added sum() test

Author: skirpichev

Lib/test/test_builtin.py

@@ -35,6 +35,7 @@
 from test.support.import_helper import import_module
 from test.support.os_helper import (EnvironmentVarGuard, TESTFN, unlink)
 from test.support.script_helper import assert_python_ok
+from test.support.testcase import ComplexesAreIdenticalMixin
 from test.support.warnings_helper import check_warnings
 from test.support import requires_IEEE_754
 from unittest.mock import MagicMock, patch
@@ -151,7 +152,7 @@ def map_char(arg):
 def pack(*args):
     return args
 
-class BuiltinTest(unittest.TestCase):
+class BuiltinTest(ComplexesAreIdenticalMixin, unittest.TestCase):
     # Helper to check picklability
     def check_iter_pickle(self, it, seq, proto):
         itorg = it
@@ -1902,6 +1903,13 @@ def __getitem__(self, index):
         self.assertEqual(sum(xs), complex(sum(z.real for z in xs),
                                           sum(z.imag for z in xs)))
 
+        # test that sum() of complex and real numbers doesn't
+        # smash sign of imaginary 0
+        self.assertComplexesAreIdentical(sum([complex(1, -0.0), 1]),
+                                         complex(2, -0.0))
+        self.assertComplexesAreIdentical(sum([complex(1, -0.0), 1.0]),
+                                         complex(2, -0.0))
+
     @requires_IEEE_754
     @unittest.skipIf(HAVE_DOUBLE_ROUNDING,
                          "sum accuracy not guaranteed on machines with double rounding")