Deep copy of a dict in python

Question

I would like to make a deep copy of a dict in python  Unfortunately the  deepcopy   method doesn t exist for the dict  How do I do that    gt  gt  gt  my dict     a    1  2  3    b    4  5  6    gt  gt  gt  my copy   my dict deepcopy   Traceback  most recent calll last     File   lt stdin gt    line 1  in  lt module gt  AttributeError   dict  object has no attribute  deepcopy   gt  gt  gt  my copy   my dict copy    gt  gt  gt  my dict  a   2    7  gt  gt  gt  my copy  a   2  7   The last line should be 3   I would like that modifications in my dict don t impact the snapshot my copy   How do I do that  The solution should be compatible with Python 3 x

User · Answer

Python 3 x  from copy import deepcopy  my dict     one   1   two   2  new dict deepcopy   deepcopy my dict    Without deepcopy  I am unable to remove the hostname dictionary from within my domain dictionary   Without deepcopy I get the following error    RuntimeError  dictionary changed size during iteration       when I try to remove the desired element from my dictionary inside of another dictionary   import socket import xml etree ElementTree as ET from copy import deepcopy   domain is a dictionary object  def remove hostname domain  hostname       domain copy   deepcopy domain      for domains  hosts in domain copy items            for host  port in hosts items               if host    hostname                  del domain domains  host      return domain   Example output   orginal domains     localdomain     localhost     all    4000       new domains     localdomain          So what s going on here is I am iterating over a copy of a dictionary rather than iterating over the dictionary itself  With this method  you are able to remove elements as needed

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A simpler  in my view  solution is to create a new dictionary and update it with the contents of the old one   my dict   a  1   my copy       my copy update  my dict    my dict  a   2  my dict  a   Out 34   2  my copy  a   Out 35   1   The problem with this approach is it may not be  deep enough    i e  is not recursively deep   good enough for simple objects but not for nested dictionaries   Here is an example where it may not be deep enough   my dict1   b  2   my dict2   c  3   my dict3    b   my dict1   c  my dict2    my copy       my copy update  my dict3    my dict1  b    z   my copy Out 42     b     b    z     c     c   3     By using Deepcopy   I can eliminate the semi-shallow behavior  but I think one must decide which approach is right for your application   In most cases you may not care  but should be aware of the possible pitfalls    final example   import copy  my copy2   copy deepcopy  my dict3    my dict1  b    99   my copy2 Out 46     b     b    z     c     c   3

User · Answer

dict copy   is a shallow copy function for dictionary   id is built-in function that gives you the address of variable  First you need to understand  why is this particular problem is happening    In  1   my dict     a    1  2  3    b    4  5  6    In  2   my copy   my dict copy    In  3   id my dict  Out 3   140190444167808  In  4   id my copy  Out 4   140190444170328  In  5   id my copy  a    Out 5   140190444024104  In  6   id my dict  a    Out 6   140190444024104   The address of the list present in both the dicts for key  a  is pointing to same location   Therefore when you change value of the list in my dict  the list in my copy changes as well     Solution for data structure mentioned in the question   In  7   my copy    key  value    for key  value in my dict items     In  8   id my copy  a    Out 8   140190444024176   Or you can use deepcopy as mentioned above

User · Answer

I like and learned a lot from Lasse V  Karlsen  I modified it into the following example  which highlights pretty well the difference between shallow dictionary copies and deep copies       import copy      my dict     a    1  2  3    b    4  5  6       my copy   copy copy my dict      my deepcopy   copy deepcopy my dict    Now if you change      my dict  a   2    7   and do      print  my copy a 2     my copy  a   2    whereas my deepcopy a 2      my deepcopy  a   2     you get       gt  gt  my copy a 2    7  whereas my deepcopy a 2    3

User · Answer

How about   import copy d           d2   copy deepcopy d    Python 2 or 3   Python 3 2  r32 88445  Feb 20 2011  21 30 00   MSC v 1500 64 bit  AMD64   on win32 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  import copy  gt  gt  gt  my dict     a    1  2  3    b    4  5  6    gt  gt  gt  my copy   copy deepcopy my dict   gt  gt  gt  my dict  a   2    7  gt  gt  gt  my copy  a   2  3  gt  gt  gt

[python] Deep copy of a dict in python

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