Python字典 | setdefault() 方法

Python中的Dictionary是数据值的无序集合，用于像 map 一样存储数据值，与其他仅保存单个值作为元素的 Data Types 不同，Dictionary 保存 key : value 对。
在Python Dictionary 中， setdefault()方法返回一个键的值（如果该键在字典中）。如果没有，它会将带有值的键插入到字典中。

Syntax: dict.setdefault(key, default_value)
Parameters: It takes two parameters: 
key – Key to be searched in the dictionary. 
default_value (optional) – Key with a value default_value is inserted to the dictionary if key is not in the dictionary. If not provided, the default_value will be None.
Returns: 
Value of the key if it is in the dictionary. 
None if key is not in the dictionary and default_value is not specified. 
default_value if key is not in the dictionary and default_value is specified.

示例 #1：

Python3

# Python program to show working
# of setdefault() method in Dictionary
 
# Dictionary with single item
Dictionary1 = { 'A': 'Geeks', 'B': 'For', 'C': 'Geeks'}
 
# using setdefault() method
Third_value = Dictionary1.setdefault('C')
print("Dictionary:", Dictionary1)
print("Third_value:", Third_value)

输出：

Dictionary: {'A': 'Geeks', 'C': 'Geeks', 'B': 'For'}
Third_value: Geeks

示例 #2：当 key 不在字典中时。

Python3

# Python program to show working
# of setdefault() method in Dictionary
 
# Dictionary with single item
Dictionary1 = { 'A': 'Geeks', 'B': 'For'}
 
# using setdefault() method
# when key is not in the Dictionary
Third_value = Dictionary1.setdefault('C')
print("Dictionary:", Dictionary1)
print("Third_value:", Third_value)
 
# using setdefault() method
# when key is not in the Dictionary
# but default value is provided
Fourth_value = Dictionary1.setdefault('D', 'Geeks')
print("Dictionary:", Dictionary1)
print("Fourth_value:", Fourth_value)

输出：

Dictionary: {'A': 'Geeks', 'B': 'For', 'C': None}
Third_value: None
Dictionary: {'A': 'Geeks', 'B': 'For', 'C': None, 'D': 'Geeks'}
Fourth_value: Geeks