python中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

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