TST: Add regression tests for old issues

pandas/tests/frame/methods/test_combine_first.py

@@ -517,3 +517,12 @@ def test_combine_first_duplicates_rows_for_nan_index_values():
     )
     combined = df1.combine_first(df2)
     tm.assert_frame_equal(combined, expected)
+
+
+def test_combine_first_int64_not_cast_to_float64():
+    # GH 28613
+    df_1 = DataFrame({"A": [1, 2, 3], "B": [4, 5, 6]})
+    df_2 = DataFrame({"A": [1, 20, 30], "B": [40, 50, 60], "C": [12, 34, 65]})
+    result = df_1.combine_first(df_2)
+    expected = DataFrame({"A": [1, 2, 3], "B": [4, 5, 6], "C": [12, 34, 65]})
+    tm.assert_frame_equal(result, expected)

pandas/tests/frame/methods/test_reset_index.py

@@ -15,11 +15,13 @@
     CategoricalIndex,
     DataFrame,
     Index,
+    Interval,
     IntervalIndex,
     MultiIndex,
     RangeIndex,
     Series,
     Timestamp,
+    cut,
     date_range,
 )
 import pandas._testing as tm
@@ -682,3 +684,16 @@ def test_drop_pos_args_deprecation():
         result = df.reset_index("a", False)
     expected = DataFrame({"a": [1, 2, 3]})
     tm.assert_frame_equal(result, expected)
+
+
+def test_reset_index_interval_columns_object_cast():
+    # GH 19136
+    df = DataFrame(
+        np.eye(2), index=Index([1, 2], name="Year"), columns=cut([1, 2], [0, 1, 2])
+    )
+    result = df.reset_index()
+    expected = DataFrame(
+        [[1, 1.0, 0.0], [2, 0.0, 1.0]],
+        columns=Index(["Year", Interval(0, 1), Interval(1, 2)]),
+    )
+    tm.assert_frame_equal(result, expected)

pandas/tests/groupby/test_apply.py

@@ -1199,3 +1199,51 @@ def test_apply_index_key_error_bug(index_values):
         lambda df: Series([df["b"].mean()], index=["b_mean"])
     )
     tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "arg,idx",
+    [
+        [
+            [
+                1,
+                2,
+                3,
+            ],
+            [
+                0.1,
+                0.3,
+                0.2,
+            ],
+        ],
+        [
+            [
+                1,
+                2,
+                3,
+            ],
+            [
+                0.1,
+                0.2,
+                0.3,
+            ],
+        ],
+        [
+            [
+                1,
+                4,
+                3,
+            ],
+            [
+                0.1,
+                0.4,
+                0.2,
+            ],
+        ],
+    ],
+)
+def test_apply_nonmonotonic_float_index(arg, idx):
+    # GH 34455
+    expected = DataFrame({"col": arg}, index=idx)
+    result = expected.groupby("col").apply(lambda x: x)
+    tm.assert_frame_equal(result, expected)

pandas/tests/reshape/concat/test_categorical.py

@@ -223,3 +223,18 @@ def test_categorical_index_upcast(self):
         exp = Series([1, 2, 4, 3], index=["foo", "bar", "baz", "bar"])
 
         tm.assert_equal(res, exp)
+
+    def test_categorical_missing_from_one_frame(self):
+        # GH 25412
+        df1 = DataFrame({"f1": [1, 2, 3]})
+        df2 = DataFrame({"f1": [2, 3, 1], "f2": Series([4, 4, 4]).astype("category")})
+        result = pd.concat([df1, df2], sort=True)
+        dtype = CategoricalDtype([4])
+        expected = DataFrame(
+            {
+                "f1": [1, 2, 3, 2, 3, 1],
+                "f2": Categorical.from_codes([-1, -1, -1, 0, 0, 0], dtype=dtype),
+            },
+            index=[0, 1, 2, 0, 1, 2],
+        )
+        tm.assert_frame_equal(result, expected)

pandas/tests/scalar/timestamp/test_arithmetic.py

@@ -310,3 +310,12 @@ def test_addsub_m8ndarray_tzaware(self, shape):
         msg = r"unsupported operand type\(s\) for -: 'numpy.ndarray' and 'Timestamp'"
         with pytest.raises(TypeError, match=msg):
             other - ts
+
+    def test_subtract_different_utc_objects(self, utc_fixture, utc_fixture2):
+        # GH 32619
+        dt = datetime(2021, 1, 1)
+        ts1 = Timestamp(dt, tz=utc_fixture)
+        ts2 = Timestamp(dt, tz=utc_fixture2)
+        result = ts1 - ts2
+        expected = Timedelta(0)
+        assert result == expected