Make __getinitargs__ an abstractmethod

pymbolic/primitives.py

@@ -21,6 +21,7 @@ THE SOFTWARE.
 """
 
 from sys import intern
+from abc import ABC, abstractmethod
 import pymbolic.traits as traits
 
 
@@ -180,7 +181,7 @@ def disable_subscript_by_getitem():
     pass
 
 
-class Expression:
+class Expression(ABC):
     """Superclass for parts of a mathematical expression. Overrides operators
     to implicitly construct :class:`Sum`, :class:`Product` and other expressions.
 
@@ -226,6 +227,10 @@ class Expression:
 
     # {{{ init arg names (override by subclass)
 
+    @abstractmethod
+    def __getinitargs__(self):
+        pass
+
     @property
     def init_arg_names(self):
         raise NotImplementedError
@@ -539,9 +544,6 @@ class Expression:
             self._hash_value = self.get_hash()
             return self._hash_value
 
-    def __getinitargs__(self):
-        raise NotImplementedError
-
     def __getstate__(self):
         return self.__getinitargs__()