In the module docstring, clarify that this is used to register pickle

support for extension types, not classes.

pickle():  If the type is a class or if the reduction function is not
           callable, raise a TypeError.

constructor():  If the constructor is not callable, raise TypeError.

This (partially) closes SourceForge patch #101859.

Lib/copy_reg.py

-"""Helper to provide extensibility for pickle/cPickle."""
+"""Helper to provide extensibility for pickle/cPickle.
+
+This is only useful to add pickle support for extension types defined in
+C, not for instances of user-defined classes.
+"""
+
+from types import ClassType as _ClassType
 
 dispatch_table = {}
 safe_constructors = {}
 
-def pickle(ob_type, pickle_function, constructor_ob = None):
+def pickle(ob_type, pickle_function, constructor_ob=None):
+    if type(ob_type) is _ClassType:
+        raise TypeError("copy_reg is not intended for use with classes")
+
+    if not callable(pickle_function):
+        raise TypeError("reduction functions must be callable")
     dispatch_table[ob_type] = pickle_function
 
     if constructor_ob is not None:
         constructor(constructor_ob)
 
 def constructor(object):
+    if not callable(object):
+        raise TypeError("constructors must be callable")
     safe_constructors[object] = 1
 
 # Example: provide pickling support for complex numbers.
@@ -18,4 +31,3 @@ def pickle_complex(c):
     return complex, (c.real, c.imag)
 
 pickle(type(1j), pickle_complex, complex)
-