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

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})

这个factory_function可以是list、set、str等等,当key不存在时,返回的是factory_function的默认值,比如list对应[ ],str对应的是空字符串,set对应set( ),int对应0