Python creating a dictionary of lists

Question

I want to create a dictionary whose values are lists   For example        1    1      2    1   2      3    2       If I do   d   dict   a     1    2   for i in a      for j in range int i   int i    2            d j  append i    I get a KeyError  because d      isn t a list   In this case  I can add the following code after the assignment of a to initialize the dictionary    for x in range 1  4       d x    list     Is there a better way to do this   Lets say I don t know the keys I am going to need until I am in the second for loop   For example   class relation      scope list   list       d   dict   for relation in relation list      for scope item in relation scope list          d scope item  append relation    An alternative would then be replacing   d scope item  append relation    with   if d has key scope item       d scope item  append relation  else      d scope item     relation     What is the best way to handle this   Ideally  appending would  just work    Is there some way to express that I want a dictionary of empty lists  even if I don t know every key when I first create the list

User · Accepted Answer

You can use defaultdict    gt  gt  gt  from collections import defaultdict  gt  gt  gt  d   defaultdict list   gt  gt  gt  a     1    2    gt  gt  gt  for i in a        for j in range int i   int i    2           d j  append i       gt  gt  gt  d defaultdict  lt type  list  gt    1    1    2    1    2    3    2      gt  gt  gt  d items     1    1      2    1    2      3    2

User · Answer

You can build it with list comprehension like this    gt  gt  gt  dict  i  range int i   int i    2   for i in   1    2      1    1  2    2    2  3     And for the second part of your question use defaultdict   gt  gt  gt  from collections import defaultdict  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

User · Answer

You can use setdefault   d   dict   a     1    2   for i in a      for j in range int i   int i    2            d setdefault j      append i   print d    prints  1    1    2    1    2    3    2      The rather oddly-named setdefault function says  Get the value with this key  or if that key isn t there  add this value and then return it    As others have rightly pointed out  defaultdict is a better and more modern choice   setdefault is still useful in older versions of Python  prior to 2 5

User · Answer

Personally  I just use JSON to convert things to strings and back   Strings I understand   import json s      yellow   1     blue   2     yellow   3     blue   4     red   1   mydict      hash   json dumps s  mydict hash     whatever  print mydict       yellow   1     blue   2     yellow   3     blue   4     red   1      whatever

User · Answer

easy way is   a    1 2  d      for i in a    d i   i     print d    1    1      2   2

User · Answer

Your question has already been answered  but IIRC you can replace lines like   if d has key scope item     with   if scope item in d    That is  d references d keys   in that construction  Sometimes defaultdict isn t the best option  for example  if you want to execute multiple lines of code after the else associated with the above if   and I find the in syntax easier to read

[python] Python creating a dictionary of lists

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