Don't reindex an new value on setitem if the original dataframe was e…

…mpty (#8026)

This PR resolves #8023. The pandas compatibility patch in #7948 (and the backport to 0.19 in #7957) also needs to handle the case of assigning to an empty DataFrame.

Url: #8026

python/cudf/cudf/core/dataframe.py

@@ -7953,7 +7953,7 @@ def _setitem_with_dataframe(
             "Number of Input Columns must be same replacement Dataframe"
         )
 
-    if not input_df.index.equals(replace_df.index):
+    if len(input_df) != 0 and not input_df.index.equals(replace_df.index):
         replace_df = replace_df.reindex(input_df.index)
 
     for col_1, col_2 in zip(input_cols, replace_df.columns):

python/cudf/cudf/tests/test_dataframe.py

@@ -1070,6 +1070,13 @@ def test_dataframe_setitem_index_len1():
     np.testing.assert_equal(gdf.b.to_array(), [0])
 
 
+def test_empty_dataframe_setitem_df():
+    gdf1 = cudf.DataFrame()
+    gdf2 = cudf.DataFrame({"a": [1, 2, 3, 4, 5]})
+    gdf1["a"] = gdf2["a"]
+    assert_eq(gdf1, gdf2)
+
+
 def test_assign():
     gdf = cudf.DataFrame({"x": [1, 2, 3]})
     gdf2 = gdf.assign(y=gdf.x + 1)