How does collections.defaultdict work?

Question

I\'ve read the examples in python docs, but still can\'t figure out what this method means. Can somebody help? Here are two examples from the python docs

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Answer 1

I think its best used in place of a switch case statement. Imagine if we have a switch case statement as below:

option = 1

switch(option) {
    case 1: print '1st option'
    case 2: print '2nd option'
    case 3: print '3rd option'
    default: return 'No such option'
}

There is no switch case statements available in python. We can achieve the same by using defaultdict.

from collections import defaultdict

def default_value(): return "Default Value"
dd = defaultdict(default_value)

dd[1] = '1st option'
dd[2] = '2nd option'
dd[3] = '3rd option'

print(dd[4])    
print(dd[5])    
print(dd[3])

It prints:

Default Value
Default Value
3rd option

In the above snippet dd has no keys 4 or 5 and hence it prints out a default value which we have configured in a helper function. This is quite nicer than a raw dictionary where a KeyError is thrown if key is not present. From this it is evident that defaultdict more like a switch case statement where we can avoid a complicated if-elif-elif-else blocks.

One more good example that impressed me a lot from this site is:

>>> from collections import defaultdict
>>> food_list = 'spam spam spam spam spam spam eggs spam'.split()
>>> food_count = defaultdict(int) # default value of int is 0
>>> for food in food_list:
...     food_count[food] += 1 # increment element's value by 1
...
defaultdict(, {'eggs': 1, 'spam': 7})
>>>

If we try to access any items other than eggs and spam we will get a count of 0.