Combine two dictionaries, concatenate string values?

Question

Related: Is there any pythonic way to combine two dicts (adding values for keys that appear in both)?

I'd like to merge two string:string dictionaries, and concatenate the values. The above post recommends using collections.Counter, but it doesn't handle string concatenation.

>>> from collections import Counter
>>> a = Counter({'foo':'bar', 'baz':'bazbaz'})
>>> b = Counter({'foo':'baz'})
>>> a + b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/collections.py", line 569, in __add__
TypeError: cannot concatenate 'str' and 'int' objects

(My guess is Counter tries to set b['baz'] to 0.)

I'd like to get a result of {'foo':'barbaz', 'baz':'bazbaz'}. Concatenation order doesn't matter to me. What is a clean, Pythonic way to do this?

What will be the expected output if the second dict looks like : `{'foo':'baz','spam':'eggs'}`? — Ashwini Chaudhary, Jul 11 '13 at 22:59
@AshwiniChaudhary {'foo':'barbaz', 'baz':'bazbaz', 'spam':'eggs'} — jbreed, Jul 11 '13 at 23:22
`collections.Counter` is **specifically** for dictionaries where the value is an *integer that conceptually represents a count*. It cannot help with anything else. — Karl Knechtel, Jul 27 '22 at 03:11

Ashwini Chaudhary · Accepted Answer · 2014-09-21T19:41:53.820

Dict-comprehension:

>>> d = {'foo': 'bar', 'baz': 'bazbaz'}
>>> d1 = {'foo': 'baz'}
>>> keys = d.viewkeys() | d1.viewkeys()
>>> {k : d.get(k, '') + d1.get(k, '') for k in keys}
{'foo': 'barbaz', 'baz': 'bazbaz'}

For Python 2.6 and earlier:

>>> dict((k, d.get(k, '') + d1.get(k, '')) for k in keys)
{'foo': 'barbaz', 'baz': 'bazbaz'}

This will work for any number of dicts:

def func(*dicts):
    keys = set().union(*dicts)
    return {k: "".join(dic.get(k, '') for dic in dicts)  for k in keys}
... 
>>> d = {'foo': 'bar', 'baz': 'bazbaz'}
>>> d1 = {'foo': 'baz','spam': 'eggs'}
>>> d2 = {'foo': 'foofoo', 'spam': 'bar'}
>>> func(d, d1, d2)
{'foo': 'barbazfoofoo', 'baz': 'bazbaz', 'spam': 'eggsbar'}

Jon Clements · Answer 2 · 2013-07-11T23:07:44.753

Can write a generic helper, such as:

a = {'foo':'bar', 'baz':'bazbaz'}
b = {'foo':'baz'}

def concatd(*dicts):
    if not dicts:
        return {} # or should this be None or an exception?
    fst = dicts[0]
    return {k: ''.join(d.get(k, '') for d in dicts) for k in fst}

print concatd(a, b)
# {'foo': 'barbaz', 'baz': 'bazbaz'}

c = {'foo': '**not more foo!**'}
print concatd(a, b, c)
# {'foo': 'barbaz**not more foo!**', 'baz': 'bazbaz'}

score 0 · Answer 3 · answered Dec 11 '19 at 14:30

One can use defaultdict to achieve this:

from collections import defaultdict

a = {'foo': 'bar', 'baz': 'bazbaz'}
b = {'foo': 'baz'}

new_dict = defaultdict(str)
for key, value in a.items():
    new_dict[key] += value
for key, value in b.items():
    new_dict[key] += value

print(new_dict)
# defaultdict(<class 'str'>, {'foo': 'barbaz', 'baz': 'bazbaz'})
print(dict(new_dict))
# {'foo': 'barbaz', 'baz': 'bazbaz'}

If there are many dicts to join, we could use itertools.chain.from_iterable:

from collections import defaultdict
from itertools import chain

a = {'foo': 'bar', 'baz': 'bazbaz'}
b = {'foo': 'baz'}
c = {'baz': '123'}
dicts = [a, b, c]

new_dict = defaultdict(str)
for key, value in chain.from_iterable(map(dict.items, dicts)):
    new_dict[key] += value

print(dict(new_dict))
# {'foo': 'barbaz', 'baz': 'bazbaz123'}

Combine two dictionaries, concatenate string values?

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