Fix warnings in test_joining.py (#12304)

Contributes to #9999 and #10363.

Authors:
  - Vyas Ramasubramani (https://github.com/vyasr)

Approvers:
  - https://github.com/brandon-b-miller
  - Matthew Roeschke (https://github.com/mroeschke)

URL: #12304

python/cudf/cudf/core/join/_join_helpers.py

@@ -2,7 +2,6 @@
 
 from __future__ import annotations
 
-import warnings
 from collections import abc
 from typing import TYPE_CHECKING, Any, Tuple, cast
 
@@ -98,14 +97,8 @@ def _match_join_keys(
     ):
         common_type = max(ltype, rtype)
 
-    if how == "left":
-        if rcol.fillna(0).can_cast_safely(ltype):
-            return lcol, rcol.astype(ltype)
-        else:
-            warnings.warn(
-                f"Can't safely cast column from {rtype} to {ltype}, "
-                f"upcasting to {common_type}."
-            )
+    if how == "left" and rcol.fillna(0).can_cast_safely(ltype):
+        return lcol, rcol.astype(ltype)
 
     return lcol.astype(common_type), rcol.astype(common_type)
 

python/cudf/cudf/core/join/join.py

@@ -1,6 +1,7 @@
 # Copyright (c) 2020-2022, NVIDIA CORPORATION.
 from __future__ import annotations
 
+import warnings
 from typing import Any, ClassVar, List, Optional
 
 import cudf
@@ -411,6 +412,18 @@ def _validate_merge_params(
                         "lsuffix and rsuffix are not defined"
                     )
 
+        if (
+            isinstance(lhs, cudf.DataFrame)
+            and isinstance(rhs, cudf.DataFrame)
+            and lhs._data.nlevels != rhs._data.nlevels
+        ):
+            warnings.warn(
+                "merging between different levels is deprecated and will be "
+                f"removed in a future version. ({lhs._data.nlevels} levels on "
+                f"the left, {rhs._data.nlevels} on the right)",
+                FutureWarning,
+            )
+
 
 class MergeSemi(Merge):
     _joiner: ClassVar[staticmethod] = staticmethod(libcudf.join.semi_join)

python/cudf/cudf/tests/test_joining.py

@@ -14,6 +14,7 @@
     NUMERIC_TYPES,
     assert_eq,
     assert_exceptions_equal,
+    expect_warning_if,
 )
 
 _JOIN_TYPES = ("left", "inner", "outer", "right", "leftanti", "leftsemi")
@@ -1123,11 +1124,21 @@ def test_typecast_on_join_overflow_unsafe(dtypes):
     lhs = cudf.DataFrame({"a": [1, 2, 3, 4, 5]}, dtype=dtype_l)
     rhs = cudf.DataFrame({"a": [1, 2, 3, 4, dtype_l_max + 1]}, dtype=dtype_r)
 
-    with pytest.warns(
+    p_lhs = lhs.to_pandas()
+    p_rhs = rhs.to_pandas()
+
+    with expect_warning_if(
+        (dtype_l.kind == "f" and dtype_r.kind in {"i", "u"})
+        or (dtype_l.kind in {"i", "u"} and dtype_r.kind == "f"),
         UserWarning,
-        match=(f"Can't safely cast column" f" from {dtype_r} to {dtype_l}"),
     ):
-        merged = lhs.merge(rhs, on="a", how="left")  # noqa: F841
+        expect = p_lhs.merge(p_rhs, on="a", how="left")
+    got = lhs.merge(rhs, on="a", how="left")
+
+    # The dtypes here won't match exactly because pandas does some unsafe
+    # conversions (with a warning that we are catching above) that we don't
+    # want to match.
+    assert_join_results_equal(expect, got, how="left", check_dtype=False)
 
 
 @pytest.mark.parametrize(
@@ -2148,11 +2159,13 @@ def test_join_multiindex_empty():
     lhs = pd.DataFrame({"a": [1, 2, 3], "b": [2, 3, 4]}, index=["a", "b", "c"])
     lhs.columns = pd.MultiIndex.from_tuples([("a", "x"), ("a", "y")])
     rhs = pd.DataFrame(index=["a", "c", "d"])
-    expect = lhs.join(rhs, how="inner")
+    with pytest.warns(FutureWarning):
+        expect = lhs.join(rhs, how="inner")
 
     lhs = cudf.from_pandas(lhs)
     rhs = cudf.from_pandas(rhs)
-    got = lhs.join(rhs, how="inner")
+    with pytest.warns(FutureWarning):
+        got = lhs.join(rhs, how="inner")
 
     assert_join_results_equal(expect, got, how="inner")