How does collections defaultdict work

Question

I ve read the examples in python docs  but still can t figure out what this method means  Can somebody help  Here are two examples from the python docs   gt  gt  gt  from collections import defaultdict   gt  gt  gt  s    mississippi   gt  gt  gt  d   defaultdict int   gt  gt  gt  for k in s          d k     1      gt  gt  gt  d items      i   4     p   2     s   4     m   1     and   gt  gt  gt  s      yellow   1     blue   2     yellow   3     blue   4     red   1    gt  gt  gt  d   defaultdict list   gt  gt  gt  for k  v in s          d k  append v       gt  gt  gt  d items      blue    2  4      red    1      yellow    1  3      the parameters int and list are for what

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defaultdict   The standard dictionary includes the method setdefault   for retrieving a value and establishing a default if the value does not exist  By contrast  defaultdict lets the caller specify the default value to be returned  up front when the container is initialized    as defined by Doug Hellmann in The Python Standard Library by Example  How to use defaultdict  Import defaultdict   gt  gt  gt  from collections import defaultdict   Initialize defaultdict  Initialize it by passing      callable as its first argument mandatory     gt  gt  gt  d int   defaultdict int   gt  gt  gt  d list   defaultdict list   gt  gt  gt  def foo            return  default value        gt  gt  gt  d foo   defaultdict foo   gt  gt  gt  d int defaultdict  lt type  int  gt        gt  gt  gt  d list defaultdict  lt type  list  gt        gt  gt  gt  d foo defaultdict  lt function foo at 0x7f34a0a69578 gt              kwargs as its second argument optional     gt  gt  gt  d int   defaultdict int  a 10  b 12  c 13   gt  gt  gt  d int defaultdict  lt type  int  gt     a   10   c   13   b   12     or   gt  gt  gt  kwargs     a  10  b  12  c  13   gt  gt  gt  d int   defaultdict int    kwargs   gt  gt  gt  d int defaultdict  lt type  int  gt     a   10   c   13   b   12     How does it works  As is a child class of standard dictionary  it can perform all the same functions   But in case of passing an unknown key it returns the default value instead of error  For ex    gt  gt  gt  d int  a   10  gt  gt  gt  d int  d   0  gt  gt  gt  d int defaultdict  lt type  int  gt     a   10   c   13   b   12   d   0     In case you want to change default value overwrite default factory    gt  gt  gt  d int default factory   lambda  1  gt  gt  gt  d int  e   1  gt  gt  gt  d int defaultdict  lt function  lt lambda gt  at 0x7f34a0a91578 gt     a   10   c   13   b   12   e   1   d   0     or   gt  gt  gt  def foo            return 2  gt  gt  gt  d int default factory   foo  gt  gt  gt  d int  f   2  gt  gt  gt  d int defaultdict  lt function foo at 0x7f34a0a0a140 gt     a   10   c   13   b   12   e   1   d   0   f   2     Examples in the Question  Example 1  As int has been passed as default factory  any unknown key will return 0 by default   Now as the string is passed in the loop  it will increase the count of those alphabets in d    gt  gt  gt  s    mississippi   gt  gt  gt  d   defaultdict int   gt  gt  gt  d default factory  lt type  int  gt   gt  gt  gt  for k in s          d k     1  gt  gt  gt  d items      i   4     p   2     s   4     m   1    gt  gt  gt  d defaultdict  lt type  int  gt     i   4   p   2   s   4   m   1     Example 2  As a list has been passed as default factory  any unknown non-existent  key will return     ie  list  by default   Now as the list of tuples is passed in the loop  it will append the value in the d color    gt  gt  gt  s      yellow   1     blue   2     yellow   3     blue   4     red   1    gt  gt  gt  d   defaultdict list   gt  gt  gt  d default factory  lt type  list  gt   gt  gt  gt  for k  v in s          d k  append v   gt  gt  gt  d items      blue    2  4      red    1      yellow    1  3     gt  gt  gt  d defaultdict  lt type  list  gt     blue    2  4    red    1    yellow    1  3

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In short   defaultdict int  - the argument int indicates that the values will be int type   defaultdict list  - the argument list indicates that the values will be list type

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Well  defaultdict can also raise keyerror in the following case       from collections import defaultdict     d   defaultdict       print d 3    raises keyerror   Always remember to give argument to the defaultdict like defaultdict int

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I think its best used in place of a switch case statement  Imagine if we have a switch case statement as below   option   1  switch option        case 1  print  1st option      case 2  print  2nd option      case 3  print  3rd option      default  return  No such option      There is no switch case statements available in python  We can achieve the same by using defaultdict   from collections import defaultdict  def default value    return  Default Value  dd   defaultdict default value   dd 1     1st option  dd 2     2nd option  dd 3     3rd option   print dd 4       print dd 5       print dd 3     It prints   Default Value Default Value 3rd option   In the above snippet dd has no keys 4 or 5 and hence it prints out a default value which we have configured in a helper function  This is quite nicer than a raw dictionary where a KeyError is thrown if key is not present  From this it is evident that defaultdict more like a switch case statement where we can avoid a complicated if-elif-elif-else blocks   One more good example that impressed me a lot from this site is    gt  gt  gt  from collections import defaultdict  gt  gt  gt  food list    spam spam spam spam spam spam eggs spam  split    gt  gt  gt  food count   defaultdict int    default value of int is 0  gt  gt  gt  for food in food list          food count food     1   increment element s value by 1     defaultdict  lt type  int  gt     eggs   1   spam   7    gt  gt  gt    If we try to access any items other than eggs and spam we will get a count of 0

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Usually  a Python dictionary throws a KeyError if you try to get an item with a key that is not currently in the dictionary   The defaultdict in contrast will simply create any items that you try to access  provided of course they do not exist yet    To create such a  default  item  it calls the function object that you pass to the constructor  more precisely  it s an arbitrary  callable  object  which includes function and type objects    For the first example  default items are created using int    which will return the integer object 0   For the second example  default items are created using list    which returns a new empty list object

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There is a great explanation of defaultdicts here  http   ludovf net blog python-collections-defaultdict   Basically  the parameters int and list are functions that you pass  Remember that Python accepts function names as arguments  int returns 0 by default and list returns an empty list when called with parentheses   In normal dictionaries  if in your example I try calling d a   I will get an error  KeyError   since only keys m  s  i and p exist and key a has not been initialized  But in a defaultdict  it takes a function name as an argument  when you try to use a key that has not been initialized  it simply calls the function you passed in and assigns its return value as the value of the new key

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defaultdict means that if a key is not found in the dictionary  then instead of a KeyError being thrown  a new entry is created  The type of this new entry is given by the argument of defaultdict   For example   somedict      print somedict 3     KeyError  someddict   defaultdict int  print someddict 3     print int    thus 0

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The defaultdict tool is a container in the collections class of Python  It s similar to the usual dictionary  dict  container  but it has one difference  The value fields  data type is specified upon initialization   For example   from collections import defaultdict  d   defaultdict list   d  python   append  awesome    d  something-else   append  not relevant    d  python   append  language    for i in d items         print i   This prints     python     awesome    language      something-else     not relevant

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The standard dictionary includes the method setdefault   for retrieving a value and establishing a default if the value does not exist  By contrast  defaultdict lets the caller specify the default up front when the container is initialized   import collections  def default factory        return  default value   d   collections defaultdict default factory  foo  bar   print  d    d print  foo   gt    d  foo   print  bar   gt    d  bar     This works well as long as it is appropriate for all keys to have the same default  It can be especially useful if the default is a type used for aggregating or accumulating values  such as a list  set  or even int  The standard library documentation includes several examples of using defaultdict this way     python collections defaultdict py  d  defaultdict  lt function default factory at 0x100468c80 gt     foo    bar    foo   gt  bar bar   gt  default value

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The documentation and the explanation are pretty much self-explanatory   http   docs python org library collections html collections defaultdict  The type function int str etc   passed as an argument is used to initialize a default value for any given key where the key is not present in the dict

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My own 2    you can also subclass defaultdict   class MyDict defaultdict       def   missing   self  key           value    None  None          self key    value         return value   This could come in handy for very complex cases

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The behavior of defaultdict can be easily mimicked using dict setdefault instead of d key  in every call   In other words  the code   from collections import defaultdict  d   defaultdict list   print d  key                             empty list    d  key   append 1                        adding constant 1 to the list print d  key                             list containing the constant  1    is equivalent to   d   dict    print d setdefault  key   list           empty list    d setdefault  key   list    append 1     adding constant 1 to the list print d setdefault  key   list           list containing the constant  1    The only difference is that  using defaultdict  the list constructor is called only once  and using dict setdefault the list constructor is called more often  but the code may be rewriten to avoid this  if really needed    Some may argue there is a performance consideration  but this topic is a minefield   This post shows there isn t a big performance gain in using defaultdict  for example   IMO  defaultdict is a collection that adds more confusion than benefits to the code   Useless for me  but others may think different

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Without defaultdict  you can probably assign new values to unseen keys but you cannot modify it  For example   import collections d   collections defaultdict int  for i in range 10     d i     i print d    Output  defaultdict  lt class  int  gt    0  0  1  1  2  2  3  3  4  4  5  5  6  6  7  7  8  8  9  9    import collections d      for i in range 10     d i     i print d    Output  Traceback  most recent call last   File  python   line 4  in  lt module gt  KeyError  0

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Dictionaries are a convenient way to store data for later retrieval by name  key   Keys must be unique  immutable objects  and are typically strings  The values in a dictionary can be anything  For many applications  the values are simple types such as integers and strings   It gets more interesting when the values in a dictionary are collections  lists  dicts  etc   In this case  the value  an empty list or dict  must be initialized the first time a given key is used  While this is relatively easy to do manually  the defaultdict type automates and simplifies these kinds of operations   A defaultdict works exactly like a normal dict  but it is initialized with a function     default factory     that takes no arguments and provides the default value for a nonexistent key   A defaultdict will never raise a KeyError  Any key that does not exist gets the value returned by the default factory   from collections import defaultdict ice cream   defaultdict lambda   Vanilla    ice cream  Sarah      Chunky Monkey  ice cream  Abdul      Butter Pecan   print ice cream  Sarah     gt  gt  gt Chunky Monkey  print ice cream  Joe     gt  gt  gt Vanilla   Here is another example on How using defaultdict  we can reduce complexity  from collections import defaultdict   Time complexity O n 2  def delete nth naive array  n       ans          for num in array          if ans count num   lt  n              ans append num      return ans    Time Complexity O n   using hash tables  def delete nth array n       result          counts   defaultdict int       for i in array          if counts i   lt  n              result append i              counts i     1     return result   x    1 2 3 1 2 1 2 3  print delete nth x  n 2   print delete nth naive x  n 2     In conclusion  whenever you need a dictionary  and each element   s value should start with a default value  use a defaultdict

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Since the question is about  how it works   some readers may want to see more nuts and bolts  Specifically  the method in question is the   missing   key  method  See  https   docs python org 2 library collections html defaultdict-objects    More concretely  this answer shows how to make use of   missing   key  in a practical way  https   stackoverflow com a 17956989 1593924  To clarify what  callable  means  here s an interactive session  from 2 7 6 but should work in v3 too     gt  gt  gt  x   int  gt  gt  gt  x  lt type  int  gt   gt  gt  gt  y   int 5   gt  gt  gt  y 5  gt  gt  gt  z   x 5   gt  gt  gt  z 5   gt  gt  gt  from collections import defaultdict  gt  gt  gt  dd   defaultdict int   gt  gt  gt  dd defaultdict  lt type  int  gt        gt  gt  gt  dd   defaultdict x   gt  gt  gt  dd defaultdict  lt type  int  gt        gt  gt  gt  dd  a   0  gt  gt  gt  dd defaultdict  lt type  int  gt     a   0     That was the most typical use of defaultdict  except for the pointless use of the x variable   You can do the same thing with 0 as the explicit default value  but not with a simple value    gt  gt  gt  dd2   defaultdict 0   Traceback  most recent call last     File   lt pyshell 7 gt    line 1  in  lt module gt      dd2   defaultdict 0  TypeError  first argument must be callable   Instead  the following works because it passes in a simple function  it creates on the fly a nameless function which takes no arguments and always returns 0     gt  gt  gt  dd2   defaultdict lambda  0   gt  gt  gt  dd2 defaultdict  lt function  lt lambda gt  at 0x02C4C130 gt        gt  gt  gt  dd2  a   0  gt  gt  gt  dd2 defaultdict  lt function  lt lambda gt  at 0x02C4C130 gt     a   0    gt  gt  gt     And with a different default value    gt  gt  gt  dd3   defaultdict lambda  1   gt  gt  gt  dd3 defaultdict  lt function  lt lambda gt  at 0x02C4C170 gt        gt  gt  gt  dd3  a   1  gt  gt  gt  dd3 defaultdict  lt function  lt lambda gt  at 0x02C4C170 gt     a   1    gt  gt  gt

[python] How does collections.defaultdict work?

defaultdict

How to use defaultdict

Import defaultdict

Initialize defaultdict

How does it works

Examples in the Question

Examples related to python

Examples related to dictionary

Examples related to default-value

Examples related to defaultdict