how to concatenate two dictionaries to create a new one in Python

Question

Say I have three dicts   d1  1 2 3 4  d2  5 6 7 9  d3  10 8 13 22    How do I create a new d4 that combines these three dictionaries  i e    d4  1 2 3 4 5 6 7 9 10 8 13 22

User · Answer

Slowest and doesn t work in Python3  concatenate the items and call dict on the resulting list     python -mtimeit -s d1  1 2 3 4   d2  5 6 7 9   d3  10 8 13 22      d4   dict d1 items     d2 items     d3 items      100000 loops  best of 3  4 93 usec per loop  Fastest  exploit the dict constructor to the hilt  then one update     python -mtimeit -s d1  1 2 3 4   d2  5 6 7 9   d3  10 8 13 22      d4   dict d1    d2   d4 update d3    1000000 loops  best of 3  1 88 usec per loop  Middling  a loop of update calls on an initially-empty dict     python -mtimeit -s d1  1 2 3 4   d2  5 6 7 9   d3  10 8 13 22      d4        for d in  d1  d2  d3   d4 update d    100000 loops  best of 3  2 67 usec per loop  Or  equivalently  one copy-ctor and two updates     python -mtimeit -s d1  1 2 3 4   d2  5 6 7 9   d3  10 8 13 22      d4   dict d1    for d in  d2  d3   d4 update d    100000 loops  best of 3  2 65 usec per loop    I recommend approach  2   and I particularly recommend avoiding  1   which also takes up O N  extra auxiliary memory for the concatenated list of items temporary data structure

User · Answer

Here s a one-liner  imports don t count    that can easily be generalized to concatenate N dictionaries   Python 3  from itertools import chain dict chain from iterable d items   for d in  d1  d2  d3      and   from itertools import chain def dict union  args       return dict chain from iterable d items   for d in args     Python 2 6  amp  2 7  from itertools import chain dict chain from iterable d iteritems   for d in  d1  d2  d3     Output    gt  gt  gt  from itertools import chain  gt  gt  gt  d1  1 2 3 4   gt  gt  gt  d2  5 6 7 9   gt  gt  gt  d3  10 8 13 22   gt  gt  gt  dict chain from iterable d iteritems   for d in  d1  d2  d3     1  2  3  4  5  6  7  9  10  8  13  22    Generalized to concatenate N dicts   from itertools import chain def dict union  args       return dict chain from iterable d iteritems   for d in args     I m a little late to this party  I know  but I hope this helps someone

User · Answer

You can use the update   method to build a new dictionary containing all the items   dall      dall update d1  dall update d2  dall update d3    Or  in a loop   dall      for d in  d1  d2  d3     dall update d

User · Answer

Use the dict constructor  d1  1 2 3 4  d2  5 6 7 9  d3  10 8 13 22   d4   reduce lambda x y  dict x    y    d1  d2  d3     As a function  from functools import partial dict merge   partial reduce  lambda a b  dict a    b     The overhead of creating intermediate dictionaries can be eliminated by using thedict update   method   from functools import reduce def update d  other   d update other   return d d4   reduce update   d1  d2  d3

User · Answer

d4   dict d1 items     d2 items     d3 items      alternatively  and supposedly faster    d4   dict d1  d4 update d2  d4 update d3    Previous SO question that both of these answers came from is here

[python] how to concatenate two dictionaries to create a new one in Python?

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