https://docs.python.org/<version>/library/copy.html

"Two problems often exist with deep copy operations that don’t exist with shallow copy operations:

Recursive objects (compound objects that, directly or indirectly, contain a reference to themselves) may cause a recursive loop.
Because deep copy copies everything it may copy too much, e.g., administrative data structures that should be shared even between copies."

I believe the last line in above paragraph is missing a 'not' ahead of 'be shared'. 

It should read:
"administrative data structures that should be not shared even between copies"