python: change SpaceGroup.__eq__

instead of comparing addresses, compare Hall symbols

With multiple modules/C++ libraries, we might have multiple
copies of inline variables from C++ headers and comparing
addresses won't work.

python/sym.cpp

@@ -185,9 +185,13 @@ void add_symmetry(py::module& m) {
             h(i, j) = hkl[j];
         }
     }, py::arg("miller_array").noconvert())
-    // In Python, SpaceGroup always points to an entry in spacegroup_tables::main.
-    .def("__eq__", [](const SpaceGroup& a, const SpaceGroup& b) { return &a == &b; },
-         py::is_operator())
+    // Check equality by comparing Hall symbol strings.
+    // In Python, SpaceGroup always points to an entry in spacegroup_tables::main,
+    // so Hall symbols are consistent. The same settings with different H-M names
+    // (see space group 65 in the ITB list) are considered equal.
+    .def("__eq__", [](const SpaceGroup& a, const SpaceGroup& b) {
+        return strcmp(a.hall, b.hall) == 0;
+    }, py::is_operator())
     .def("__repr__", [](const SpaceGroup &self) {
         return "<gemmi.SpaceGroup(\"" + self.xhm() + "\")>";
     })

tests/test_symmetry.py

@@ -326,17 +326,21 @@ def check_xhm(name, xhm):
         self.assertEqual(gemmi.SpaceGroup(4005).hm, 'I 1 2 1')
         self.assertIsNone(gemmi.find_spacegroup_by_name('abc'))
 
-    def test_equality(self):
-        # Tests if all functions return a reference, because __eq__ compares
-        # pointers to C++ objects.
+    def test_identity(self):
+        # Checks that all functions return a reference, not a copy
         p1 = gemmi.find_spacegroup_by_number(1)
-        self.assertEqual(gemmi.find_spacegroup_by_name('P1'), p1)
-        self.assertEqual(gemmi.SpaceGroup(1), p1)
-        self.assertEqual(gemmi.SpaceGroup('P 1'), p1)
+        self.assertTrue(gemmi.find_spacegroup_by_name('P1') is p1)
+        #self.assertTrue(gemmi.SpaceGroup(1) is p1)
+        #self.assertTrue(gemmi.SpaceGroup('P 1') is p1)
         gops = gemmi.GroupOps([gemmi.Op()])
-        self.assertEqual(gemmi.find_spacegroup_by_ops(gops), p1)
-        self.assertEqual(next(gemmi.spacegroup_table()), p1)
-        self.assertEqual(next(gemmi.spacegroup_table_itb()), p1)
+        self.assertTrue(gemmi.find_spacegroup_by_ops(gops) is p1)
+        self.assertTrue(next(gemmi.spacegroup_table()) is p1)
+        self.assertTrue(next(gemmi.spacegroup_table_itb()) is p1)
+
+        a = gemmi.SpaceGroup("C c c a:1")
+        b = gemmi.SpaceGroup("C c c b:1")
+        self.assertTrue(a == b)
+        self.assertTrue(a is not b)
 
     def test_groupops(self):
         gops = gemmi.GroupOps([gemmi.Op(t) for t in ['x, y, z',