Fix .transform crash when SeriesGroupBy is empty (#26208)

doc/source/whatsnew/v0.25.0.rst

@@ -391,6 +391,7 @@ Groupby/Resample/Rolling
 - Bug in :meth:`pandas.core.window.Rolling.min` and :meth:`pandas.core.window.Rolling.max` that caused a memory leak (:issue:`25893`)
 - Bug in :meth:`pandas.core.groupby.GroupBy.idxmax` and :meth:`pandas.core.groupby.GroupBy.idxmin` with datetime column would return incorrect dtype (:issue:`25444`, :issue:`15306`)
 - Bug in :meth:`pandas.core.groupby.GroupBy.cumsum`, :meth:`pandas.core.groupby.GroupBy.cumprod`, :meth:`pandas.core.groupby.GroupBy.cummin` and :meth:`pandas.core.groupby.GroupBy.cummax` with categorical column having absent categories, would return incorrect result or segfault (:issue:`16771`)
+- Bug in :meth:`pandas.core.groupby.SeriesGroupBy.transform` where transforming an empty SeriesGroupBy would raise error. Now returns empty Series, just like .apply and .agg. (:issue:`26208`)
 
 
 Reshaping

pandas/core/groupby/generic.py

@@ -917,8 +917,11 @@ def transform(self, func, *args, **kwargs):
             s = klass(res, indexer)
             results.append(s)
 
-        from pandas.core.reshape.concat import concat
-        result = concat(results).sort_index()
+        if results:
+            from pandas.core.reshape.concat import concat
+            result = concat(results).sort_index()
+        else:
+            result = Series([])
 
         # we will only try to coerce the result type if
         # we have a numeric dtype, as these are *always* udfs

pandas/tests/groupby/test_transform.py

@@ -880,3 +880,13 @@ def test_transform_absent_categories(func):
     result = getattr(df.y.groupby(df.x), func)()
     expected = df.y
     assert_series_equal(result, expected)
+
+
+def test_transform_empty_df():
+    # 26208
+    # handle empty SeriesGroupBy
+    d = pd.DataFrame({1: [], 2: []})
+    g = d.groupby(1)
+    result = g[2].transform(lambda x: x)
+    expected = pd.Series([], name=2)
+    assert_series_equal(result, expected)