Python extend for a dictionary

Question

What is the best way to extend a dictionary with another one while avoiding the use of a for loop  For instance   gt  gt  gt  a      quot a quot    1   quot b quot    2    gt  gt  gt  b      quot c quot    3   quot d quot    4    gt  gt  gt  a   a   1   b   2   gt  gt  gt  b   c   3   d   4   Result     quot a quot    1   quot b quot    2   quot c quot    3   quot d quot    4    Something like  a extend b     This does not work

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You can also use python s collections Chainmap which was introduced in python 3 3   from collections import Chainmap c   Chainmap a  b  c  a     returns 1   This has a few possible advantages  depending on your use-case  They are explained in more detail here  but I ll give a brief overview    A chainmap only uses views of the dictionaries  so no data is actually copied  This results in faster chaining  but slower lookup  No keys are actually overwritten so  if necessary  you know whether the data comes from a or b    This mainly makes it useful for things like configuration dictionaries

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a update b    Will add keys and values from b to a  overwriting if there s already a value for a key

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As others have mentioned  a update b  for some dicts a and b will achieve the result you ve asked for in your question  However  I want to point out that many times I have seen the extend method of mapping set objects desire that in the syntax a extend b   a s values should NOT be overwritten by b s values  a update b  overwrites a s values  and so isn t a good choice for extend   Note that some languages call this method defaults or inject  as it can be thought of as a way of injecting b s values  which might be a set of default values  in to a dictionary without overwriting values that might already exist   Of course  you could simple note that a extend b  is nearly the same as b update a   a b  To remove the assignment  you could do it thus   def extend a b          Create a new dictionary with a s properties extended by b      without overwriting        gt  gt  gt  extend   a  1  b  2    b  3  c  4         a   1   c   4   b   2              return dict b   a    Thanks to Tom Leys for that smart idea using a side-effect-less dict constructor for extend

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In case you need it as a Class  you can extend it with dict and use update method   Class a dict       some stuff   self update b

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Have you tried using dictionary comprehension with dictionary mapping   a     a   1   b   2  b     c   3   d   4   c      a    b    c     a   1   b   2   c   3   d   4    Another way of doing is by Using dict iterable    kwarg   c   dict a    b    c     a   1   b   2   c   3   d   4    In Python 3 9 you can add two dict using union   operator    use the merging operator   c   a   b   c     a   1   b   2   c   3   d   4

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A beautiful gem in this closed question   The  oneliner way   altering neither of the input dicts  is  basket   dict basket one    basket two    Learn what   basket two  the     means here   In case of conflict  the items from basket two will override the ones from basket one   As one-liners go  this is pretty readable and transparent  and I have no compunction against using it any time a dict that s a mix of two others comes in handy  any reader who has trouble understanding it will in fact be very well served by the way this prompts him or her towards learning about dict and the    form -    So  for example  uses like   x   mungesomedict dict adict    anotherdict     are reasonably frequent occurrences in my code   Originally submitted by Alex Martelli  Note  In Python 3  this will only work if every key in basket two is a string

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a update b    Latest Python Standard Library Documentation

[python] Python "extend" for a dictionary

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