Dict merge in a dict comprehension

Question

In python 3.5, we can merge dicts by using double-splat unpacking

>>> d1 = {1: 'one', 2: 'two'}
>>> d2 = {3: 'three'}
>>> {**d1, **d2}
{1: 'one', 2: 'two', 3: 'three'}

Cool. It doesn’t seem to generalise to dynamic use cases, though:

>>> ds = [d1, d2]
>>> {**d for d in ds}
SyntaxError: dict unpacking cannot be used in dict comprehension

Instead we have to do reduce(lambda x,y: {**x, **y}, ds, {}), which seems a lot uglier. Why the “one obvious way to do it” is not allowed by the parser, when there doesn’t seem to be any ambiguity in that expression?

Asked By: wim

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Source

Answer 1

To me, the obvious way is:

d_out = {}
for d in ds:
    d_out.update(d)

This is quick and probably quite performant. I don’t know that I can speak for the python developers, but I don’t know that your expected version is more easy to read. For example, your comprehension looks more like a set-comprehension to me due to the lack of a :. FWIW, I don’t think there is any technical reason (e.g. parser ambiguity) that they couldn’t add that form of comprehension unpacking.

Apparently, these forms were proposed, but didn’t have universal enough support to warrant implementing them (yet).

Answered By: mgilson

Answer 2

It’s not exactly an answer to your question but I’d consider using ChainMap to be an idiomatic and elegant way to do what you propose (merging dictionaries in-line):

>>> from collections import ChainMap
>>> d1 = {1: 'one', 2: 'two'}
>>> d2 = {3: 'three'}
>>> ds = [d1, d2]
>>> dict(ChainMap(*ds))
{1: 'one', 2: 'two', 3: 'three'}

Although it’s not a particularly transparent solution, since many programmers might not know exactly how a ChainMap works. Note that (as @AnttiHaapala points out) "first found is used" so, depending on your intentions you might need to make a call to reversed before passing your dicts into ChainMap.

>>> d2 = {3: 'three', 2: 'LOL'}
>>> ds = [d1, d2]
>>> dict(ChainMap(*ds))
{1: 'one', 2: 'two', 3: 'three'}

>>> dict(ChainMap(*reversed(ds)))
{1: 'one', 2: 'LOL', 3: 'three'}

Answered By: machine yearning

Answer 3

You could define this function:

from collections import ChainMap
def mergeDicts(l):
    return dict(ChainMap(*reversed(list(l))))

You can then use it like this:

>>> d1 = {1: 'one', 2: 'two'}
>>> d2 = {3: 'three'}
>>> ds = [d1, d2]
>>> mergeDicts(ds)
{1: 'one', 2: 'two', 3: 'three'}

Answered By: Pierre Carbonnelle

Answer 4

You could use itertools.chain or itertools.chain.from_iterable:

import itertools

ds = [{'a': 1, 'b': 2}, {'c': 30, 'b': 40}]

merged_d = dict(itertools.chain(*(d.items() for d in ds)))
print(merged_d)  # {'a': 1, 'b': 40, 'c': 30}

Answered By: Conchylicultor

Answer 5

Based on this solution and also mentioned by @ilgia-everilä, but making it Py2 compatible and still avoiding intermediate structures. Encapsulating it inside a function makes its use quite readable.

def merge_dicts(*dicts, **extra):
    """
    >>> merge_dicts(dict(a=1, b=1), dict(b=2, c=2), dict(c=3, d=3), d=4, e=4)
    {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 4}
    """
    return dict((
        (k,v)
        for d in dicts
        for k,v in d.items()
    ), **extra)

Answered By: vokimon

Answer 6

Idiomatic, without ChainMap:

>>> d1 = {1: 'one', 2: 'two'}
>>> d2 = {3: 'three'}
>>> {k: v for d in [d1, d2] for k, v in d.items()}
{1: 'one', 2: 'two', 3: 'three'}

Answered By: Lars Blumberg

Dict merge in a dict comprehension

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