***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.

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

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]})

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