Add cross functionality for join

Author: phofl

pandas/core/frame.py

@@ -8069,6 +8069,15 @@ def _join_compat(
             other = DataFrame({other.name: other})
 
         if isinstance(other, DataFrame):
+            if how == "cross":
+                return merge(
+                    self,
+                    other,
+                    how=how,
+                    on=on,
+                    suffixes=(lsuffix, rsuffix),
+                    sort=sort,
+                )
             return merge(
                 self,
                 other,

pandas/tests/reshape/merge/test_join.py

@@ -4,6 +4,7 @@
 import pandas as pd
 from pandas import DataFrame, Index, MultiIndex, Series, concat, merge
 import pandas._testing as tm
+from pandas.errors import MergeError
 from pandas.tests.reshape.merge.test_merge import NGROUPS, N, get_test_data
 
 a_ = np.array
@@ -803,3 +804,24 @@ def test_join_inner_multiindex_deterministic_order():
         index=MultiIndex.from_tuples([(2, 1, 4, 3)], names=("b", "a", "d", "c")),
     )
     tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    ("input_col", "output_cols"), [("b", ["a", "b"]), ("a", ["a_x", "a_y"])]
+)
+def test_join_cross(input_col, output_cols):
+    # GH#5401
+    left = DataFrame({"a": [1, 3]})
+    right = DataFrame({input_col: [3, 4]})
+    result = left.join(right, how="cross", lsuffix="_x", rsuffix="_y")
+    expected = DataFrame({output_cols[0]: [1, 1, 3, 3], output_cols[1]: [3, 4, 3, 4]})
+    tm.assert_frame_equal(result, expected)
+
+
+def test_join_cross_error_reporting():
+    # GH#5401
+    left = DataFrame({"a": [1, 3]})
+    right = DataFrame({"a": [3, 4]})
+    msg = "Can not pass any merge columns when using cross as merge method"
+    with pytest.raises(MergeError, match=msg):
+        left.join(right, how="cross", on="a")