How can I create a copy of an object in Python? How can I create a copy of an object in Python? python python

How can I create a copy of an object in Python?


To get a fully independent copy of an object you can use the copy.deepcopy() function.

For more details about shallow and deep copying please refer to the other answers to this question and the nice explanation in this answer to a related question.


How can I create a copy of an object in Python?

So, if I change values of the fields of the new object, the old object should not be affected by that.

You mean a mutable object then.

In Python 3, lists get a copy method (in 2, you'd use a slice to make a copy):

>>> a_list = list('abc')>>> a_copy_of_a_list = a_list.copy()>>> a_copy_of_a_list is a_listFalse>>> a_copy_of_a_list == a_listTrue

Shallow Copies

Shallow copies are just copies of the outermost container.

list.copy is a shallow copy:

>>> list_of_dict_of_set = [{'foo': set('abc')}]>>> lodos_copy = list_of_dict_of_set.copy()>>> lodos_copy[0]['foo'].pop()'c'>>> lodos_copy[{'foo': {'b', 'a'}}]>>> list_of_dict_of_set[{'foo': {'b', 'a'}}]

You don't get a copy of the interior objects. They're the same object - so when they're mutated, the change shows up in both containers.

Deep copies

Deep copies are recursive copies of each interior object.

>>> lodos_deep_copy = copy.deepcopy(list_of_dict_of_set)>>> lodos_deep_copy[0]['foo'].add('c')>>> lodos_deep_copy[{'foo': {'c', 'b', 'a'}}]>>> list_of_dict_of_set[{'foo': {'b', 'a'}}]

Changes are not reflected in the original, only in the copy.

Immutable objects

Immutable objects do not usually need to be copied. In fact, if you try to, Python will just give you the original object:

>>> a_tuple = tuple('abc')>>> tuple_copy_attempt = a_tuple.copy()Traceback (most recent call last):  File "<stdin>", line 1, in <module>AttributeError: 'tuple' object has no attribute 'copy'

Tuples don't even have a copy method, so let's try it with a slice:

>>> tuple_copy_attempt = a_tuple[:]

But we see it's the same object:

>>> tuple_copy_attempt is a_tupleTrue

Similarly for strings:

>>> s = 'abc'>>> s0 = s[:]>>> s == s0True>>> s is s0True

and for frozensets, even though they have a copy method:

>>> a_frozenset = frozenset('abc')>>> frozenset_copy_attempt = a_frozenset.copy()>>> frozenset_copy_attempt is a_frozensetTrue

When to copy immutable objects

Immutable objects should be copied if you need a mutable interior object copied.

>>> tuple_of_list = [],>>> copy_of_tuple_of_list = tuple_of_list[:]>>> copy_of_tuple_of_list[0].append('a')>>> copy_of_tuple_of_list(['a'],)>>> tuple_of_list(['a'],)>>> deepcopy_of_tuple_of_list = copy.deepcopy(tuple_of_list)>>> deepcopy_of_tuple_of_list[0].append('b')>>> deepcopy_of_tuple_of_list(['a', 'b'],)>>> tuple_of_list(['a'],)

As we can see, when the interior object of the copy is mutated, the original does not change.

Custom Objects

Custom objects usually store data in a __dict__ attribute or in __slots__ (a tuple-like memory structure.)

To make a copyable object, define __copy__ (for shallow copies) and/or __deepcopy__ (for deep copies).

from copy import copy, deepcopyclass Copyable:    __slots__ = 'a', '__dict__'    def __init__(self, a, b):        self.a, self.b = a, b    def __copy__(self):        return type(self)(self.a, self.b)    def __deepcopy__(self, memo): # memo is a dict of id's to copies        id_self = id(self)        # memoization avoids unnecesary recursion        _copy = memo.get(id_self)        if _copy is None:            _copy = type(self)(                deepcopy(self.a, memo),                 deepcopy(self.b, memo))            memo[id_self] = _copy         return _copy

Note that deepcopy keeps a memoization dictionary of id(original) (or identity numbers) to copies. To enjoy good behavior with recursive data structures, make sure you haven't already made a copy, and if you have, return that.

So let's make an object:

>>> c1 = Copyable(1, [2])

And copy makes a shallow copy:

>>> c2 = copy(c1)>>> c1 is c2False>>> c2.b.append(3)>>> c1.b[2, 3]

And deepcopy now makes a deep copy:

>>> c3 = deepcopy(c1)>>> c3.b.append(4)>>> c1.b[2, 3]


Shallow copy with copy.copy()

#!/usr/bin/env python3import copyclass C():    def __init__(self):        self.x = [1]        self.y = [2]# It copies.c = C()d = copy.copy(c)d.x = [3]assert c.x == [1]assert d.x == [3]# It's shallow.c = C()d = copy.copy(c)d.x[0] = 3assert c.x == [3]assert d.x == [3]

Deep copy with copy.deepcopy()

#!/usr/bin/env python3import copyclass C():    def __init__(self):        self.x = [1]        self.y = [2]c = C()d = copy.deepcopy(c)d.x[0] = 3assert c.x == [1]assert d.x == [3]

Documentation: https://docs.python.org/3/library/copy.html

Tested on Python 3.6.5.


matomo