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:

Python3

# Python program to demonstrate
# dictionary


Dict = {1: 'Geeks', 2: 'For', 3: 'Geeks'}
print("Dictionary:")
print(Dict)
print(Dict[1])

# Uncommenting this print(Dict[4])
# will raise a KeyError as the
# 4 is not present in the dictionary

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 dictionary class that returns a dictionary-like object. The functionality of both dictionaries and defaultdict are almost same except for the fact that defaultdict 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:

Python3

# Python program to demonstrate
# defaultdict


from collections import defaultdict


# Function to return a default
# values for keys that is not
# present
def def_value():
    return "Not Present"

# Defining the dict
d = defaultdict(def_value)
d["a"] = 1
d["b"] = 2

print(d["a"])
print(d["b"])
print(d["c"])

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:

Python3

# 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"])

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:

Python3

# 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'))

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:

Python3

# Python program to demonstrate
# defaultdict


from collections import defaultdict


# Defining a dict
d = defaultdict(list)

for i in range(5):
    d[i].append(i)

print("Dictionary with values as list:")
print(d)

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:

Python3

# Python program to demonstrate
# defaultdict


from collections import defaultdict


# Defining the dict
d = defaultdict(int)

L = [1, 2, 3, 4, 2, 4, 1, 2]

# Iterate through the list
# for keeping the count
for i in L:

    # The default value is 0
    # so there is no need to
    # enter the key first
    d[i] += 1

print(d)

Output:

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