As Mike Fletcher pointed out, a __deepcopy__() method should be called

with the memo as an argument.

diff --git a/Lib/copy.py b/Lib/copy.py
index 1fbd48178af536cdfbd0f47ec2c07e962b0710bd..1bdd4e15d6dd13832fefee749b4fe94af0783bb7 100644 (file)
--- a/Lib/copy.py
+++ b/Lib/copy.py
@@ -215,7 +215,7 @@ def _keep_alive(x, memo):
 
 def _deepcopy_inst(x, memo):
        if hasattr(x, '__deepcopy__'):
-               return x.__deepcopy__()
+               return x.__deepcopy__(memo)
        if hasattr(x, '__getinitargs__'):
                args = x.__getinitargs__()
                _keep_alive(args, memo)