hi. after using deepcopy() on a nested dict/list structure, 
i noticed that modifications to the deepcopied structure were 
affecting the original. this looks to me like a serious bug:

>>> import copy
>>> foo = { 'a':[1,2,3], 'b':{'c':[4,5]} }
>>> bar = copy.deepcopy(foo)
>>> id(foo)
4297360512
>>> id(bar)
4297373104
>>> id(foo['a'])
4299410752
>>> id(bar['a'])
4299760200
>>> id(foo['b'])
4297371984
>>> id(bar['b'])
4297373920
>>> id(foo['b']['c'])
4299721040
>>> id(bar['b']['c'])
4299761496
>>> id(foo['b']['c'][0])
4297074656
>>> id(bar['b']['c'][0])
4297074656

Why do you say "modifications to the deepcopied structure were 
affecting the original"? Your code sample doesn't include any modifications to the deepcopied structure.

Try modifying it, and watch the original remaining unchanged.

thanks for the quick response. 

that was just my working up a simplified repro, but you are 
correct: on modification there, it gets a new id() location.

totally not what i would've expected (python flags
it for copying when it changes, to save space?)

however i am still seeing the problem in my code, so
i will continue to whittle it down to a simpler repro,
and hopefully add it here later today.

Numbers are immutable, and hence don't need to be copied. In fact, it is impossible to create two int object that both have the value 4, but are different objects:

py> 2+2 is 3+1
True

learn something everyday. i have found the bug in my code, 
deepcopy() is not to blame. thx for your time!