Defaultdict in Python

Dictionary in Python is an unordered collection of data values that are used to store data values like a map. Unlike other Data Types that hold only single value as an element, the Dictionary holds key:value pair. In Dictionary, the key must be unique and immutable. This means that a Python Tuple can be a key whereas a Python List can not. A Dictionary can be created by placing a sequence of elements within curly {} braces, separated by ‘comma’.

Example:

Output:

Dictionary:
{1: 'Geeks', 2: 'For', 3: 'Geeks'}
Geeks

Traceback (most recent call last):
  File "/home/1ca83108cc81344dc7137900693ced08.py", line 11, in 
    print(Dict[4])
KeyError: 4

Sometimes, when the KeyError is raised, it might become a problem. To overcome this Python introduces another dictionary like container known as Defaultdict which is present inside the collections module.

Note: For more information, refer to Python Dictionary.

DefaultDict

Defaultdict is a container like dictionaries present in the module collections. Defaultdict is a sub-class of the dict class that returns a dictionary-like object. The functionality of both dictionaries and defualtdict are almost same except for the fact that defualtdict never raises a KeyError. It provides a default value for the key that does not exists.

Syntax: defaultdict(default_factory)

Parameters:

• default_factory: A function returning the default value for the dictionary defined. If this argument is absent then the dictionary raises a KeyError.

Example:

Output:

1
2
Not Present

Inner Working of defaultdict

Defaultdict adds one writable instance variable and one method in addition to the standard dictionary operations. The instance variable is the default_factory parameter and the method provided is __missing__.

• Default_factory: It is a function returning the default value for the dictionary defined. If this argument is absent then the dictionary raises a KeyError.

  Example:

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  # Python program to demonstrate # default_factory argument of  # defaultdict       from collections import defaultdict           # Defining the dict and passing  # lambda as default_factory argument d = defaultdict(lambda: "Not Present") d["a"] = 1 d["b"] = 2    print(d["a"]) print(d["b"]) print(d["c"])

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  Output:

  
  
  
  

  1
  2
  Not Present
  

• __missing__(): This function is used to provide the default value for the dictionary. This function takes default_factory as an argument and if this argument is None, a KeyError is raised otherwise it provides a default value for the given key. This method is basically called by the __getitem__() method of the dict class when the requested key is not found. __getitem__() raises or return the value returned by the __missing__(). method.

  Example:

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  # Python program to demonstrate # defaultdict       from collections import defaultdict           # Defining the dict d = defaultdict(lambda: "Not Present") d["a"] = 1 d["b"] = 2    # Provides the default value  # for the key print(d.__missing__('a')) print(d.__missing__('d'))

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  Output:

  Not Present
  Not Present
  

Using List as default_factory

When the list class is passed as the default_factory argument, then a defaultdict is created with the values that are list.

Example:

Output:

Dictionary with values as list:
defaultdict(<class 'list'>, {0: [0], 1: [1], 2: [2], 3: [3], 4: [4]})

Using int as default_factory

When the int class is passed as the default_factory argument, then a defaultdict is created with default value as zero.

Example:

Output:

defaultdict(<class 'int'>, {1: 2, 2: 3, 3: 1, 4: 2})

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