Python copy & deepcopy with dicts and slots

copy_deepcopy.py

import copy
from itertools import chain


class CopyTestClass(object):
    def __init__(self, q, w):
        self._a = q
        self._b = w
        self._cache = {}

    def __copy__(self):
        # Create a new instance
        cls = self.__class__
        result = cls.__new__(cls)

        # Copy all attributes
        result.__dict__.update(self.__dict__)

        # Return updated instance
        return result

    def __deepcopy__(self, memo):
        # Create a new instance
        cls = self.__class__
        result = cls.__new__(cls)

        # Don't copy self reference
        memo[id(self)] = result

        # Don't copy the cache - if it exists
        if hasattr(self, "_cache"):
            memo[id(self._cache)] = self._cache.__new__(dict)

        # Deep copy all other attributes
        for k, v in self.__dict__.items():
            setattr(result, k, copy.deepcopy(v, memo))

        # Return updated instance
        return result


class CopyTestClassWithSlots(object):
    __slots__ = ('_a', '_b', '_cache')

    def __init__(self, q, w):
        self._a = q
        self._b = w
        self._cache = {}

    def __copy__(self):
        # Create a new instance
        cls = self.__class__
        result = cls.__new__(cls)

        # Get all __slots__ of the derived class
        slots = chain.from_iterable(getattr(s, '__slots__', []) for s in self.__class__.__mro__)
        
        # Copy all attributes
        for var in slots:
            setattr(result, var, copy.copy(getattr(self, var)))

        # Return updated instance
        return result

    def __deepcopy__(self, memo):
        # Create a new instance
        cls = self.__class__
        result = cls.__new__(cls)

        # Don't copy self reference
        memo[id(self)] = result

        # Don't copy the cache - if it exists
        if hasattr(self, "_cache"):
            memo[id(self._cache)] = self._cache.__new__(dict)

        # Get all __slots__ of the derived class
        slots = chain.from_iterable(getattr(s, '__slots__', []) for s in self.__class__.__mro__)
        
        # Deep copy all other attributes
        for var in slots:
            setattr(result, var, copy.deepcopy(getattr(self, var), memo))

        # Return updated instance
        return result


# Testing deep copy with __dict__
test1 = CopyTestClass(q=10, w=[1.0 for _ in range(10)])
test1._cache["coins"] = 120
test1c = copy.deepcopy(test1)

# Testing copy with __slots__
test2 = CopyTestClass(q=25, w=None)
test2c = copy.copy(test2)

# Testing deep copy with __slots__
test3 = CopyTestClass(q=[1.0 for _ in range(4)], w=None)
test3._b = "a string"
test3._cache['test_me'] = 1020
test3c = copy.deepcopy(test3)