CODEWITHSUNDEEP
pythonpython-3.xclassdeep-copy
Miguel
Miguel

Reputation: 1345

deepcopy override clarification

In "How to override the copy/deepcopy operations for a Python object?" post, it is written in one of the answers that:

    def __deepcopy__(self, memo):
    cls = self.__class__
    result = cls.__new__(cls)
    memo[id(self)] = result
    for k, v in self.__dict__.items():
        setattr(result, k, deepcopy(v, memo))
    return result

Is a possible way of overriding the deepcopy method. I do not understand what each of the lines does. What is the purpose of this implementation? Can't I just create my own copy method with something specific for my class like the following?

def my_copy(self, original_obj):
    obj = MyClass()
    obj.a = (copy of a from the other object)
    obj.b = (copy of b from the other object)
    ...
    return obj

Upvotes: 2

Views: 2435

Answers (1)

CodeSamurai777
CodeSamurai777

Reputation: 3355

You are right you could to this in the way you presented. However this would be specific to your object implementation. The example you posted is much more generic and could handle copying many different classes of objects. It could also be made as a mixin to easy add it to your class.

The code presented, do this by:

def __deepcopy__(self, memo):
        cls = self.__class__ # Extract the class of the object
        result = cls.__new__(cls) # Create a new instance of the object based on extracted class
        memo[id(self)] = result
        for k, v in self.__dict__.items():
            setattr(result, k, deepcopy(v, memo)) # Copy over attributes by copying directly or in case of complex objects like lists for exaample calling the `__deepcopy()__` method defined by them. Thus recursively copying the whole tree of objects.
        return result

Note also that if your class consisted of complex attributes like lists you would also need to call directly deepcopy on them or otherwise you would end up with a shallow copy for some of the attributes.

EDIT

memo is a dict, where id-to-object correspondence is kept to reconstruct complex object graphs perfectly.

Upvotes: 3

Related Questions