Python "extend" for a dictionary


What is the best way to extend a dictionary with another one while avoiding the use of a for loop? For instance:

>>> a = { "a" : 1, "b" : 2 }
>>> b = { "c" : 3, "d" : 4 }
>>> a
{'a': 1, 'b': 2}
>>> b
{'c': 3, 'd': 4}


{ "a" : 1, "b" : 2, "c" : 3, "d" : 4 }

Something like:

a.extend(b)  # This does not work
Asked By: FerranB




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

Answered By: vartec

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.

Answered By: Tom Leys

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.

    >>> 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.

Answered By: eblume

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}
Answered By: Vlad Bezden

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.

Answered By: Ivo Merchiers

Notice that since Python 3.9 a much easier syntax was introduced (Union Operators):

d1 = {'a': 1}
d2 = {'b': 2}

extended_dict = d1 | d2
>> {'a':1, 'b': 2}

Pay attention: in case first dict shared keys with second dict, position matters!

d1 = {'b': 1}
d2 = {'b': 2}
d1 | d2 
>> {'b': 2} 

Relevant PEP

Answered By: Aaron_ab

In terms of efficiency, it seems faster to use the unpack operation, compared with the update method.

Here an image of a test I did:
enter image description here

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