How can I add new keys to a dictionary

Question

Is it possible to add a key to a Python dictionary after it has been created   It doesn t seem to have an  add   method

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Let s pretend you want to live in the immutable world and do NOT want to modify the original but want to create a new dict that is the result of adding a new key to the original   In Python 3 5  you can do   params     a   1   b   2  new params      params      c   3     The Python 2 equivalent is   params     a   1   b   2  new params   dict params      c   3     After either of these   params is still equal to   a   1   b   2   and  new params is equal to   a   1   b   2   c   3   There will be times when you don t want to modify the original  you only want the result of adding to the original   I find this a refreshing alternative to the following   params     a   1   b   2  new params   params copy   new params  c     3   or  params     a   1   b   2  new params   params copy   new params update   c   3     Reference  https   stackoverflow com a 2255892 514866

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So many answers and still everybody forgot about the strangely named  oddly behaved  and yet still handy dict setdefault    This  value   my dict setdefault key  default    basically just does this   try      value   my dict key  except KeyError    key not found     value   my dict key    default   e g    gt  gt  gt  mydict     a  1   b  2   c  3   gt  gt  gt  mydict setdefault  d   4  4   returns new value at mydict  d    gt  gt  gt  print mydict    a  1   b  2   c  3   d  4    a new key value pair was indeed added   but see what happens when trying it on an existing key     gt  gt  gt  mydict setdefault  a   111  1   old value was returned  gt  gt  gt  print mydict    a  1   b  2   c  3   d  4    existing key was ignored

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The conventional syntax is d key    value  but if your keyboard is missing the square bracket keys you could also do  d   setitem   key  value   In fact  defining   getitem   and   setitem   methods is how you can make your own class support the  square bracket syntax  See https   python developpez com cours DiveIntoPython php endiveintopython object oriented framework special class methods php

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To add multiple keys simultaneously  use dict update      gt  gt  gt  x    1 2   gt  gt  gt  print x   1  2    gt  gt  gt  d    3 4  5 6  7 8   gt  gt  gt  x update d   gt  gt  gt  print x   1  2  3  4  5  6  7  8    For adding a single key  the accepted answer has less computational overhead

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add dictionary key  value class   class myDict dict        def   init   self           self   dict        def add self  key  value            self key    value   add new key and value overwriting any exiting same key         if self get key   None              print  key   key   already used     report if key already used         self setdefault key  value    if key exit do nothing      example  myd   myDict   name    fred   myd add  apples  6  print   n   myd  myd add  bananas  3  print   n   myd  myd add  jack   7  print   n   myd  myd add name  myd  print   n   myd  myd add  apples   23  print   n   myd  myd add name  2  print myd

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This popular question addresses functional methods of merging dictionaries a and b   Here are some of the more straightforward methods  tested in Python 3      c   dict  a    b      see also https   stackoverflow com q 2255878 c   dict  list a items      list b items      c   dict  i for d in  a b  for i in d items       Note  The first method above only works if the keys in b are strings   To add or modify a single element  the b dictionary would contain only that one element     c   dict  a      d   dog        returns a dictionary based on  a    This is equivalent to     def functional dict add  dictionary  key  value       temp   dictionary copy      temp key    value    return temp  c   functional dict add  a   d    dog

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I think it would also be useful to point out Python s collections module that consists of many useful dictionary subclasses and wrappers that simplify the addition and modification of data types in a dictionary  specifically defaultdict      dict subclass that calls a factory function to supply missing values   This is particularly useful if you are working with dictionaries that always consist of the same data types or structures  for example a dictionary of lists     gt  gt  gt  from collections import defaultdict  gt  gt  gt  example   defaultdict int   gt  gt  gt  example  key      1  gt  gt  gt  example  key   defaultdict  lt class  int  gt     key   1     If the key does not yet exist  defaultdict assigns the value given  in our case 10  as the initial value to the dictionary  often used inside loops   This operation therefore does two things  it adds a new key to a dictionary  as per question   and assigns the value if the key doesn t yet exist  With the standard dictionary  this would have raised an error as the    operation is trying to access a value that doesn t yet exist    gt  gt  gt  example   dict    gt  gt  gt  example  key      1 Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  KeyError   key    Without the use of defaultdict  the amount of code to add a new element would be much greater and perhaps looks something like      This type of code would often be inside a loop if  key  not in example      example  key     0    add key and initial value to dict  could also be a list example  key      1    this is implementing a counter   defaultdict can also be used with complex data types such as list and set    gt  gt  gt  example   defaultdict list   gt  gt  gt  example  key   append 1   gt  gt  gt  example defaultdict  lt class  list  gt     key    1      Adding an element automatically initialises the list

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quot Is it possible to add a key to a Python dictionary after it has been created  It doesn t seem to have an  add   method  quot   Yes it is possible  and it does have a method that implements this  but you don t want to use it directly  To demonstrate how and how not to use it  let s create an empty dict with the dict literal      my dict       Best Practice 1  Subscript notation To update this dict with a single new key and value  you can use the subscript notation  see Mappings here  that provides for item assignment  my dict  new key      new value   my dict is now    new key    new value    Best Practice 2  The update method - 2 ways We can also update the dict with multiple values efficiently as well using the update method   We may be unnecessarily creating an extra dict here  so we hope our dict has already been created and came from or was used for another purpose  my dict update   key 2    value 2    key 3    value 3     my dict is now    key 2    value 2    key 3    value 3    new key    new value    Another efficient way of doing this with the update method is with keyword arguments  but since they have to be legitimate python words  you can t have spaces or special symbols or start the name with a number  but many consider this a more readable way to create keys for a dict  and here we certainly avoid creating an extra unnecessary dict  my dict update foo  bar   foo2  baz    and my dict is now    key 2    value 2    key 3    value 3    new key    new value      foo    bar    foo2    baz    So now we have covered three Pythonic ways of updating a dict   Magic method    setitem    and why it should be avoided There s another way of updating a dict that you shouldn t use  which uses the   setitem   method  Here s an example of how one might use the   setitem   method to add a key-value pair to a dict  and a demonstration of the poor performance of using it   gt  gt  gt  d       gt  gt  gt  d   setitem    foo    bar    gt  gt  gt  d   foo    bar      gt  gt  gt  def f            d              for i in xrange 100               d  foo     i       gt  gt  gt  def g            d              for i in xrange 100               d   setitem    foo   i        gt  gt  gt  import timeit  gt  gt  gt  number   100  gt  gt  gt  min timeit repeat f  number number   0 0020880699157714844  gt  gt  gt  min timeit repeat g  number number   0 005071878433227539  So we see that using the subscript notation is actually much faster than using   setitem    Doing the Pythonic thing  that is  using the language in the way it was intended to be used  usually is both more readable and computationally efficient

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If you re not joining two dictionaries  but adding new key-value pairs to a dictionary  then using the subscript notation seems like the best way   import timeit  timeit timeit  dictionary     karga   1   darga   2   dictionary update   aaa   123123   asd   233      gt  gt  0 49582505226135254  timeit timeit  dictionary     karga   1   darga   2   dictionary  aaa     123123  dictionary  asd     233     gt  gt  0 20782899856567383   However  if you d like to add  for example  thousands of new key-value pairs  you should consider using the update   method

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Here is an easy way  your dict      your dict  someKey      someValue   This will add a new key  value pair in the your dict dictionary with key   someKey and value   somevalue You can also use this way to update the value of the key somekey if that already exists in the your dict

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your dict       To add new key   your dict key  value  your dict update key value

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dictionary key    value

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I feel like consolidating info about Python dictionaries  Creating an empty dictionary data        OR data   dict    Creating a dictionary with initial values data     a   1   b   2   c   3    OR data   dict a 1  b 2  c 3    OR data    k  v for k  v in    a   1     b  2     c  3     Inserting Updating a single value data  a     1    Updates if  a  exists  else adds  a    OR data update   a   1     OR data update dict a 1     OR data update a 1   Inserting Updating multiple values data update   c  3  d  4      Updates  c  and adds  d   Python  3 9   The update operator    now works for dictionaries  data      c  3  d  4   Creating a merged dictionary without modifying originals data3      data3 update data     Modifies data3  not data data3 update data2     Modifies data3  not data2  Python  3 5   This uses a new feature called dictionary unpacking  data      data1    data2    data3   Python  3 9   The merge operator   now works for dictionaries  data   data1     c  3  d  4   Deleting items in dictionary del data key     Removes specific element in a dictionary data pop key     Removes the key  amp  returns the value data clear      Clears entire dictionary  Check if a key is already in dictionary key in data  Iterate through pairs in a dictionary for key in data    Iterates just through the keys  ignoring the values for key  value in d items      Iterates through the pairs for key in d keys      Iterates just through key  ignoring the values for value in d values      Iterates just through value  ignoring the keys  Create a dictionary from two lists data   dict zip list with keys  list with values

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Here s another way that I didn t see here     gt  gt  gt  foo   dict a 1 b 2   gt  gt  gt  foo   a   1   b   2   gt  gt  gt  goo   dict c 3   foo   gt  gt  gt  goo   c   3   a   1   b   2    You can use the dictionary constructor and implicit expansion to reconstruct a dictionary  Moreover  interestingly  this method can be used to control the positional order during dictionary construction  post Python 3 6   In fact  insertion order is guaranteed for Python 3 7 and above    gt  gt  gt  foo   dict a 1 b 2 c 3 d 4   gt  gt  gt  new dict    k  v for k  v in list foo items     2    gt  gt  gt  new dict   a   1   b   2   gt  gt  gt  new dict update newvalue 99   gt  gt  gt  new dict   a   1   b   2   newvalue   99   gt  gt  gt  new dict update  k  v for k  v in list foo items    2      gt  gt  gt  new dict   a   1   b   2   newvalue   99   c   3   d   4   gt  gt  gt     The above is using dictionary comprehension

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first to check whether the key already exists  a  1 2 3 4  a get 1  2 a get 5  None   then you can add the new key and value

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You can create one   class myDict dict        def   init   self           self   dict        def add self  key  value           self key    value     example  myd   myDict   myd add  apples  6  myd add  bananas  3  print myd    Gives    gt  gt  gt     apples   6   bananas   3

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If you want to add a dictionary within a dictionary you can do it this way    Example  Add a new entry to your dictionary  amp  sub dictionary  dictionary      dictionary  new key      some new entry    add new dictionary entry dictionary  dictionary within a dictionary          this is required by python dictionary  dictionary within a dictionary    sub dict       other     dictionary   print  dictionary    Output     new key    some new entry    dictionary within a dictionary     sub dict     other    dictionarly       NOTE  Python requires that you first add a sub    dictionary  dictionary within a dictionary          before adding entries

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You create a new key value pair on a dictionary by assigning a value to that key d     key    value   print d       key    value    d  mynewkey      mynewvalue   print d       key    value    mynewkey    mynewvalue    If the key doesn t exist  it s added and points to that value  If it exists  the current value it points to is overwritten

[python] How can I add new keys to a dictionary?

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