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”).

Solution

• 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.

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]})

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}})

Discussion

• We compare 2 ways of constructing a multivalued dictionary. d = {} for key, value in pairs: if key not in d:

    d[key] = []
  

  d[key].append(value)
• The defauldict is much simpler d = defaultdict(list) for key, value in pairs: d[key].append(value)