Understanding dict copy - shallow or deep

Question

While reading up the documentation for dict copy    it says that it makes a shallow copy of the dictionary  Same goes for the book I am following  Beazley s Python Reference   which says       The m copy   method makes a shallow   copy of the items contained in a   mapping object and places them in a   new mapping object    Consider this    gt  gt  gt  original   dict a 1  b 2   gt  gt  gt  new   original copy    gt  gt  gt  new update   c   3    gt  gt  gt  original   a   1   b   2   gt  gt  gt  new   a   1   c   3   b   2    So I assumed this would update the value of original  and add  c   3  also since I was doing a shallow copy  Like if you do it for a list    gt  gt  gt  original    1  2  3   gt  gt  gt  new   original  gt  gt  gt  new append 4   gt  gt  gt  new  original   1  2  3  4    1  2  3  4     This works as expected   Since both are shallow copies  why is that the dict copy   doesn t work as I expect it to  Or my understanding of shallow vs deep copying is flawed

User · Answer

It s not a matter of deep copy or shallow copy  none of what you re doing is deep copy   Here    gt  gt  gt  new   original    you re creating a new reference to the the list dict referenced by original   while here    gt  gt  gt  new   original copy    gt  gt  gt    or  gt  gt  gt  new   list original    dict original    you re creating a new list dict which is filled with a copy of the references of objects contained in the original container

User · Answer

Adding to kennytm s answer  When you do a shallow copy parent copy   a new dictionary is created with same keys but the values are not copied they are referenced If you add a new value to parent copy it won t effect parent because parent copy is a new dictionary not reference   parent    1   1 2 3   parent copy   parent copy   parent reference   parent  print id parent  id parent copy  id parent reference   140690938288400 140690938290536 140690938288400  print id parent 1   id parent copy 1   id parent reference 1    140690938137128 140690938137128 140690938137128  parent copy 1  append 4  parent copy 2      new    print parent  parent copy  parent reference   1   1  2  3  4    1   1  2  3  4   2    new     1   1  2  3  4     The hash id  value of parent 1   parent copy 1  are identical which implies   1 2 3  of parent 1  and parent copy 1  stored at id 140690938288400   But hash of parent and parent copy are different which implies  They are different dictionaries and parent copy is a new dictionary having values reference to values of parent

User · Answer

Take this example   original   dict a 1  b 2  c dict d 4  e 5   new   original copy     Now let s change a value in the  shallow   first  level   new  a     10   new     a   10   b   2   c     d   4   e   5     original     a   1   b   2   c     d   4   e   5     no change in original  since   a   is an immutable integer   Now let s change a value one level deeper   new  c    d     40   new     a   10   b   2   c     d   40   e   5     original     a   1   b   2   c     d   40   e   5     new  c   points to the same original  d   mutable dictionary  so it will be changed

User · Answer

Contents are shallow copied   So if the original dict contains a list or another dictionary  modifying one them in the original or its shallow copy will modify them  the list or the dict  in the other

User · Answer

new  and  original  are different dicts  that s why you can update just one of them   The items are shallow-copied  not the dict itself

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By  shallow copying  it means the content of the dictionary is not copied by value  but just creating a new reference    gt  gt  gt  a    1   1 2 3    gt  gt  gt  b   a copy    gt  gt  gt  a  b   1   1  2  3     1   1  2  3     gt  gt  gt  a 1  append 4   gt  gt  gt  a  b   1   1  2  3  4     1   1  2  3  4      In contrast  a deep copy will copy all contents by value    gt  gt  gt  import copy  gt  gt  gt  c   copy deepcopy a   gt  gt  gt  a  c   1   1  2  3  4     1   1  2  3  4     gt  gt  gt  a 1  append 5   gt  gt  gt  a  c   1   1  2  3  4  5     1   1  2  3  4      So    b   a  Reference assignment  Make a and b points to the same object    b   a copy    Shallow copying  a and b will become two isolated objects  but their contents still share the same reference   b   copy deepcopy a   Deep copying  a and b s structure and content become completely isolated

User · Answer

In your second part  you should use new   original copy     copy and   are different things

[python] Understanding dict.copy() - shallow or deep?

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