Skip to content
Related Articles

Related Articles

Improve Article
Defaultdict in Python
  • Difficulty Level : Medium
  • Last Updated : 17 May, 2020

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:




# 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 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:




# 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:




    # 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:




    # 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:




# 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:




# 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})

 Attention geek! Strengthen your foundations with the Python Programming Foundation Course and learn the basics.  

To begin with, your interview preparations Enhance your Data Structures concepts with the Python DS Course. And to begin with your Machine Learning Journey, join the Machine Learning – Basic Level Course

My Personal Notes arrow_drop_up
Recommended Articles
Page :