Mapping Keys to Multiple Values in a Dictionary

multiValuesDic.py

"""
If you want to map keys to multiple values, you need to store the multiple values in another container, like a list or set.
For example:

d = {
   'a': [1, 2, 3],
   'b': [4, 5]
   }
"""
from collections import defaultdict

# pay attention: a feature of defaultdict is that it automatically initializes the first value 
# so you can simple focus on adding items.
d = defaultdict(list)
d['a'].append(1)
d['a'].append(2)
d['a'].append(3)

"""
without defaultdict, initialization of the first value can be messy.
"""
d = {}
for key, value in pairs:
  if key not in d:
    d[key] = []
  d[key].append(value)
  
# with defaultdict, it simply leads to much cleaner code

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