Increment key if key already in dict

Solution 1:

A cleaner way is to use a mapping between key and a list of values for it. Use defaultdict for this. If a key isn't present, it will call the defaultfactory function* and returns it.

• here a default factory function is the list function that returns an empty list

from collections import defaultdict

# foo=[("a", "bar"), ("a", "bazz")] # some random input that maps keys to values, and keys repeat
d = defaultdict(list) # return empty list if key doesn't exist
for k, val in foo:
    d[k].append(val)
# If you don't want a simple dict instead of defaultdict
d = dict(d)
print(d)
>>>{'a': ['bar', 'bazzz']}

You can also do this without defaultdict with dict's setdefault method.

# foo=[("a", "bar"), ("a", "bazz")]


d = {}
for k, val in foo:
    # set [] as the default value for k if k doesn't already exist, and return the value of k
    d.setdefault(k,[]).append(val)

print(d)
>>>{'a': ['bar', 'bazzz']}