Defaultdict is available as part of collections module in Python that overcome some drawbacks of using native dict. Let us see why and when to use a defaultdict.
Consider a scenario when you are trying to access a key which is not present in dict. It will straightaway throw a keyerror. Again, it can be explicitly handled but then there needs to be an additional check. This can be avoided by using a defaultdict.
>>> d = {"a": 1}
>>> d["a"]
1
>>> d["b"]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'b'
Above piece of code will throw a key error when a key that is not present in the dict is accessed. If defaultdict is used for the same, it will behave like this:
>>> from collections import defaultdict
>>> dd = defaultdict(dict)
>>> dd.update({"a": 1})
>>> dd["a"]
1
>>> dd["b"]
{}
In this case KeyError was not encountered. Other operations can also be done in similar fashion without worrying about KeyError.
Another example where defaultdict is a savior is when updating a nested dict in which parent is not present.
>>> d = {"fruit": {}}
>>> d["fruit"]["mango"] = 10
>>> d
{'fruit': {'mango': 10}}
>>> d["vegetable"]["potato"] = 5
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'vegetable'
When replaced with defaultdict,
>>> from collections import defaultdict
>>> dd = defaultdict(dict)
>>> dd.update({"fruit": {}})
>>> dd
defaultdict(<type 'dict'>, {'fruit': {}})
>>> dd["fruit"]["mango"] = 10
>>> dd["vegetable"]["potato"] = 5
>>> dd
defaultdict(<type 'dict'>, {'vegetable': {'potato': 5}, 'fruit': {'mango': 10}})
In this case no error was found. In fact, a new dict was created vegetable automatically with child having potato key.
So, whenever there is a possibility of getting a keyerror in any piece of code, it is better to use defaultdict instead of dict.
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