[ENH] Method for adding functionality to pandas groupby

CHANGELOG.md

@@ -2,7 +2,9 @@
 
 ## [Unreleased]
 -   [ENH] Added `row_count` parameter for janitor.conditional_join - Issue #1269 @samukweku
--   [ENG] Reverse deprecation of `pivot_wider()` -- Issue #1464
+-   [ENH] Reverse deprecation of `pivot_wider()` -- Issue #1464
+-   [ENH] Add accessor and method for pandas DataFrameGroupBy objects. - Issue #587 @samukweku
+
 ## [v0.31.0] - 2025-03-07
 
 -   [ENH] Added support for pd.Series.select - Issue #1394 @samukweku

janitor/functions/select.py

@@ -327,17 +327,18 @@ def select_rows(
     return _select(df, rows=list(args), invert=invert)
 
 
+@pf.register_groupby_method
 @pf.register_dataframe_method
 @pf.register_series_method
 @deprecated_alias(rows="index")
 def select(
-    df: pd.DataFrame | pd.Series,
+    df: pd.DataFrame | pd.Series | DataFrameGroupBy,
     *args: tuple,
     index: Any = None,
     columns: Any = None,
     axis: str = "columns",
     invert: bool = False,
-) -> pd.DataFrame | pd.Series:
+) -> pd.DataFrame | pd.Series | DataFrameGroupBy:
     """Method-chainable selection of rows and/or columns.
 
     It accepts a string, shell-like glob strings `(*string*)`,
@@ -371,6 +372,8 @@ def select(
             - `rows` keyword deprecated in favour of `index`.
         - 0.31.0
             - Add support for pd.Series.
+        - 0.32.0
+            - Add support for DataFrameGroupBy.
 
     Examples:
         >>> import pandas as pd
@@ -436,6 +439,10 @@ def select(
     Returns:
         A pandas DataFrame or Series with the specified rows and/or columns selected.
     """  # noqa: E501
+    if args and isinstance(df, DataFrameGroupBy):
+        return get_columns(group=df, label=list(args))
+    if isinstance(df, DataFrameGroupBy):
+        return get_columns(group=df, label=[columns])
     if args:
         check("invert", invert, [bool])
         if (index is not None) or (columns is not None):
@@ -478,6 +485,12 @@ def get_index_labels(
     return index[_select_index(arg, df, axis)]
 
 
+@refactored_function(
+    message=(
+        "This function will be deprecated in a 1.x release. "
+        "Please use `jn.select` instead."
+    )
+)
 def get_columns(
     group: DataFrameGroupBy | SeriesGroupBy, label: Any
 ) -> DataFrameGroupBy | SeriesGroupBy:
@@ -488,6 +501,11 @@ def get_columns(
 
     !!! info "New in version 0.25.0"
 
+    !!!note
+
+        This function will be deprecated in a 1.x release.
+        Please use `jn.select` instead.
+
     Args:
         group: A Pandas GroupBy object.
         label: column(s) to select.

tests/functions/test_select_columns.py

@@ -479,6 +479,20 @@ def test_select_groupby(dataframe):
     assert_frame_equal(expected, actual)
 
 
+def test_select_groupby_args(dataframe):
+    """Test output on a grouped object"""
+    expected = dataframe.select_dtypes("number").groupby(dataframe["a"]).sum()
+    actual = dataframe.groupby("a").select(is_numeric_dtype).sum()
+    assert_frame_equal(expected, actual)
+
+
+def test_select_groupby_columns(dataframe):
+    """Test output on a grouped object"""
+    expected = dataframe.select_dtypes("number").groupby(dataframe["a"]).sum()
+    actual = dataframe.groupby("a").select(columns=is_numeric_dtype).sum()
+    assert_frame_equal(expected, actual)
+
+
 def test_select_str_multiindex(multiindex):
     """Test str selection on a MultiIndex - exact match"""
     expected = multiindex.select_columns("bar")