How can I create a copy of an object in Python

Question

I would like to create a copy of an object  I want the new object to possess all properties of the old object  values of the fields   But I want to have independent objects  So  if I change values of the fields of the new object  the old object should not be affected by that

User · Answer

I believe the following should work with many well-behaved classed in Python:

def copy(obj):
    return type(obj)(obj)

(Of course, I am not talking here about "deep copies," which is a different story, and which may be not a very clear concept -- how deep is deep enough?)

According to my tests with Python 3, for immutable objects, like tuples or strings, it returns the same object (because there is no need to make a shallow copy of an immutable object), but for lists or dictionaries it creates an independent shallow copy.

Of course this method only works for classes whose constructors behave accordingly. Possible use cases: making a shallow copy of a standard Python container class.

User · Answer

Shallow copy with copy copy       usr bin env python3  import copy  class 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    3 assert c x     3  assert d x     3    Deep copy with copy deepcopy       usr bin env python3 import copy class C        def   init   self           self x    1          self y    2  c   C   d   copy deepcopy c  d x 0    3 assert c x     1  assert d x     3    Documentation  https   docs python org 3 library copy html  Tested on Python 3 6 5

User · Answer

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    gt  gt  gt  a list   list  abc    gt  gt  gt  a copy of a list   a list copy    gt  gt  gt  a copy of a list is a list False  gt  gt  gt  a copy of a list    a list True  Shallow Copies Shallow copies are just copies of the outermost container  list copy is a shallow copy   gt  gt  gt  list of dict of set      foo   set  abc      gt  gt  gt  lodos copy   list of dict of set copy    gt  gt  gt  lodos copy 0   foo   pop    c   gt  gt  gt  lodos copy    foo     b    a      gt  gt  gt  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   gt  gt  gt  lodos deep copy   copy deepcopy list of dict of set   gt  gt  gt  lodos deep copy 0   foo   add  c    gt  gt  gt  lodos deep copy    foo     c    b    a      gt  gt  gt  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   gt  gt  gt  a tuple   tuple  abc    gt  gt  gt  tuple copy attempt   a tuple copy   Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt  AttributeError   tuple  object has no attribute  copy   Tuples don t even have a copy method  so let s try it with a slice   gt  gt  gt  tuple copy attempt   a tuple     But we see it s the same object   gt  gt  gt  tuple copy attempt is a tuple True  Similarly for strings   gt  gt  gt  s    abc   gt  gt  gt  s0   s     gt  gt  gt  s    s0 True  gt  gt  gt  s is s0 True  and for frozensets  even though they have a copy method   gt  gt  gt  a frozenset   frozenset  abc    gt  gt  gt  frozenset copy attempt   a frozenset copy    gt  gt  gt  frozenset copy attempt is a frozenset True  When to copy immutable objects Immutable objects should be copied if you need a mutable interior object copied   gt  gt  gt  tuple of list        gt  gt  gt  copy of tuple of list   tuple of list     gt  gt  gt  copy of tuple of list 0  append  a    gt  gt  gt  copy of tuple of list    a      gt  gt  gt  tuple of list    a      gt  gt  gt  deepcopy of tuple of list   copy deepcopy tuple of list   gt  gt  gt  deepcopy of tuple of list 0  append  b    gt  gt  gt  deepcopy of tuple of list    a    b      gt  gt  gt  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  deepcopy  class 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   gt  gt  gt  c1   Copyable 1   2    And copy makes a shallow copy   gt  gt  gt  c2   copy c1   gt  gt  gt  c1 is c2 False  gt  gt  gt  c2 b append 3   gt  gt  gt  c1 b  2  3   And deepcopy now makes a deep copy   gt  gt  gt  c3   deepcopy c1   gt  gt  gt  c3 b append 4   gt  gt  gt  c1 b  2  3

User · Answer

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

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

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