BUG: fix Series.where(cond) when cond is empty (#34595)

Author: simonjayhawkins

doc/source/whatsnew/v1.1.0.rst

@@ -1013,6 +1013,7 @@ Reshaping
 - Ensure only named functions can be used in :func:`eval()` (:issue:`32460`)
 - Bug in :func:`Dataframe.aggregate` and :func:`Series.aggregate` was causing recursive loop in some cases (:issue:`34224`)
 - Fixed bug in :func:`melt` where melting MultiIndex columns with ``col_level`` > 0 would raise a ``KeyError`` on ``id_vars`` (:issue:`34129`)
+- Bug in :meth:`Series.where` with an empty Series and empty ``cond`` having non-bool dtype (:issue:`34592`)
 
 Sparse
 ^^^^^^

pandas/core/generic.py

@@ -8831,16 +8831,17 @@ def _where(
 
         msg = "Boolean array expected for the condition, not {dtype}"
 
-        if not isinstance(cond, ABCDataFrame):
-            # This is a single-dimensional object.
-            if not is_bool_dtype(cond):
-                raise ValueError(msg.format(dtype=cond.dtype))
-        elif not cond.empty:
-            for dt in cond.dtypes:
-                if not is_bool_dtype(dt):
-                    raise ValueError(msg.format(dtype=dt))
+        if not cond.empty:
+            if not isinstance(cond, ABCDataFrame):
+                # This is a single-dimensional object.
+                if not is_bool_dtype(cond):
+                    raise ValueError(msg.format(dtype=cond.dtype))
+            else:
+                for dt in cond.dtypes:
+                    if not is_bool_dtype(dt):
+                        raise ValueError(msg.format(dtype=dt))
         else:
-            # GH#21947 we have an empty DataFrame, could be object-dtype
+            # GH#21947 we have an empty DataFrame/Series, could be object-dtype
             cond = cond.astype(bool)
 
         cond = -cond if inplace else cond

pandas/tests/series/indexing/test_where.py

@@ -443,3 +443,10 @@ def test_where_sparse():
     result = ser.where(ser >= 2, 0)
     expected = pd.Series(pd.arrays.SparseArray([0, 2]))
     tm.assert_series_equal(result, expected)
+
+
+def test_where_empty_series_and_empty_cond_having_non_bool_dtypes():
+    # https://github.com/pandas-dev/pandas/issues/34592
+    ser = Series([], dtype=float)
+    result = ser.where([])
+    tm.assert_series_equal(result, ser)