Python: make the cofactor methods return Optionals as described in th…

…e docstrings

Also don't raise a `MemoryError` for terminals but actually return None

bindings/python/oxidd/bcdd.py

@@ -196,20 +196,25 @@ def manager(self) -> BCDDManager:
         return BCDDManager._from_raw(_lib.oxidd_bcdd_containing_manager(self._func))
 
     @override
-    def cofactors(self) -> tuple[Self, Self]:
+    def cofactors(self) -> Optional[tuple[Self, Self]]:
         raw_pair = _lib.oxidd_bcdd_cofactors(self._func)
+        if raw_pair.first._p == _ffi.NULL:
+            return None
+        assert raw_pair.second._p != _ffi.NULL
         return (
             self.__class__._from_raw(raw_pair.first),
             self.__class__._from_raw(raw_pair.second),
         )
 
     @override
-    def cofactor_true(self) -> Self:
-        return self.__class__._from_raw(_lib.oxidd_bcdd_cofactor_true(self._func))
+    def cofactor_true(self) -> Optional[Self]:
+        raw = _lib.oxidd_bcdd_cofactor_true(self._func)
+        return self.__class__._from_raw(raw) if raw._p != _ffi.NULL else None
 
     @override
-    def cofactor_false(self) -> Self:
-        return self.__class__._from_raw(_lib.oxidd_bcdd_cofactor_false(self._func))
+    def cofactor_false(self) -> Optional[Self]:
+        raw = _lib.oxidd_bcdd_cofactor_false(self._func)
+        return self.__class__._from_raw(raw) if raw._p != _ffi.NULL else None
 
     @override
     def level(self) -> Optional[int]:

bindings/python/oxidd/bdd.py

@@ -194,20 +194,25 @@ def manager(self) -> BDDManager:
         return BDDManager._from_raw(_lib.oxidd_bdd_containing_manager(self._func))
 
     @override
-    def cofactors(self) -> tuple[Self, Self]:
+    def cofactors(self) -> Optional[tuple[Self, Self]]:
         raw_pair = _lib.oxidd_bdd_cofactors(self._func)
+        if raw_pair.first._p == _ffi.NULL:
+            return None
+        assert raw_pair.second._p != _ffi.NULL
         return (
             self.__class__._from_raw(raw_pair.first),
             self.__class__._from_raw(raw_pair.second),
         )
 
     @override
-    def cofactor_true(self) -> Self:
-        return self.__class__._from_raw(_lib.oxidd_bdd_cofactor_true(self._func))
+    def cofactor_true(self) -> Optional[Self]:
+        raw = _lib.oxidd_bdd_cofactor_true(self._func)
+        return self.__class__._from_raw(raw) if raw._p != _ffi.NULL else None
 
     @override
-    def cofactor_false(self) -> Self:
-        return self.__class__._from_raw(_lib.oxidd_bdd_cofactor_false(self._func))
+    def cofactor_false(self) -> Optional[Self]:
+        raw = _lib.oxidd_bdd_cofactor_false(self._func)
+        return self.__class__._from_raw(raw) if raw._p != _ffi.NULL else None
 
     @override
     def level(self) -> Optional[int]:

bindings/python/oxidd/protocols.py

@@ -131,7 +131,7 @@ class BooleanFunction(Function, Protocol):
     """Boolean function represented as decision diagram"""
 
     @abstractmethod
-    def cofactors(self) -> tuple[Self, Self]:
+    def cofactors(self) -> Optional[tuple[Self, Self]]:
         r"""Get the cofactors ``(f_true, f_false)`` of ``self``
 
         Let f(x₀, …, xₙ) be represented by ``self``, where x₀ is (currently) the
@@ -157,7 +157,7 @@ def cofactors(self) -> tuple[Self, Self]:
         raise NotImplementedError
 
     @abstractmethod
-    def cofactor_true(self) -> Self:
+    def cofactor_true(self) -> Optional[Self]:
         """Get the cofactor ``f_true`` of ``self``
 
         This method is slightly more efficient than :meth:`Self::cofactors` in
@@ -172,7 +172,7 @@ def cofactor_true(self) -> Self:
         raise NotImplementedError
 
     @abstractmethod
-    def cofactor_false(self) -> Self:
+    def cofactor_false(self) -> Optional[Self]:
         """Get the cofactor ``f_true`` of ``self``
 
         This method is slightly more efficient than :meth:`Self::cofactors` in

bindings/python/oxidd/zbdd.py

@@ -192,20 +192,25 @@ def manager(self) -> ZBDDManager:
         return ZBDDManager._from_raw(_lib.oxidd_zbdd_containing_manager(self._func))
 
     @override
-    def cofactors(self) -> tuple[Self, Self]:
+    def cofactors(self) -> Optional[tuple[Self, Self]]:
         raw_pair = _lib.oxidd_zbdd_cofactors(self._func)
+        if raw_pair.first._p == _ffi.NULL:
+            return None
+        assert raw_pair.second._p != _ffi.NULL
         return (
             self.__class__._from_raw(raw_pair.first),
             self.__class__._from_raw(raw_pair.second),
         )
 
     @override
-    def cofactor_true(self) -> Self:
-        return self.__class__._from_raw(_lib.oxidd_zbdd_cofactor_true(self._func))
+    def cofactor_true(self) -> Optional[Self]:
+        raw = _lib.oxidd_zbdd_cofactor_true(self._func)
+        return self.__class__._from_raw(raw) if raw._p != _ffi.NULL else None
 
     @override
-    def cofactor_false(self) -> Self:
-        return self.__class__._from_raw(_lib.oxidd_zbdd_cofactor_false(self._func))
+    def cofactor_false(self) -> Optional[Self]:
+        raw = _lib.oxidd_zbdd_cofactor_false(self._func)
+        return self.__class__._from_raw(raw) if raw._p != _ffi.NULL else None
 
     @override
     def level(self) -> Optional[int]: