How do I initialize a dictionary of empty lists in Python

Question

My attempt to programmatically create a dictionary of lists is failing to allow me to individually address dictionary keys  Whenever I create the dictionary of lists and try to append to one key  all of them are updated  Here s a very simple test case   data      data   data fromkeys range 2      data 1  append  hello   print data   Actual result   0    hello    1    hello     Expected result   0      1    hello     Here s what works  data    0    1     data 1  append  hello   print data   Actual and Expected Result   0      1    hello     Why is the fromkeys method not working as expected

User · Accepted Answer

Passing    as second argument to dict fromkeys   gives a rather useless result     all values in the dictionary will be the same list object   In Python 2 7 or above  you can use a dicitonary comprehension instead   data    k     for k in range 2     In earlier versions of Python  you can use  data   dict  k      for k in range 2

User · Answer

You could use this   data  1      hello

User · Answer

Use defaultdict instead   from collections import defaultdict data   defaultdict list  data 1  append  hello     This way you don t have to initialize all the keys you want to use to lists beforehand   What is happening in your example is that you use one  mutable  list   alist    1  data   dict fromkeys range 2   alist  alist append 2  print data   would output  0   1  2   1   1  2

User · Answer

You are populating your dictionaries with references to a single list so when you update it  the update is reflected across all the references  Try a dictionary comprehension instead  See Create a dictionary with list comprehension in Python  d    k   v for k in blah blah blah

[python] How do I initialize a dictionary of empty lists in Python?

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