BUG: fix valueError for groupby with empty inputs when sample method is called (#48484)

* fix valueError for empty inputs for groupby sample * add to whatsnew/v1.6.0.rst doc * refactor test for groupby sample * fix duplicate assert in test

Author: ntachukwu

doc/source/whatsnew/v1.6.0.rst

@@ -189,7 +189,7 @@ Plotting
 
 Groupby/resample/rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^
--
+- Bug in :meth:`DataFrameGroupBy.sample` raises ``ValueError`` when the object is empty (:issue:`48459`)
 -
 
 Reshaping

pandas/core/groupby/groupby.py

@@ -4256,6 +4256,9 @@ def sample(
  2 blue 2
  0 red 0
  """ # noqa:E501
+ if self._selected_obj.empty:
+ # GH48459 prevent ValueError when object is empty
+ return self._selected_obj
  size = sample.process_sampling_size(n, frac, replace)
  if weights is not None:
  weights_arr = sample.preprocess_weights(

pandas/tests/groupby/test_sample.py

@@ -142,3 +142,13 @@ def test_groupby_sample_with_selections():
  result = df.groupby("a")[["b", "c"]].sample(n=None, frac=None)
  expected = DataFrame({"b": [1, 2], "c": [1, 2]}, index=result.index)
  tm.assert_frame_equal(result, expected)
+
+
+def test_groupby_sample_with_empty_inputs():
+ # GH48459
+ df = DataFrame({"a": [], "b": []})
+ groupby_df = df.groupby("a")
+
+ result = groupby_df.sample()
+ expected = df
+ tm.assert_frame_equal(result, expected)