dirty fix and add integration test

Author: jdoepfert

pandas/core/algorithms.py

@@ -70,9 +70,9 @@ def _ensure_data(values, dtype=None):
             # we are actually coercing to uint64
             # until our algos support uint8 directly (see TODO)
             return np.asarray(values).astype('uint64'), 'bool', 'uint64'
-        elif is_signed_integer_dtype(values) or is_signed_integer_dtype(dtype):
+        elif is_signed_integer_dtype(values) and is_signed_integer_dtype(dtype):
             return _ensure_int64(values), 'int64', 'int64'
-        elif (is_unsigned_integer_dtype(values) or
+        elif (is_unsigned_integer_dtype(values) and
               is_unsigned_integer_dtype(dtype)):
             return _ensure_uint64(values), 'uint64', 'uint64'
         elif is_float_dtype(values) or is_float_dtype(dtype):

pandas/tests/test_algos.py

@@ -555,6 +555,19 @@ def test_empty(self, empty):
         result = algos.isin(vals, empty)
         tm.assert_numpy_array_equal(expected, result)
 
+    def test_regression_issue_19356(self):
+        # Regression test for GH19356
+        l = [-9, -0.5]
+        expected = np.array([True, False])
+
+        series_float = pd.Series([-9.0, 0.0])
+        result_float = series_float.isin(l)
+        tm.assert_numpy_array_equal(expected, result_float.values)
+
+        series_int = pd.Series([-9, 0])
+        result_int = series_int.isin(l)
+        tm.assert_numpy_array_equal(expected, result_int.values)
+
 
 class TestValueCounts(object):