Slicing a dictionary

Question

I have a dictionary  and would like to pass a part of it to a function  that part being given by a list  or tuple  of keys  Like so     the dictionary d    1 2  3 4  5 6  7 8     the subset of keys I m interested in l    1 5    Now  ideally I d like to be able to do this    gt  gt  gt  d l   1 2  5 6        but that s not working  since it will look for a key named  1 5   And d 1 5  isn t even valid Python  though it seems it would be handy    I know I can do this    gt  gt  gt  dict   key  value  for key value in d iteritems   if key in l    1  2  5  6    or this    gt  gt  gt  dict   key  d key   for key in l     which is more compact     but I feel there must be a  better  way of doing this  Am I missing a more elegant solution    I m using Python 2 7

User · Answer

set intersection and dict comprehension can be used here    the dictionary d    1 2  3 4  5 6  7 8     the subset of keys I m interested in l    1 5    gt  gt  gt  key d key  for key in set l   amp  set d    1  2  5  6

User · Answer

You should be iterating over the tuple and checking if the key is in the dict not the other way around  if you don t check if the key exists and it is not in the dict you are going to get a key error   print  k d k  for k in l if k in d     Some timings     k d k  for k in set d  intersection l    In  22     timeit                         l   xrange 100000   k d k  for k in l            100 loops  best of 3  11 5 ms per loop  In  23     timeit                         l   xrange 100000   k d k  for k in set d  intersection l             10 loops  best of 3  20 4 ms per loop  In  24     timeit                         l   xrange 100000  l   set l                                 key  d key  for key in d viewkeys    amp  l            10 loops  best of 3  24 7 ms per  In  25     timeit                          l   xrange 100000   k d k  for k in l if k in d            100 loops  best of 3  17 9 ms per loop   I don t see how  k d k  for k in l  is not readable or elegant and if all elements are in d then it is pretty efficient

User · Answer

the dictionary  d    1 2  3 4  5 6  7 8    the subset of keys I m interested in  l    1 5    answer   key  d key  for key in l

User · Answer

Use a set to intersect on the dict viewkeys   dictionary view   l    1  5   key  d key  for key in d viewkeys    amp  l    This is Python 2 syntax  in Python 3 use d keys      This still uses a loop  but at least the dictionary comprehension is a lot more readable  Using set intersections is very efficient  even if d or l is large   Demo    gt  gt  gt  d    1 2  3 4  5 6  7 8   gt  gt  gt  l    1  5   gt  gt  gt   key  d key  for key in d viewkeys    amp  l   1  2  5  6

User · Answer

On Python 3 you can use the itertools islice to slice the dict items   iterator  import itertools  d    1  2  3  4  5  6   dict itertools islice d items    2     1  2  3  4    Note  this solution does not take into account specific keys  It slices by internal ordering of d  which in Python 3 7  is guaranteed to be insertion-ordered

User · Answer

Write a dict subclass that accepts a list of keys as an  item  and returns a  slice  of the dictionary   class SliceableDict dict       default   None     def   getitem   self  key           if isinstance key  list       use one return statement below               uses default value if a key does not exist             return  k  self get k  self default  for k in key                raises KeyError if a key does not exist             return  k  self k  for k in key                omits key if it does not exist             return  k  self k  for k in key if k in self          return dict get self  key    Usage   d   SliceableDict  1 2  3 4  5 6  7 8   d  1  5        1  2  5  6    Or if you want to use a separate method for this type of access  you can use   to accept any number of arguments   class SliceableDict dict       def slice self   keys           return  k  self k  for k in keys            or one of the others from the first example  d   SliceableDict  1 2  3 4  5 6  7 8   d slice 1  5         1  2  5  6  keys   1  5 d slice  keys       same

[python] Slicing a dictionary

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