Python defaultdict - simplest demonstration of why it matters

simple_demo_defaultdict.py

# Python 3.7 was used for this gist - it should work on many different versions (not going to enumerate them here)

# I don't really like the docs for defaultdict. maybe they're more rigorous than this definition
# i think this micro-example captures the essense of when/why to use defaultdict.


import collections

# create a normal dictionary and a defaultdict so we can compare functionality
d = {}
dd = collections.defaultdict(list)

# both dicts are empty at this point
# attempt to append a value to a key within the dict (note that the key does not yet exist in either case)
# d['x'].append('hello')   # <-- KeyError; this doesn't work...
dd['x'].append('world')    # <-- dd['x'] is now a list equal to `['world']`

# print our dicts to see what they contain
print(d)    # <-- empty because we commented it out to prevent it from erroring
print(dd)   # <-- ['world'] 


# why is this cool does this matter?
# If we want to capture a list for each key in our dictionary, then defaultdict helps so we don't have to test for key existence