For some context into the error:

$ python3.5 -m pdb cachecopy.py 
> /home/jaraco/cachecopy.py(1)<module>()
-> import copy
(Pdb) c
Traceback (most recent call last):
  File "/usr/lib/python3.5/pdb.py", line 1661, in main
    pdb._runscript(mainpyfile)
  File "/usr/lib/python3.5/pdb.py", line 1542, in _runscript
    self.run(statement)
  File "/usr/lib/python3.5/bdb.py", line 431, in run
    exec(cmd, globals, locals)
  File "<string>", line 1, in <module>
  File "/home/jaraco/cachecopy.py", line 1, in <module>
    import copy
  File "/usr/lib/python3.5/copy.py", line 182, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "/usr/lib/python3.5/copy.py", line 293, in _reconstruct
    y = callable(*args)
  File "/usr/lib/python3.5/copyreg.py", line 88, in __newobj__
    return cls.__new__(cls, *args)
TypeError: Required argument 'user_function' (pos 1) not found
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
> /usr/lib/python3.5/copyreg.py(88)__newobj__()
-> return cls.__new__(cls, *args)
(Pdb) cls
<class 'functools._lru_cache_wrapper'>
(Pdb) args
cls = <class 'functools._lru_cache_wrapper'>
args = ()