python的默认字典defaultdict

  在字典中查找某个值时，若key不存在时则会返回一个KeyError错误而不是一个默认值，这时候可以使用defaultdict函数。

注意：使用dict[key]=value时，若key不存在则报错；使用dict.get(key)时，若key不存在则会返回一个默认值。

defaultdict接受一个工厂函数作为参数，如下来构造：

dict =defaultdict( factory_function)
        factory_function可以是list、set、str等等，作用是当key不存在时，返回的是工厂函数的默认值，比如list对应[ ]，str对应的是空字符串，set对应set( )，int对应0。
        defaultdict是python内建dict类的一个字类，功能与dict相同，但它带有一个默认的值，若key值不存在时返回一个默认的值。

原文链接：https://blog.csdn.net/weixin_44799217/article/details/124380270

测试一

from collections import defaultdict

dict1 = defaultdict(int)
dict2 = defaultdict(set)
dict3 = defaultdict(str)
dict4 = defaultdict(list)
dict4[1] = '666'
print(dict1[0])
print(dict2[0])
print(dict3[0])
print(dict4[0])
print(dict4[1])

输出:

0
set()

[]
666

测试二

from collections import defaultdict
def log_missing():
    print('Key added')
    return 0

current={'green' : 12, 'blue': 3}
increments = [
    ('red', 5), ('blue', 17), ('orange', 9),
]
result = defaultdict(log_missing, current)

print('Before: ', result, dict(result))
for key, amount in increments:
    result[key] += amount
    print('After: ', result, dict(result))

输出:

2022-08-09 06:59:10.687841: Hi there! 
2022-08-09 06:59:10.898213: Hi again! 
Before:  defaultdict(<function log_missing at 0x0000026AB9005488>, {'green': 12, 'blue': 3}) {'green': 12, 'blue': 3}
Key added
After:  defaultdict(<function log_missing at 0x0000026AB9005488>, {'green': 12, 'blue': 3, 'red': 5}) {'green': 12, 'blue': 3, 'red': 5}
After:  defaultdict(<function log_missing at 0x0000026AB9005488>, {'green': 12, 'blue': 20, 'red': 5}) {'green': 12, 'blue': 20, 'red': 5}
Key added
After:  defaultdict(<function log_missing at 0x0000026AB9005488>, {'green': 12, 'blue': 20, 'red': 5, 'orange': 9}) {'green': 12, 'blue': 20, 'red': 5, 'orange': 9}

内嵌defaultdict的钩子函数

from collections import defaultdict


def increment_with_report(current, increments):
    added_count = 0
    
    def missing():
        nonlocal added_count#  Stateful closure
        added_count += 1
        return 0
    result = defaultdict(missing, current)
    for key, amount in increments:
        result[key] += amount
    return result, added_count


current = {'green': 12, 'blue': 3}
increments = [
    ('red', 5), ('blue', 17), ('orange', 9),
]

result, count = increment_with_report(current, increments)
assert count == 2
print(result, count)

输出:

2022-08-09 07:03:43.805543: Hi there! 
2022-08-09 07:03:44.022240: Hi again! 
defaultdict(<function increment_with_report.<locals>.missing at 0x0000017272FC5510>, {'green': 12, 'blue': 20, 'red': 5, 'orange': 9}) 2