Python dunders __copy__ and __deepcopy__

They are invoked, but not directly by the language runtime, but by the copy and deepcopy functions of the copy module of the library.

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At the end of the documentation it is explained:

  

In order for a class to define its own copy implementation, it can   define special methods __copy__() and __deepcopy__() . The former is   called to implement the shallow copy operation; no additional   arguments are passed. The latter is called to implement the deep copy   operation; it is passed one argument, the memo dictionary. If the    __deepcopy__() implementation needs to make a deep copy of a component, it should call the deepcopy () function with the component   the first argument and the memo dictionary the second argument.

Translating:

  

"For a class to define its own copy implementation, it can   define the special methods __copy__() and __deepcopy__() . O   first it is called for the operation of copy race; no argument   is passed. The latter is called to implement the   deep copy; is passed an argument, the dictionary of "memo." If the    __deepcopy__() implementation needs to do a deep copy   of some other component, it should call the deepcopy() function with   the first component and the "memo" dictionary as second   argument "

It is not mandatory for any system or code base to use the copy.copy of the default library to copy its own objects, but if it does, the protocol is already set - and if everyone use, because of that protocol, the copies will work fine. If you try to use django, and copying an instantiated object without using copy.copy may have problems by not passing through the special methods defined in the objects.

Oh, just to demonstrate the calls, in the ipython terminal you can do:

In [19]: from copy import copy, deepcopy

In [20]: class Teste:
    ...:     def __copy__(self):
    ...:         print("copy")
    ...:         return Teste()
    ...:     def __deepcopy__(self, memo=None):
    ...:         print("deepcopy")
    ...:         return Teste()
    ...:     

In [21]: copy(Teste())
copy
Out[21]: <__main__.Teste at 0x7f6c19e97fd0>

In [22]: deepcopy(Teste())
deepcopy
Out[22]: <__main__.Teste at 0x7f6c19e99e48>