How do I merge dictionaries together in Python

Question

d3   dict d1    d2    I understand that this merges the dictionary  But  is it unique  What if d1 has the same key as d2 but different value   I would like d1 and d2 to be merged  but d1 has priority if there is duplicate key

User · Answer

My solution is to define a merge function  It s not sophisticated and just cost one line  Here s the code in Python 3   from functools import reduce from operator import or   def merge  dicts       return   k  reduce lambda d  x  x get k  d   dicts  None  for k in reduce or   map lambda x  x keys    dicts   set        Tests   gt  gt  gt  d    0  0  1  1  2  4  3  9  4  16   gt  gt  gt  d letters    0   a   1   b   2   c   3   d   4   e   5   f   6   g   7   h   8   i   9   j   10   k   11   l   12   m   13   n   14   o   15   p   16   q   17   r   18   s   19   t   20   u   21   v   22   w   23   x   24   y   25   z   26   A   27   B   28   C   29   D   30   E   31   F   32   G   33   H   34   I   35   J   36   K   37   L   38   M   39   N   40   O   41   P   42   Q   43   R   44   S   45   T   46   U   47   V   48   W   49   X   50   Y   51   Z    gt  gt  gt  merge d  d letters   0   a   1   b   2   c   3   d   4   e   5   f   6   g   7   h   8   i   9   j   10   k   11   l   12   m   13   n   14   o   15   p   16   q   17   r   18   s   19   t   20   u   21   v   22   w   23   x   24   y   25   z   26   A   27   B   28   C   29   D   30   E   31   F   32   G   33   H   34   I   35   J   36   K   37   L   38   M   39   N   40   O   41   P   42   Q   43   R   44   S   45   T   46   U   47   V   48   W   49   X   50   Y   51   Z    gt  gt  gt  merge d letters  d   0  0  1  1  2  4  3  9  4  16  5   f   6   g   7   h   8   i   9   j   10   k   11   l   12   m   13   n   14   o   15   p   16   q   17   r   18   s   19   t   20   u   21   v   22   w   23   x   24   y   25   z   26   A   27   B   28   C   29   D   30   E   31   F   32   G   33   H   34   I   35   J   36   K   37   L   38   M   39   N   40   O   41   P   42   Q   43   R   44   S   45   T   46   U   47   V   48   W   49   X   50   Y   51   Z    gt  gt  gt  merge d   0  0  1  1  2  4  3  9  4  16   gt  gt  gt  merge d letters   0   a   1   b   2   c   3   d   4   e   5   f   6   g   7   h   8   i   9   j   10   k   11   l   12   m   13   n   14   o   15   p   16   q   17   r   18   s   19   t   20   u   21   v   22   w   23   x   24   y   25   z   26   A   27   B   28   C   29   D   30   E   31   F   32   G   33   H   34   I   35   J   36   K   37   L   38   M   39   N   40   O   41   P   42   Q   43   R   44   S   45   T   46   U   47   V   48   W   49   X   50   Y   51   Z    gt  gt  gt  merge        It works for arbitrary number of dictionary arguments  Were there any duplicate keys in those dictionary  the key from the rightmost dictionary in the argument list wins

User · Answer

You can use the  update   method if you don t need the original d2 any more      Update the dictionary with the key value pairs from other  overwriting existing keys  Return None    E g     gt  gt  gt  d1     a   1   b   2    gt  gt  gt  d2     b   1   c   3   gt  gt  gt  d2 update d1   gt  gt  gt  d2   a   1   c   3   b   2    Update   Of course you can copy the dictionary first in order to create a new merged one  This might or might not be necessary  In case you have compound objects  objects that contain other objects  like lists or class instances  in your dictionary  copy deepcopy should also be considered

User · Answer

If you want d1 to have priority in the conflicts  do   d3   d2 copy   d3 update d1    Otherwise  reverse d2 and d1

User · Answer

Starting in Python 3 9  the operator   creates a new dictionary with the merged keys and values from two dictionaries     d1      a   1   b   2     d2      b   1   c   3   d3   d2   d1   d3    b   2   c   3   a   1    This      Creates a new dictionary d3 with the merged keys and values of d2 and d1  The values of d1 take priority when d2 and d1 share keys      Also note the    operator which modifies d2 by merging d1 in  with priority on d1 values     d1      a   1   b   2     d2      b   1   c   3   d2    d1   d2    b   2   c   3   a   1

User · Answer

Trey Hunner has a nice blog post outlining several options for merging multiple dictionaries  including  for python3 3   ChainMap and dictionary unpacking

User · Answer

In Python2   d1   a  1  b  2  d2   a  10  c  3    d1 overrides d2    dict d2   d1      a   1   c   3   b   2    d2 overrides d1   dict d1   d2      a   10   c   3   b   2    This behavior is not just a fluke of implementation  it is guaranteed in the documentation      If a key is specified both in the   positional argument and as a keyword   argument  the value associated with   the keyword is retained in the   dictionary

User · Answer

I believe that  as stated above  using d2 update d1  is the best approach and that you can also copy d2 first if you still need it   Although  I want to point out that dict d1    d2  is actually a bad way to merge dictionnaries in general since keyword arguments need to be strings  thus it will fail if you have a dict such as       1   foo     2   bar

[python] How do I merge dictionaries together in Python?

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