feat: add nunique method to Series/DataFrameGroupby (#256)

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Fixes #<issue_number_goes_here> 🦕

Author: TrevorBergeron

bigframes/core/groupby/__init__.py

@@ -179,6 +179,9 @@ def any(self) -> df.DataFrame:
     def count(self) -> df.DataFrame:
         return self._aggregate_all(agg_ops.count_op)
 
+    def nunique(self) -> df.DataFrame:
+        return self._aggregate_all(agg_ops.nunique_op)
+
     def cumsum(self, *args, numeric_only: bool = False, **kwargs) -> df.DataFrame:
         if not numeric_only:
             self._raise_on_non_numeric("cumsum")
@@ -442,6 +445,9 @@ def max(self, *args) -> series.Series:
     def count(self) -> series.Series:
         return self._aggregate(agg_ops.count_op)
 
+    def nunique(self) -> series.Series:
+        return self._aggregate(agg_ops.nunique_op)
+
     def sum(self, *args) -> series.Series:
         return self._aggregate(agg_ops.sum_op)
 

tests/system/small/test_groupby.py

@@ -69,11 +69,13 @@ def test_dataframe_groupby_median(scalars_df_index, scalars_pandas_df_index):
     ("operator"),
     [
         (lambda x: x.count()),
+        (lambda x: x.nunique()),
         (lambda x: x.any()),
         (lambda x: x.all()),
     ],
     ids=[
         "count",
+        "nunique",
         "any",
         "all",
     ],

third_party/bigframes_vendored/pandas/core/groupby/__init__.py

@@ -363,6 +363,15 @@ def agg(self, func):
         """
         raise NotImplementedError(constants.ABSTRACT_METHOD_ERROR_MESSAGE)
 
+    def nunique(self):
+        """
+        Return number of unique elements in the group.
+
+        Returns:
+            Series: Number of unique values within each group.
+        """
+        raise NotImplementedError(constants.ABSTRACT_METHOD_ERROR_MESSAGE)
+
 
 class DataFrameGroupBy(GroupBy):
     def agg(self, func, **kwargs):
@@ -391,3 +400,12 @@ def agg(self, func, **kwargs):
             DataFrame
         """
         raise NotImplementedError(constants.ABSTRACT_METHOD_ERROR_MESSAGE)
+
+    def nunique(self):
+        """
+        Return DataFrame with counts of unique elements in each position.
+
+        Returns:
+            DataFrame
+        """
+        raise NotImplementedError(constants.ABSTRACT_METHOD_ERROR_MESSAGE)