python: make sere twa::acc() survives its automaton

Fixes #468.

* python/spot/__init__.py: Add wrapper around twa::acc() and
twa::get_acceptance() to store the automaton into the acceptance
proxy, therefore ensuring that the automaton survives that proxy.
* tests/python/setacc.py: Test it.

python/spot/__init__.py

@@ -137,6 +137,31 @@ def __om_init_new(self, str=None):
 option_map.__init__ = __om_init_new
 
 
+__twa__acc1 = twa.acc
+__twa__acc2 = twa.get_acceptance
+
+
+# We store the automaton into the acceptance condition
+# returned by acc so that it does not crash when
+# taking the reference to a temporary, as in
+#    spot.translate('Foo').acc()
+# Issue #468.
+def __twa_acc1_tmp(self):
+    a = __twa__acc1(self)
+    a._aut = self
+    return a
+
+
+def __twa_acc2_tmp(self):
+    a = __twa__acc2(self)
+    a._aut = self
+    return a
+
+
+twa.acc = __twa_acc1_tmp
+twa.get_acceptance = __twa_acc2_tmp
+
+
 @_extend(twa, ta)
 class twa:
     def _repr_svg_(self, opt=None):
@@ -181,7 +206,6 @@ class twa:
                 self.highlight_edge(val, color)
         return self
 
-
 @_extend(twa)
 class twa:
     def to_str(a, format='hoa', opt=None):

tests/python/setacc.py

@@ -96,3 +96,13 @@ except RuntimeError as e:
         pass
     else:
         raise e
+
+
+# issue #468
+from gc import collect
+acc = spot.translate('a').acc()
+collect()
+assert acc == spot.acc_cond('Inf(0)')
+acc = spot.translate('b').get_acceptance()
+collect()
+assert acc == spot.acc_code('Inf(0)')