BUG: Keep float dtype in merge on int and float column

Author: reidy-p

pandas/core/reshape/merge.py

@@ -906,13 +906,20 @@ def _maybe_coerce_merge_keys(self):
                 continue
 
             # if we are numeric, then allow differing
-            # kinds to proceed, eg. int64 and int8
+            # kinds to proceed, eg. int64 and int8, int and float
             # further if we are object, but we infer to
             # the same, then proceed
             if is_numeric_dtype(lk) and is_numeric_dtype(rk):
                 if lk.dtype.kind == rk.dtype.kind:
                     continue
 
+                # check whether ints and floats
+                if is_integer_dtype(rk) and is_float_dtype(lk):
+                    continue
+
+                if is_float_dtype(rk) and is_integer_dtype(lk):
+                    continue
+
                 # let's infer and see if we are ok
                 if lib.infer_dtype(lk) == lib.infer_dtype(rk):
                     continue

pandas/tests/reshape/test_merge.py

@@ -13,7 +13,11 @@
 from pandas.core.reshape.merge import merge, MergeError
 from pandas.util.testing import assert_frame_equal, assert_series_equal
 from pandas.core.dtypes.dtypes import CategoricalDtype
-from pandas.core.dtypes.common import is_categorical_dtype, is_object_dtype
+from pandas.core.dtypes.common import (
+    is_categorical_dtype,
+    is_object_dtype,
+    is_float_dtype,
+)
 from pandas import DataFrame, Index, MultiIndex, Series, Categorical
 import pandas.util.testing as tm
 from pandas.api.types import CategoricalDtype as CDT
@@ -1408,6 +1412,25 @@ def test_join_multi_dtypes(self, d1, d2):
         expected.sort_values(['k1', 'k2'], kind='mergesort', inplace=True)
         tm.assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize('int_vals, float_vals', [
+        ([1, 2, 3], [1.0, 2.0, 3.0]),
+        ([1, 2, 3], [1.0, 3.0]),
+        ([1, 2], [1.0, 2.0, 3.0]),
+        ([1, 2, 3], [1.1, 2.5, 3.0]),
+    ])
+    def test_merge_on_ints_floats(self, int_vals, float_vals):
+        # GH 16572
+        # Check that float column is not cast to object if
+        # merging on float and int
+        A = DataFrame({'X': int_vals})
+        B = DataFrame({'Y': float_vals})
+
+        res = A.merge(B, left_on='X', right_on='Y')
+        assert is_float_dtype(res['Y'].dtype)
+
+        res = B.merge(A, left_on='Y', right_on='X')
+        assert is_float_dtype(res['Y'].dtype)
+
 
 @pytest.fixture
 def left():