Default dict example

def_dict_ex.py

from collections import defaultdict

fruits = [("berry", "strawberry"), ("citrus", "orange"), \
          ("citrus", "lemon"), ("stone fruit", "nectarine"), \
          ("berry", "blue berry") ]

# Suppose we want to build a dictionary using the first entry of each tuple as a key...

fruit_dict = {}
for tup in fruits:
  if tup[0] not in fruit_dict:
    fruit_dict[tup[0]] = [tup[1]]
  else:
    fruit_dict[tup[0]].append(tup[1])

for fruit_type, fruit in fruit_dict.iteritems():
  print fruit_type, ': ', ', '.join(fruit)

# Same implementation using defaultdict

fruit_dict = defaultdict(list)
for tup in fruits:
  fruit_dict[tup[0]].append(tup[1])

for fruit_type, fruit in fruit_dict.iteritems():
  print fruit_type, ': ', ', '.join(fruit)
  
# By way of explanation, defaultdict takes a function (acting as a default value factory).
# in this case 'list' is provided. Instead of throwing a key error if a key isn't found,
# a new key will have the initial value provided by the factory function.
# Here we've specified the function 'list', so whenever we operate on a new key, it takes
# an empty list as its default value.