew/v3.0.0.rst
@@ -62,6 +62,7 @@ Other enhancements
 - :meth:`Series.plot` now correctly handle the ``ylabel`` parameter for pie charts, allowing for explicit control over the y-axis label (:issue:`58239`)
 - :meth:`DataFrame.plot.scatter` argument ``c`` now accepts a column of strings, where rows with the same string are colored identically (:issue:`16827` and :issue:`16485`)
 - :class:`DataFrameGroupBy` and :class:`SeriesGroupBy` methods ``sum``, ``mean``, ``median``, ``prod``, ``min``, ``max``, ``std``, ``var`` and ``sem`` now accept ``skipna`` parameter (:issue:`15675`)
+- :class:`Rolling` and :class:`Expanding` now support ``nunique`` (:issue:`26958`)
 - :class:`Rolling` and :class:`Expanding` now support aggregations ``first`` and ``last`` (:issue:`33155`)
 - :func:`read_parquet` accepts ``to_pandas_kwargs`` which are forwarded to :meth:`pyarrow.Table.to_pandas` which enables passing additional keywords to customize the conversion to pandas, such as ``maps_as_pydicts`` to read the Parquet map data type as python dictionaries (:issue:`56842`)
 - :meth:`.DataFrameGroupBy.transform`, :meth:`.SeriesGroupBy.transform`, :meth:`.DataFrameGroupBy.agg`, :meth:`.SeriesGroupBy.agg`, :meth:`.SeriesGroupBy.apply`, :meth:`.DataFrameGroupBy.apply` now support ``kurt`` (:issue:`40139`)
diff --git a/pandas/_libs/window/aggregations.pyi b/pandas/_libs/window/aggregations.pyi
index ee735761e3..b4bdd7e05c 100644
--- a/pandas/_libs/window/aggregations.pyi
+++ b/pandas/_libs/window/aggregations.pyi
@@ -89,6 +89,12 @@ def roll_rank(
     method: WindowingRankType,
     ascending: bool,
 ) -> np.ndarray: ...  # np.ndarray[float]
+def roll_nunique(
+    values: np.ndarray,  # const float64_t[:]
+    start: np.ndarray,  # np.ndarray[np.int64]
+    end: np.ndarray,  # np.ndarray[np.int64]
+    minp: int,  # int64_t
+) -> np.ndarray: ...  # np.ndarray[float]
 def roll_apply(
     obj: object,
     start: np.ndarray,  # np.ndarray[np.int64]
diff --git a/pandas/_libs/window/aggregations.pyx b/pandas/_libs/window/aggregations.pyx
index d33c840371..2baed13cbd 100644
--- a/pandas/_libs/window/aggregations.pyx
+++ b/pandas/_libs/window/aggregations.pyx
@@ -6,6 +6,7 @@ from libc.math cimport (
     sqrt,
 )
 from libcpp.deque cimport deque
+from libcpp.unordered_map cimport unordered_map
 
 from pandas._libs.algos cimport TiebreakEnumType
 
@@ -1470,6 +1471,66 @@ def roll_rank(const float64_t[:] values, ndarray[int64_t] start,
     return np.asarray(output)
 
 
+def roll_nunique(const float64_t[:] values, ndarray[int64_t] start,
+                 ndarray[int64_t] end, int64_t minp) -> np.ndarray:
+    """
+    Rolling number of unique elements in the window
+    """
+    cdef:
+        Py_ssize_t i, j, s, e, N = len(start)
+        int64_t nobs = 0
+        float64_t val
+        float64_t[::1] output
+        unordered_map[float64_t, int64_t] value_counts
+
+    is_monotonic_increasing_bounds = is_monotonic_increasing_start_end_bounds(
+        start, end
+    )
+    output = np.empty(N, dtype=np.float64)
+    value_counts = unordered_map[float64_t, int64_t]()
+
+    with nogil:
+        for i in range(N):
+            s = start[i]
+            e = end[i]
+
+            if i == 0 or not is_monotonic_increasing_bounds or s >= end[i - 1]:
+                if i != 0:
+                    nobs = 0
+                    value_counts.clear()
+
+                # setup
+                for j in range(s, e):
+                    val = values[j]
+                    if val == val:
+                        nobs += 1
+                        value_counts[val] += 1
+
+            else:
+                # calculate deletes
+                for j in range(start[i - 1], s):
+                    val = values[j]
+                    if val == val:
+                        value_counts[val] -= 1
+                        if value_counts[val] == 0:
+                            value_counts.erase(val)
+                        nobs -= 1
+
+                # calculate adds
+                for j in range(end[i - 1], e):
+                    val = values[j]
+                    if val == val:
+                        nobs += 1
+                        value_counts[val] += 1
+
+            if nobs >= minp:
+                output[i] = value_counts.size()
+            else:
+                output[i] = NaN
+
+    return np.asarray(output)
+
+
 def roll_apply(object obj,
                ndarray[int64_t] start, ndarray[int64_t] end,
                int64_t minp,
diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 33745438e2..dbf2090e53 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -2628,7 +2628,15 @@ class ExtensionArray:
             if op.how not in ["any", "all"]:
                 # Fail early to avoid conversion to object
                 op._get_cython_function(op.kind, op.how, np.dtype(object), False)
-            npvalues = self.to_numpy(object, na_value=np.nan)
+
+            arr = self
+            if op.how == "sum":
+                # https://github.com/pandas-dev/pandas/issues/60229
+                # All NA should result in the empty string.
+                assert "skipna" in kwargs
+                if kwargs["skipna"] and min_count == 0:
+                    arr = arr.fillna("")
+            npvalues = arr.to_numpy(object, na_value=np.nan)
         else:
             raise NotImplementedError(
                 f"function is not implemented for this dtype: {self.dtype}"
diff --git a/pandas/core/window/expanding.py b/pandas/core/window/expanding.py
index 81c89e1ef5..bff3485c9c 100644
--- a/pandas/core/window/expanding.py
+++ b/pandas/core/window/expanding.py
@@ -927,6 +927,41 @@ class Expanding(RollingAndExpandingMixin):
             numeric_only=numeric_only,
         )
 
+    @doc(
+        template_header,
+        ".. versionadded:: 3.0.0 \n\n",
+        create_section_header("Parameters"),
+        kwargs_numeric_only,
+        create_section_header("Returns"),
+        template_returns,
+        create_section_header("See Also"),
+        template_see_also,
+        create_section_header("Examples"),
+        dedent(
+            """
+        >>> s = pd.Series([1, 4, 2, 3, 5, 3])
+        >>> s.expanding().nunique()
+        0    1.0
+        1    2.0
+        2    3.0
+        3    4.0
+        4    5.0
+        5    5.0
+        dtype: float64
+        """
+        ).replace("\n", "", 1),
+        window_method="expanding",
+        aggregation_description="nunique",
+        agg_method="nunique",
+    )
+    def nunique(
+        self,
+        numeric_only: bool = False,
+    ):
+        return super().nunique(
+            numeric_only=numeric_only,
+        )
+
     @doc(
         template_header,
         create_section_header("Parameters"),
diff --git a/pandas/core/window/rolling.py b/pandas/core/window/rolling.py
index 69fce8cf21..03534bbee4 100644
--- a/pandas/core/window/rolling.py
+++ b/pandas/core/window/rolling.py
@@ -1799,6 +1799,16 @@ class RollingAndExpandingMixin(BaseWindow):
 
         return self._apply(window_func, name="rank", numeric_only=numeric_only)
 
+    def nunique(
+        self,
+        numeric_only: bool = False,
+    ):
+        window_func = partial(
+            window_aggregations.roll_nunique,
+        )
+
+        return self._apply(window_func, name="nunique", numeric_only=numeric_only)
+
     def cov(
         self,
         other: DataFrame | Series | None = None,
@@ -2855,6 +2865,43 @@ class Rolling(RollingAndExpandingMixin):
             numeric_only=numeric_only,
         )
 
+    @doc(
+        template_header,
+        ".. versionadded:: 3.0.0 \n\n",
+        create_section_header("Parameters"),
+        kwargs_numeric_only,
+        create_section_header("Returns"),
+        template_returns,
+        create_section_header("See Also"),
+        template_see_also,
+        create_section_header("Examples"),
+        dedent(
+            """
+        >>> s = pd.Series([1, 4, 2, np.nan, 3, 3, 4, 5])
+        >>> s.rolling(3).nunique()
+        0    NaN
+        1    NaN
+        2    3.0
+        3    NaN
+        4    NaN
+        5    NaN
+        6    2.0
+        7    3.0
+        dtype: float64
+        """
+        ).replace("\n", "", 1),
+        window_method="rolling",
+        aggregation_description="nunique",
+        agg_method="nunique",
+    )
+    def nunique(
+        self,
+        numeric_only: bool = False,
+    ):
+        return super().nunique(
+            numeric_only=numeric_only,
+        )
+
     @doc(
         template_header,
         create_section_header("Parameters"),


Test patch:
diff --git a/pandas/tests/frame/test_reductions.py b/pandas/tests/frame/test_reductions.py
index 64e686d25f..127f0fc50a 100644
--- a/pandas/tests/frame/test_reductions.py
+++ b/pandas/tests/frame/test_reductions.py
@@ -835,6 +835,16 @@ class TestDataFrameAnalytics:
         expected = Series([], index=index, dtype=expected_dtype)
         tm.assert_series_equal(result, expected)
 
+    @pytest.mark.parametrize("min_count", [0, 1])
+    def test_axis_1_sum_na(self, string_dtype_no_object, skipna, min_count):
+        # https://github.com/pandas-dev/pandas/issues/60229
+        dtype = string_dtype_no_object
+        df = DataFrame({"a": [pd.NA]}, dtype=dtype)
+        result = df.sum(axis=1, skipna=skipna, min_count=min_count)
+        value = "" if skipna and min_count == 0 else pd.NA
+        expected = Series([value], dtype=dtype)
+        tm.assert_series_equal(result, expected)
+
     @pytest.mark.parametrize("method, unit", [("sum", 0), ("prod", 1)])
     @pytest.mark.parametrize("numeric_only", [None, True, False])
     def test_sum_prod_nanops(self, method, unit, numeric_only):
diff --git a/pandas/tests/groupby/test_reductions.py b/pandas/tests/groupby/test_reductions.py
index ea876cfdf4..45047fe004 100644
--- a/pandas/tests/groupby/test_reductions.py
+++ b/pandas/tests/groupby/test_reductions.py
@@ -955,6 +955,20 @@ def test_min_empty_string_dtype(func, string_dtype_no_object):
     tm.assert_frame_equal(result, expected)
 
 
+@pytest.mark.parametrize("min_count", [0, 1])
+def test_string_dtype_empty_sum(string_dtype_no_object, skipna, min_count):
+    # https://github.com/pandas-dev/pandas/issues/60229
+    dtype = string_dtype_no_object
+    df = DataFrame({"a": ["x"], "b": [pd.NA]}, dtype=dtype)
+    gb = df.groupby("a")
+    result = gb.sum(skipna=skipna, min_count=min_count)
+    value = "" if skipna and min_count == 0 else pd.NA
+    expected = DataFrame(
+        {"b": value}, index=pd.Index(["x"], name="a", dtype=dtype), dtype=dtype
+    )
+    tm.assert_frame_equal(result, expected)
+
+
 def test_max_nan_bug():
     df = DataFrame(
         {
diff --git a/pandas/tests/resample/test_base.py b/pandas/tests/resample/test_base.py
index b2d9f6c0e3..0db5c0c82d 100644
--- a/pandas/tests/resample/test_base.py
+++ b/pandas/tests/resample/test_base.py
@@ -223,6 +223,31 @@ def test_resample_empty_series(freq, index, resample_method):
     assert result.index.freq == expected.index.freq
 
 
+@pytest.mark.parametrize("min_count", [0, 1])
+def test_resample_empty_sum_string(string_dtype_no_object, min_count):
+    # https://github.com/pandas-dev/pandas/issues/60229
+    dtype = string_dtype_no_object
+    ser = Series(
+        pd.NA,
+        index=DatetimeIndex(
+            [
+                "2000-01-01 00:00:00",
+                "2000-01-01 00:00:10",
+                "2000-01-01 00:00:20",
+                "2000-01-01 00:00:30",
+            ]
+        ),
+        dtype=dtype,
+    )
+    rs = ser.resample("20s")
+    result = rs.sum(min_count=min_count)
+
+    value = "" if min_count == 0 else pd.NA
+    index = date_range(start="2000-01-01", freq="20s", periods=2, unit="s")
+    expected = Series(value, index=index, dtype=dtype)
+    tm.assert_series_equal(result, expected)
+
+
 @pytest.mark.parametrize(
     "freq",
     [
diff --git a/pandas/tests/resample/test_resampler_grouper.py b/pandas/tests/resample/test_resampler_grouper.py
index e7850f96b3..7870c5a9d3 100644
--- a/pandas/tests/resample/test_resampler_grouper.py
+++ b/pandas/tests/resample/test_resampler_grouper.py
@@ -494,6 +494,26 @@ def test_resample_groupby_agg_object_dtype_all_nan(consolidate):
     tm.assert_frame_equal(result, expected)
 
 
+@pytest.mark.parametrize("min_count", [0, 1])
+def test_groupby_resample_empty_sum_string(
+    string_dtype_no_object, test_frame, min_count
+):
+    # https://github.com/pandas-dev/pandas/issues/60229
+    dtype = string_dtype_no_object
+    test_frame = test_frame.assign(B=pd.array([pd.NA] * len(test_frame), dtype=dtype))
+    gbrs = test_frame.groupby("A").resample("40s")
+    result = gbrs.sum(min_count=min_count)
+
+    index = pd.MultiIndex(
+        levels=[[1, 2, 3], [pd.to_datetime("2000-01-01", unit="ns")]],
+        codes=[[0, 1, 2], [0, 0, 0]],
+        names=["A", None],
+    )
+    value = "" if min_count == 0 else pd.NA
+    expected = DataFrame({"B": value}, index=index, dtype=dtype)
+    tm.assert_frame_equal(result, expected)
+
+
 def test_groupby_resample_with_list_of_keys():
     # GH 47362
     df = DataFrame(
diff --git a/pandas/tests/window/test_cython_aggregations.py b/pandas/tests/window/test_cython_aggregations.py
index feb25a294c..39811ea3ec 100644
--- a/pandas/tests/window/test_cython_aggregations.py
+++ b/pandas/tests/window/test_cython_aggregations.py
@@ -32,6 +32,7 @@ def _get_rolling_aggregations():
             ("roll_min", window_aggregations.roll_min),
             ("roll_first", window_aggregations.roll_first),
             ("roll_last", window_aggregations.roll_last),
+            ("roll_nunique", window_aggregations.roll_nunique),
         ]
         + [
             (
diff --git a/pandas/tests/window/test_expanding.py b/pandas/tests/window/test_expanding.py
index 39cedc3b69..2c96ce01c6 100644
--- a/pandas/tests/window/test_expanding.py
+++ b/pandas/tests/window/test_expanding.py
@@ -255,6 +255,43 @@ def test_rank(window, method, pct, ascending, test_data):
     tm.assert_series_equal(result, expected)
 
 
+@pytest.mark.parametrize("window", [1, 3, 10, 20])
+@pytest.mark.parametrize("test_data", ["default", "duplicates", "nans", "precision"])
+def test_nunique(window, test_data):
+    length = 20
+    if test_data == "default":
+        ser = Series(data=np.random.default_rng(2).random(length))
+    elif test_data == "duplicates":
+        ser = Series(data=np.random.default_rng(2).choice(3, length))
+    elif test_data == "nans":
+        ser = Series(
+            data=np.random.default_rng(2).choice(
+                [1.0, 0.25, 0.75, np.nan, np.inf, -np.inf], length
+            )
+        )
+    elif test_data == "precision":
+        ser = Series(
+            data=[
+                0.3,
+                0.1 * 3,  # Not necessarily exactly 0.3
+                0.6,
+                0.2 * 3,  # Not necessarily exactly 0.6
+                0.9,
+                0.3 * 3,  # Not necessarily exactly 0.9
+                0.5,
+                0.1 * 5,  # Not necessarily exactly 0.5
+                0.8,
+                0.2 * 4,  # Not necessarily exactly 0.8
+            ],
+            dtype=np.float64,
+        )
+
+    expected = ser.expanding(window).apply(lambda x: x.nunique())
+    result = ser.expanding(window).nunique()
+
+    tm.assert_series_equal(result, expected)
+
+
 def test_expanding_corr(series):
     A = series.dropna()
     B = (A + np.random.default_rng(2).standard_normal(len(A)))[:-5]
diff --git a/pandas/tests/window/test_groupby.py b/pandas/tests/window/test_groupby.py
index 392239b8ad..1dcdad2bfd 100644
--- a/pandas/tests/window/test_groupby.py
+++ b/pandas/tests/window/test_groupby.py
@@ -96,6 +96,7 @@ class TestRolling:
             "count",
             "kurt",
             "skew",
+            "nunique",
         ],
     )
     def test_rolling(self, f, roll_frame):
@@ -1034,7 +1035,19 @@ class TestExpanding:
         return DataFrame({"A": [1] * 20 + [2] * 12 + [3] * 8, "B": np.arange(40)})
 
     @pytest.mark.parametrize(
-        "f", ["sum", "mean", "min", "max", "first", "last", "count", "kurt", "skew"]
+        "f",
+        [
+            "sum",
+            "mean",
+            "min",
+            "max",
+            "first",
+            "last",
+            "count",
+            "kurt",
+            "skew",
+            "nunique",
+        ],
     )
     def test_expanding(self, f, frame):
         g = frame.groupby("A", group_keys=False)
diff --git a/pandas/tests/window/test_rolling.py b/pandas/tests/window/test_rolling.py
index 2aaa35ec5e..8c57781c14 100644
--- a/pandas/tests/window/test_rolling.py
+++ b/pandas/tests/window/test_rolling.py
@@ -1586,6 +1586,43 @@ def test_rank(window, method, pct, ascending, test_data):
     tm.assert_series_equal(result, expected)
 
 
+@pytest.mark.parametrize("window", [1, 3, 10, 20])
+@pytest.mark.parametrize("test_data", ["default", "duplicates", "nans", "precision"])
+def test_nunique(window, test_data):
+    length = 20
+    if test_data == "default":
+        ser = Series(data=np.random.default_rng(2).random(length))
+    elif test_data == "duplicates":
+        ser = Series(data=np.random.default_rng(2).choice(3, length))
+    elif test_data == "nans":
+        ser = Series(
+            data=np.random.default_rng(2).choice(
+                [1.0, 0.25, 0.75, np.nan, np.inf, -np.inf], length
+            )
+        )
+    elif test_data == "precision":
+        ser = Series(
+            data=[
+                0.3,
+                0.1 * 3,  # Not necessarily exactly 0.3
+                0.6,
+                0.2 * 3,  # Not necessarily exactly 0.6
+                0.9,
+                0.3 * 3,  # Not necessarily exactly 0.9
+                0.5,
+                0.1 * 5,  # Not necessarily exactly 0.5
+                0.8,
+                0.2 * 4,  # Not necessarily exactly 0.8
+            ],
+            dtype=np.float64,
+        )
+
+    expected = ser.rolling(window).apply(lambda x: x.nunique())
+    result = ser.rolling(window).nunique()
+
+    tm.assert_series_equal(result, expected)
+
+
 def test_rolling_quantile_np_percentile():
     # #9413: Tests that rolling window's quantile default behavior
     # is analogous to Numpy's percentile