Add value_counts implementation for Series and as free function

docs/supported_apis/series_supported.rst

@@ -468,7 +468,7 @@ the related section on `Defaulting to pandas`_.
 +-----------------------------+---------------------------------+
 | ``valid``                   | D                               |
 +-----------------------------+---------------------------------+
-| ``value_counts``            | D                               |
+| ``value_counts``            | Y                               |
 +-----------------------------+---------------------------------+
 | ``values``                  | Y                               |
 +-----------------------------+---------------------------------+

docs/supported_apis/utilities_supported.rst

@@ -21,7 +21,7 @@ default to pandas.
 +---------------------------+---------------------------------+----------------------------------------------------+
 | `pd.unique`_              | Y                               |                                                    |
 +---------------------------+---------------------------------+----------------------------------------------------+
-| ``pd.value_counts``       | D                               |                                                    |
+| ``pd.value_counts``       | Y                               |                                                    |
 +---------------------------+---------------------------------+----------------------------------------------------+
 | `pd.cut`_                 | D                               |                                                    |
 +---------------------------+---------------------------------+----------------------------------------------------+

modin/backends/base/query_compiler.py

@@ -466,6 +466,9 @@ def unique(self, **kwargs):
 
     # END Abstract map partitions operations
 
+    def value_counts(self, **kwargs):
+        pass
+
     # Abstract map partitions across select indices
     @abc.abstractmethod
     def astype(self, col_dtypes, **kwargs):

modin/backends/pandas/query_compiler.py

@@ -509,6 +509,100 @@ def is_monotonic_decreasing(self):
         ),
     )
 
+    def value_counts(self, **kwargs):
+        """
+        Return a QueryCompiler of Series containing counts of unique values.
+
+        Returns
+        -------
+        PandasQueryCompiler
+        """
+        if kwargs.get("bins", None) is not None:
+            new_modin_frame = self._modin_frame._apply_full_axis(
+                0, lambda df: df.squeeze(axis=1).value_counts(**kwargs)
+            )
+            return self.__constructor__(new_modin_frame)
+
+        def map_func(df, *args, **kwargs):
+            return df.squeeze(axis=1).value_counts(**kwargs)
+
+        def reduce_func(df, *args, **kwargs):
+            normalize = kwargs.get("normalize", False)
+            sort = kwargs.get("sort", True)
+            ascending = kwargs.get("ascending", False)
+            dropna = kwargs.get("dropna", True)
+
+            try:
+                result = df.squeeze(axis=1).groupby(df.index, sort=False).sum()
+            # This will happen with Arrow buffer read-only errors. We don't want to copy
+            # all the time, so this will try to fast-path the code first.
+            except (ValueError):
+                result = df.copy().squeeze(axis=1).groupby(df.index, sort=False).sum()
+
+            if not dropna and np.nan in df.index:
+                result = result.append(
+                    pandas.Series(
+                        [df.squeeze(axis=1).loc[[np.nan]].sum()], index=[np.nan]
+                    )
+                )
+            if normalize:
+                result = result / df.squeeze(axis=1).sum()
+
+            result = result.sort_values(ascending=ascending) if sort else result
+
+            # We want to sort both values and indices of the result object.
+            # This function will sort indices for equal values.
+            def sort_index_for_equal_values(result, ascending):
+                """
+                Sort indices for equal values of result object.
+
+                Parameters
+                ----------
+                result : pandas.Series or pandas.DataFrame with one column
+                    The object whose indices for equal values is needed to sort.
+                ascending : boolean
+                    Sort in ascending (if it is True) or descending (if it is False) order.
+
+                Returns
+                -------
+                pandas.DataFrame
+                    A new DataFrame with sorted indices.
+                """
+                is_range = False
+                is_end = False
+                i = 0
+                new_index = np.empty(len(result), dtype=type(result.index))
+                while i < len(result):
+                    j = i
+                    if i < len(result) - 1:
+                        while result[result.index[i]] == result[result.index[i + 1]]:
+                            i += 1
+                            if is_range is False:
+                                is_range = True
+                            if i == len(result) - 1:
+                                is_end = True
+                                break
+                    if is_range:
+                        k = j
+                        for val in sorted(
+                            result.index[j : i + 1], reverse=not ascending
+                        ):
+                            new_index[k] = val
+                            k += 1
+                        if is_end:
+                            break
+                        is_range = False
+                    else:
+                        new_index[j] = result.index[j]
+                    i += 1
+                return pandas.DataFrame(result, index=new_index)
+
+            return sort_index_for_equal_values(result, ascending)
+
+        return MapReduceFunction.register(map_func, reduce_func, preserve_index=False)(
+            self, **kwargs
+        )
+
     # END MapReduce operations
 
     # Reduction operations

modin/data_management/functions/mapreducefunction.py

@@ -18,13 +18,15 @@ class MapReduceFunction(Function):
     @classmethod
     def call(cls, map_function, reduce_function, **call_kwds):
         def caller(query_compiler, *args, **kwargs):
+            preserve_index = call_kwds.pop("preserve_index", True)
             return query_compiler.__constructor__(
                 query_compiler._modin_frame._map_reduce(
                     call_kwds.get("axis")
                     if "axis" in call_kwds
                     else kwargs.get("axis"),
                     lambda x: map_function(x, *args, **kwargs),
                     lambda y: reduce_function(y, *args, **kwargs),
+                    preserve_index=preserve_index,
                 )
             )
 

modin/engines/base/frame/data.py

@@ -780,16 +780,26 @@ def _fold_reduce(self, axis, func):
         )
         return self._compute_map_reduce_metadata(axis, new_parts)
 
-    def _map_reduce(self, axis, map_func, reduce_func=None):
-        """Apply function that will reduce the data to a Pandas Series.
+    def _map_reduce(self, axis, map_func, reduce_func=None, preserve_index=True):
+        """
+        Apply function that will reduce the data to a Pandas Series.
 
-        Args:
-            axis: 0 for columns and 1 for rows. Default is 0.
-            map_func: Callable function to map the dataframe.
-            reduce_func: Callable function to reduce the dataframe. If none,
-                then apply map_func twice.
+        Parameters
+        ----------
+            axis : 0 or 1
+                0 for columns and 1 for rows.
+            map_func : callable
+                Callable function to map the dataframe.
+            reduce_func : callable
+                Callable function to reduce the dataframe.
+                If none, then apply map_func twice. Default is None.
+            preserve_index : boolean
+                The flag to preserve index for default behavior
+                map and reduce operations. Default is True.
 
-        Return:
+        Returns
+        -------
+        BasePandasFrame
             A new dataframe.
         """
         map_func = self._build_mapreduce_func(axis, map_func)
@@ -802,7 +812,20 @@ def _map_reduce(self, axis, map_func, reduce_func=None):
         reduce_parts = self._frame_mgr_cls.map_axis_partitions(
             axis, map_parts, reduce_func
         )
-        return self._compute_map_reduce_metadata(axis, reduce_parts)
+        if preserve_index:
+            return self._compute_map_reduce_metadata(axis, reduce_parts)
+        else:
+            if axis == 0:
+                new_index = ["__reduced__"]
+                new_columns = self._frame_mgr_cls.get_indices(
+                    1, reduce_parts, lambda df: df.columns
+                )
+            else:
+                new_index = self._frame_mgr_cls.get_indices(
+                    0, reduce_parts, lambda df: df.index
+                )
+                new_columns = ["__reduced__"]
+            return self.__constructor__(reduce_parts, new_index, new_columns)
 
     def _map(self, func, dtypes=None):
         """Perform a function that maps across the entire dataset.

modin/pandas/__init__.py

@@ -27,7 +27,6 @@
 
 from pandas import (
     eval,
-    value_counts,
     cut,
     factorize,
     test,
@@ -132,6 +131,7 @@
     pivot,
     to_numeric,
     unique,
+    value_counts,
 )
 from .plotting import Plotting as plotting
 from .. import execution_engine, Publisher
@@ -302,7 +302,6 @@ def _update_engine(publisher: Publisher):
     "json_normalize",
     "concat",
     "eval",
-    "value_counts",
     "cut",
     "factorize",
     "test",
@@ -382,6 +381,7 @@ def _update_engine(publisher: Publisher):
     "pivot",
     "to_numeric",
     "unique",
+    "value_counts",
     "datetime",
     "NamedAgg",
     "DEFAULT_NPARTITIONS",

modin/pandas/general.py

@@ -288,3 +288,39 @@ def unique(values):
         The unique values returned as a NumPy array.
     """
     return Series(values).unique()
+
+
+def value_counts(
+    values, sort=True, ascending=False, normalize=False, bins=None, dropna=True
+):
+    """
+    Compute a histogram of the counts of non-null values.
+
+    Parameters
+    ----------
+    values : ndarray (1-d)
+    sort : bool, default True
+        Sort by values
+    ascending : bool, default False
+        Sort in ascending order
+    normalize: bool, default False
+        If True then compute a relative histogram
+    bins : integer, optional
+        Rather than count values, group them into half-open bins,
+        convenience for pd.cut, only works with numeric data
+    dropna : bool, default True
+        Don't include counts of NaN
+
+    Returns
+    -------
+    Series
+
+    Notes
+    -----
+    The indices of resulting object will be in descending
+    (ascending, if ascending=True) order for equal values.
+    It slightly differ from pandas where indices are located in random order.
+    """
+    return Series(values).value_counts(
+        sort=sort, ascending=ascending, normalize=normalize, bins=bins, dropna=dropna,
+    )

modin/pandas/series.py

@@ -1359,13 +1359,46 @@ def update(self, other):
     def value_counts(
         self, normalize=False, sort=True, ascending=False, bins=None, dropna=True
     ):
-        return self._default_to_pandas(
-            pandas.Series.value_counts,
-            normalize=normalize,
-            sort=sort,
-            ascending=ascending,
-            bins=bins,
-            dropna=dropna,
+        """
+        Return a Series containing counts of unique values.
+
+        The resulting object will be in descending order so that the
+        first element is the most frequently-occurring element.
+        Excludes NA values by default.
+
+        Parameters
+        ----------
+        normalize : bool, default False
+            If True then the object returned will contain the relative
+            frequencies of the unique values.
+        sort : bool, default True
+            Sort by frequencies.
+        ascending : bool, default False
+            Sort in ascending order.
+        bins : int, optional
+            Rather than count values, group them into half-open bins,
+            a convenience for ``pd.cut``, only works with numeric data.
+        dropna : bool, default True
+            Don't include counts of NaN.
+
+        Returns
+        -------
+        Series
+
+        Notes
+        -----
+        The indices of resulting object will be in descending
+        (ascending, if ascending=True) order for equal values.
+        It slightly differ from pandas where indices are located in random order.
+        """
+        return self.__constructor__(
+            query_compiler=self._query_compiler.value_counts(
+                normalize=normalize,
+                sort=sort,
+                ascending=ascending,
+                bins=bins,
+                dropna=dropna,
+            )
         )
 
     def view(self, dtype=None):

modin/pandas/test/test_general.py

@@ -319,6 +319,57 @@ def test_unique():
     assert_array_equal(modin_result, pandas_result)
 
 
+@pytest.mark.parametrize("normalize, bins, dropna", [(True, 3, False)])
+def test_value_counts(normalize, bins, dropna):
+    def sort_index_for_equal_values(result, ascending):
+        is_range = False
+        is_end = False
+        i = 0
+        new_index = np.empty(len(result), dtype=type(result.index))
+        while i < len(result):
+            j = i
+            if i < len(result) - 1:
+                while result[result.index[i]] == result[result.index[i + 1]]:
+                    i += 1
+                    if is_range is False:
+                        is_range = True
+                    if i == len(result) - 1:
+                        is_end = True
+                        break
+            if is_range:
+                k = j
+                for val in sorted(result.index[j : i + 1], reverse=not ascending):
+                    new_index[k] = val
+                    k += 1
+                if is_end:
+                    break
+                is_range = False
+            else:
+                new_index[j] = result.index[j]
+            i += 1
+        return pandas.Series(result, index=new_index)
+
+    # We sort indices for pandas result because of issue #1650
+    values = np.array([3, 1, 2, 3, 4, np.nan])
+    modin_result = pd.value_counts(values, normalize=normalize, ascending=False)
+    pandas_result = sort_index_for_equal_values(
+        pandas.value_counts(values, normalize=normalize, ascending=False), False
+    )
+    df_equals(modin_result, pandas_result)
+
+    modin_result = pd.value_counts(values, bins=bins, ascending=False)
+    pandas_result = sort_index_for_equal_values(
+        pandas.value_counts(values, bins=bins, ascending=False), False
+    )
+    df_equals(modin_result, pandas_result)
+
+    modin_result = pd.value_counts(values, dropna=dropna, ascending=True)
+    pandas_result = sort_index_for_equal_values(
+        pandas.value_counts(values, dropna=dropna, ascending=True), True
+    )
+    df_equals(modin_result, pandas_result)
+
+
 def test_to_datetime():
     # DataFrame input for to_datetime
     modin_df = pd.DataFrame({"year": [2015, 2016], "month": [2, 3], "day": [4, 5]})