Appending to list in Python dictionary

Question

Is there a more elegant way to write this code    What I am doing  I have keys and dates  There can be a number of dates assigned to a key and so I am creating a dictionary of lists of dates to represent this  The following code works fine  but I was hoping for a more elegant and Pythonic method    dates dict   dict    for key   date in cur      if key in dates dict          dates dict key  append date      else          dates dict key     date     I was expecting the below to work  but I keep getting a NoneType has no attribute append error    dates dict   dict    for key   date in cur      dates dict key    dates dict get key      append date     This probably has something to do with the fact that  print    append 1    None    but why

User · Answer

dates dict key    dates dict get key      append date  sets   dates dict key  to None as list append returns None   In  5   l    1 2 3   In  6   var   l append 3   In  7   print var None   You should use collections defaultdict  import collections dates dict   collections defaultdict list

User · Answer

Is there a more elegant way to write this code    Use collections defaultdict   from collections import defaultdict  dates dict   defaultdict list  for key  date in cur      dates dict key  append date

User · Answer

list append returns None  since it is an in-place operation and you are assigning it back to dates dict key   So  the next time when you do dates dict get key      append you are actually doing None append  That is why it is failing  Instead  you can simply do  dates dict setdefault key      append date    But  we have collections defaultdict for this purpose only  You can do something like this  from collections import defaultdict dates dict   defaultdict list  for key  date in cur      dates dict key  append date    This will create a new list object  if the key is not found in the dictionary   Note  Since the defaultdict will create a new list if the key is not found in the dictionary  this will have unintented side-effects  For example  if you simply want to retrieve a value for the key  which is not there  it will create a new list and return it

[python] Appending to list in Python dictionary

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