How to copy a dictionary and only edit the copy

Question

Can someone please explain this to me  This doesn t make any sense to me   I copy a dictionary into another and edit the second and both are changed  Why is this happening    gt  gt  gt  dict1     key1    value1    key2    value2    gt  gt  gt  dict2   dict1  gt  gt  gt  dict2   key2    value2    key1    value1    gt  gt  gt  dict2  key2      WHY     gt  gt  gt  dict1   key2    WHY      key1    value1

User · Answer

gt  gt  gt  dict2   dict1   dict2 is bind to the same Dict object which binds to dict1  so if you modify dict2  you will modify the dict1   There are many ways to copy Dict object  I simply use  dict 1                 a  1              b  2            dict 2      dict 2 update dict 1

User · Answer

dict2   dict1 does not copy the dictionary  It simply gives you the programmer a second way  dict2  to refer to the same dictionary

User · Answer

Another cleaner way would be using json  see below code  gt  gt  gt  a      quot name quot   quot Onkar quot   quot Address quot     quot state quot   quot MH quot   quot country quot   quot India quot   quot innerAddress quot    quot city quot   quot Pune quot       gt  gt  gt  b   json dumps a   gt  gt  gt  b   json loads b   gt  gt  gt  id a  2334461105416  gt  gt  gt  id b  2334461105224  gt  gt  gt  a 0   quot Address quot    quot innerAddress quot    quot city quot    quot Nagpur quot   gt  gt  gt  a    name    Onkar    Address     state    MH    country    India    innerAddress     city    Nagpur       gt  gt  gt  b    name    Onkar    Address     state    MH    country    India    innerAddress     city    Pune       gt  gt  gt  id a 0   quot Address quot    quot innerAddress quot    2334460618376  gt  gt  gt  id b 0   quot Address quot    quot innerAddress quot    2334424569880  To create another dictionary do json dumps   and then json loads   on the same dictionary object  You will have separate dict object

User · Answer

When you assign dict2   dict1  you are not making a copy of dict1  it results in dict2 being just another name for dict1   To copy the mutable types like dictionaries  use copy   deepcopy of the copy module   import copy  dict2   copy deepcopy dict1

User · Answer

You can also just make a new dictionary with a dictionary comprehension   This avoids importing copy   dout   dict  k v  for k v in mydict items      Of course in python    2 7 you can do   dout    k v for k v in mydict items      But for backwards compat   the top method is better

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the following code  which is on dicts which follows json syntax more than 3 times faster than deepcopy  def CopyDict dSrc       try          return json loads json dumps dSrc       except Exception as e          Logger warning  Can t copy dict the preferred way   str dSrc           return deepcopy dSrc

User · Answer

dict1 is a symbol that references an underlying dictionary object  Assigning dict1 to dict2 merely assigns the same reference  Changing a key s value via the dict2 symbol changes the underlying object  which also affects dict1  This is confusing   It is far easier to reason about immutable values than references  so make copies whenever possible   person     name    Mary    age   25  one year later      person   age   26     does not mutate person dict   This is syntactically the same as   one year later   dict person  age 26

User · Answer

The best and the easiest ways to create a copy of a dict in both Python 2 7 and 3 are      To create a copy of simple single-level  dictionary   1  Using dict   method  instead of generating a reference that points to the existing dict   my dict1   dict   my dict1  message      Hello Python  print my dict1       message   Hello Python    my dict2   dict my dict1  print my dict2       message   Hello Python      Made changes in my dict1  my dict1  name      Emrit  print my dict1       message   Hello Python    name     Emrit   print my dict2       message   Hello Python     2  Using the built-in update   method of python dictionary    my dict2   dict   my dict2 update my dict1  print my dict2       message   Hello Python      Made changes in my dict1  my dict1  name      Emrit  print my dict1       message   Hello Python    name     Emrit   print my dict2       message   Hello Python     To create a copy of nested or complex dictionary   Use the built-in copy module  which provides a generic shallow and deep copy operations  This module is present in both Python 2 7 and 3    import copy  my dict2   copy deepcopy my dict1

User · Answer

As others have explained  the built-in dict does not do what you want  But in Python2  and probably 3 too  you can easily create a ValueDict class that copies with   so you can be sure that the original will not change   class ValueDict dict        def   ilshift   self  args           result   ValueDict self          if isinstance args  dict               dict update result  args          else              dict   setitem   result   args          return result   Pythonic LVALUE modification      def   irshift   self  args           result   ValueDict self          dict   delitem   result  args          return result   Pythonic LVALUE modification      def   setitem   self  k  v           raise AttributeError                 Use   value dict lt  lt    s         instead of   d  s              k k       def   delitem   self  k           raise AttributeError                 Use   value dict gt  gt    s    instead of   del d  s      k k       def update self  d2           raise AttributeError                 Use   value dict lt  lt  dict2   instead of   value dict update dict2         test d   ValueDict    d  lt  lt   apples   5 d  lt  lt   pears   8 print  d     d  e   d e  lt  lt   bananas   1 print  e     e print  d     d  d  gt  gt   pears  print  d     d d  lt  lt    blueberries   2   watermelons   315  print  d     d print  e     e print  e  bananas       e  bananas       result d     apples   5   pears   8  e     apples   5   pears   8   bananas   1  d     apples   5   pears   8  d     apples   5  d     watermelons   315   blueberries   2   apples   5  e     apples   5   pears   8   bananas   1  e  bananas     1    e 0  3   would give    AttributeError  Use  value dict lt  lt   0        instead of  d 0           Please refer to the lvalue modification pattern discussed here  Python 2 7 - clean syntax for lvalue modification  The key observation is that str and int behave as values in Python  even though they re actually immutable objects under the hood   While you re observing that  please also observe that nothing is magically special about str or int  dict can be used in much the same ways  and I can think of many cases where ValueDict makes sense

User · Answer

Every variable in python  stuff like dict1 or str or   builtins   is a pointer to some hidden platonic  object  inside the machine   If you set dict1   dict2 you just point dict1 to the same object  or memory location  or whatever analogy you like  as dict2  Now  the object referenced by dict1 is the same object referenced by dict2   You can check  dict1 is dict2 should be True  Also  id dict1  should be the same as id dict2    You want dict1   copy dict2   or dict1   deepcopy dict2    The difference between copy and deepcopy  deepcopy will make sure that the elements of dict2  did you point it at a list   are also copies   I don t use deepcopy much - it s usually poor practice to write code that needs it  in my opinion

User · Answer

On python 3 5  there is an easier way to achieve a shallow copy by using the    unpackaging operator  Defined by Pep 448    gt  gt  gt dict1     key1    value1    key2    value2    gt  gt  gt dict2      dict1   gt  gt  gt print dict2    key1    value1    key2    value2    gt  gt  gt dict2  key2      WHY     gt  gt  gt print dict1    key1    value1    key2    value2    gt  gt  gt print dict2    key1    value1    key2    WHY          unpackages the dictionary into a new dictionary that is then assigned to dict2    We can also confirm that each dictionary has a distinct id     gt  gt  gt id dict1   178192816   gt  gt  gt id dict2   178192600   If a deep copy is needed then copy deepcopy   is still the way to go

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While dict copy   and dict dict1  generates a copy  they are only shallow copies  If you want a deep copy  copy deepcopy dict1  is required  An example    gt  gt  gt  source     a   1   b     m   4   n   5   o   6    c   3   gt  gt  gt  copy1   x copy    gt  gt  gt  copy2   dict x   gt  gt  gt  import copy  gt  gt  gt  copy3   copy deepcopy x   gt  gt  gt  source  a     10    a change to first-level properties won t affect copies  gt  gt  gt  source   a   10   c   3   b     m   4   o   6   n   5    gt  gt  gt  copy1   a   1   c   3   b     m   4   o   6   n   5    gt  gt  gt  copy2   a   1   c   3   b     m   4   o   6   n   5    gt  gt  gt  copy3   a   1   c   3   b     m   4   o   6   n   5    gt  gt  gt  source  b    m     40    a change to deep properties WILL affect shallow copies  b m  property  gt  gt  gt  source   a   10   c   3   b     m   40   o   6   n   5    gt  gt  gt  copy1   a   1   c   3   b     m   40   o   6   n   5    gt  gt  gt  copy2   a   1   c   3   b     m   40   o   6   n   5    gt  gt  gt  copy3    Deep copy s  b m  property is unaffected   a   1   c   3   b     m   4   o   6   n   5     Regarding shallow vs deep copies  from the Python copy module docs      The difference between shallow and deep copying is only relevant for compound objects  objects that contain other objects  like lists or class instances           A shallow copy constructs a new compound object and then  to the extent possible  inserts references into it to the objects found in the original    A deep copy constructs a new compound object and then  recursively  inserts copies into it of the objects found in the original

User · Answer

This confused me too  initially  because I was coming from a C background   In C  a variable is a location in memory with a defined type  Assigning to a variable copies the data into the variable s memory location   But in Python  variables act more like pointers to objects  So assigning one variable to another doesn t make a copy  it just makes that variable name point to the same object

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Copying by using a for loop    orig     X2   674 5   X3   245 0   copy      for key in orig      copy key    orig key   print orig      X2   674 5   X3   245 0  print copy      X2   674 5   X3   245 0  copy  X2     808 print orig      X2   674 5   X3   245 0  print copy      X2   808   X3   245 0

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You can copy and edit the newly constructed copy in one go by calling the dict constructor with additional keyword arguments    gt  gt  gt  dict1     key1    value1    key2    value2    gt  gt  gt  dict2   dict dict1  key2  WHY      gt  gt  gt  dict1   key2    value2    key1    value1    gt  gt  gt  dict2   key2    WHY      key1    value1

User · Answer

Python never implicitly copies objects  When you set dict2   dict1  you are making them refer to the same exact dict object  so when you mutate it  all references to it keep referring to the object in its current state   If you want to copy the dict  which is rare   you have to do so explicitly with  dict2   dict dict1    or  dict2   dict1 copy

User · Answer

You can use directly   dict2   eval repr dict1     where object dict2 is an independent copy of dict1  so you can modify dict2 without affecting dict1   This works for any kind of object

User · Answer

Assignment statements in Python do not copy objects  they create bindings between a target and an object    so  dict2   dict1  it results another binding between dict2and the object that dict1 refer to   if you want to copy a dict  you can use the copy module  The copy module has two interface   copy copy x  Return a shallow copy of x   copy deepcopy x  Return a deep copy of x    The difference between shallow and deep copying is only relevant for compound objects  objects that contain other objects  like lists or class instances    A shallow copy constructs a new compound object and then  to the extent possible  inserts references into it to the objects found in the original   A deep copy constructs a new compound object and then  recursively  inserts copies into it of the objects found in the original   For example  in python 2 7 9    gt  gt  gt  import copy  gt  gt  gt  a    1 2 3 4   a    b     gt  gt  gt  b   a  gt  gt  gt  c   copy copy a   gt  gt  gt  d   copy deepcopy a   gt  gt  gt  a append 5   gt  gt  gt  a 4  append  c     and the result is    gt  gt  gt  a  1  2  3  4    a    b    c    5   gt  gt  gt  b  1  2  3  4    a    b    c    5   gt  gt  gt  c  1  2  3  4    a    b    c     gt  gt  gt  d  1  2  3  4    a    b

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i ran into a peculiar behavior when trying to deep copy dictionary property of class w o assigning it to variable  new   copy deepcopy my class a  doesn t work i e  modifying new modifies my class a  but if you do old   my class a and then new   copy deepcopy old  it works perfectly i e  modifying new does not affect my class a  I am not sure why this happens  but hope it helps save some hours

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In addition to the other provided solutions  you can use    to integrate the dictionary into an empty dictionary  e g    shallow copy of other dict      other dict     Now you will have a  shallow  copy of other dict    Applied to your example    gt  gt  gt  dict1     key1    value1    key2    value2    gt  gt  gt  dict2      dict1   gt  gt  gt  dict2   key1    value1    key2    value2    gt  gt  gt  dict2  key2      WHY     gt  gt  gt  dict1   key1    value1    key2    value2    gt  gt  gt    Pointer  Difference between shallow and deep copys

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In Depth  Whenever you do dict2   dict1  dict2 refers to dict1  Both dict1 and dict2 points to the same location in the memory  This is just a normal case while working with mutable objects in python  When you are working with mutable objects in python you must be careful as it is hard to debug  Instead of using dict2   dict1  you should be using copy shallow copy  and deepcopy method from python s copy module to separate dict2 from dict1  The correct way is   gt  gt  gt  dict1     quot key1 quot    quot value1 quot    quot key2 quot    quot value2 quot    gt  gt  gt  dict2   dict1 copy    gt  gt  gt  dict2   key1    value1    key2    value2    gt  gt  gt  dict2  quot key2 quot      quot WHY  quot   gt  gt  gt  dict2   key1    value1    key2    WHY     gt  gt  gt  dict1   key1    value1    key2    value2    gt  gt  gt  id dict1  140641178056312  gt  gt  gt  id dict2  140641176198960  gt  gt  gt    As you can see the id of both dict1 and dict2 are different  which means both are pointing referencing to different locations in the memory  This solution works for dictionaries with immutable values  this is not the correct solution for those with mutable values  Eg   gt  gt  gt  import copy  gt  gt  gt  dict1     quot key1 quot     quot value1 quot    quot key2 quot     quot mutable quot   True    gt  gt  gt  dict2   dict1 copy    gt  gt  gt  dict2   key1    value1    key2     mutable   True    gt  gt  gt  dict2  quot key2 quot    quot mutable quot     False  gt  gt  gt  dict2   key1    value1    key2     mutable   False    gt  gt  gt  dict1   key1    value1    key2     mutable   False    gt  gt  gt  id dict1  140641197660704  gt  gt  gt  id dict2  140641196407832  gt  gt  gt  id dict1  quot key2 quot    140641176198960  gt  gt  gt  id dict2  quot key2 quot    140641176198960  You can see that even though we applied copy for dict1  the value of mutable is changed to false on both dict2 and dict1 even though we only change it on dict2  This is because we changed the value of a mutable dict part of the dict1  When we apply a copy on dict  it will only do a shallow copy which means it copies all the immutable values into a new dict and does not copy the mutable values but it will reference them  The ultimate solution is to do a deepycopy of dict1 to completely create a new dict with all the values copied  including mutable values   gt  gt  gt import copy  gt  gt  gt  dict1     quot key1 quot     quot value1 quot    quot key2 quot     quot mutable quot   True    gt  gt  gt  dict2   copy deepcopy dict1   gt  gt  gt  dict2   key1    value1    key2     mutable   True    gt  gt  gt  id dict1  140641196228824  gt  gt  gt  id dict2  140641197662072  gt  gt  gt  id dict1  quot key2 quot    140641178056312  gt  gt  gt  id dict2  quot key2 quot    140641197662000  gt  gt  gt  dict2  quot key2 quot    quot mutable quot     False  gt  gt  gt  dict2   key1    value1    key2     mutable   False    gt  gt  gt  dict1   key1    value1    key2     mutable   True    As you can see  id s are different  it means that dict2 is completely a new dict with all the values in dict1  Deepcopy needs to be used if whenever you want to change any of the mutable values without affecting the original dict  If not you can use shallow copy  Deepcopy is slow as it works recursively to copy any nested values in the original dict and also takes extra memory

[python] How to copy a dictionary and only edit the copy

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