Mapping Keys to Multiple Values in a Dictionary

Problem

You want to make a dictionary that maps keys to more than one value (a so-called “multidict”).

If you want to map keys to multiple values, you need to store the multiple values in another container such as a list or set.
To easily construct such dictionaries, you can use defaultdict in the collections module. A feature of defaultdict is that it automatically initializes the first value so you can simply focus on adding items.



In [1]:

    
from collections import defaultdict
d = defaultdict(list)
d['a'].append(1)
d['a'].append(2)
d['b'].append(4)

print(d)









    



defaultdict(<class 'list'>, {'a': [1, 2], 'b': [4]})



In [4]:

    
d = defaultdict(set)
d['a'].add(1)
d['a'].add(2)
d['a'].add(2)
d['b'].add(4)

print(d)









    



defaultdict(<class 'set'>, {'a': {1, 2}, 'b': {4}})

d = {} for key, value in pairs: if key not in d: d[key] = [] d[key].append(value)

d = defaultdict(list) for key, value in pairs: d[key].append(value)