diff --git a/comtypes/tools/codegenerator.py b/comtypes/tools/codegenerator.py
index 4dca21e2..7aaf5b6a 100644
--- a/comtypes/tools/codegenerator.py
+++ b/comtypes/tools/codegenerator.py
@@ -1052,23 +1052,11 @@ def CoClass(self, coclass: typedesc.CoClass) -> None:
         print(file=self.stream)
         print(file=self.stream)
 
-        for itf, idlflags in coclass.interfaces:
+        for itf, _ in coclass.interfaces:
             self.generate(itf.get_head())
-        implemented = []
-        sources = []
-        for item in coclass.interfaces:
-            # item is (interface class, impltypeflags)
-            if item[1] & 2:  # IMPLTYPEFLAG_FSOURCE
-                # source interface
-                where = sources
-            else:
-                # sink interface
-                where = implemented
-            if item[1] & 1:  # IMPLTYPEFLAG_FDEAULT
-                # The default interface should be the first item on the list
-                where.insert(0, self._to_type_name(item[0]))
-            else:
-                where.append(self._to_type_name(item[0]))
+        impl, src = typedesc.groupby_impltypeflags(coclass.interfaces)
+        implemented = [self._to_type_name(itf) for itf in impl]
+        sources = [self._to_type_name(itf) for itf in src]
 
         if implemented:
             self.last_item_class = False
diff --git a/comtypes/tools/typedesc.py b/comtypes/tools/typedesc.py
index 8bf167cc..015e009c 100644
--- a/comtypes/tools/typedesc.py
+++ b/comtypes/tools/typedesc.py
@@ -2,8 +2,9 @@
 # in typedesc_base
 
 import ctypes
-from typing import Any, List, Optional, Tuple, Union as _UnionT
+from typing import Any, List, Optional, Sequence, Tuple, Union as _UnionT
 
+from comtypes import typeinfo
 from comtypes.typeinfo import ITypeLib, TLIBATTR
 from comtypes.tools.typedesc_base import *
 
@@ -195,6 +196,10 @@ def get_head(self) -> ComInterfaceHead:
         return self.itf_head
 
 
+_ImplTypeFlags = int
+_Interface = _UnionT[ComInterface, DispInterface]
+
+
 class CoClass(object):
     def __init__(
         self, name: str, clsid: str, idlflags: List[str], tlibattr: TLIBATTR
@@ -203,7 +208,31 @@ def __init__(
         self.clsid = clsid
         self.idlflags = idlflags
         self.tlibattr = tlibattr
-        self.interfaces: List[Tuple[Any, int]] = []
+        self.interfaces: List[Tuple[_Interface, _ImplTypeFlags]] = []
 
-    def add_interface(self, itf: Any, idlflags: int) -> None:
+    def add_interface(self, itf: _Interface, idlflags: _ImplTypeFlags) -> None:
         self.interfaces.append((itf, idlflags))
+
+
+_ImplementedInterfaces = Sequence[_Interface]
+_SourceInterfaces = Sequence[_Interface]
+
+
+def groupby_impltypeflags(
+    seq: Sequence[Tuple[_Interface, _ImplTypeFlags]]
+) -> Tuple[_ImplementedInterfaces, _SourceInterfaces]:
+    implemented = []
+    sources = []
+    for itf, impltypeflags in seq:
+        if impltypeflags & typeinfo.IMPLTYPEFLAG_FSOURCE:
+            # source interface
+            where = sources
+        else:
+            # sink interface
+            where = implemented
+        if impltypeflags & typeinfo.IMPLTYPEFLAG_FDEFAULT:
+            # The default interface should be the first item on the list
+            where.insert(0, itf)
+        else:
+            where.append(itf)
+    return implemented, sources