python 字典 defaultdict

统计strings 中某个单词出现的次数

strings = ('puppy', 'kitten', 'puppy', 'puppy',
           'weasel', 'puppy', 'kitten', 'puppy')
counts = {}

for kw in strings:
    if kw not in counts:
        counts[kw] = 1
    else:
        counts[kw] += 1''

也可以使用setdefault() 方法来设置默认值

strings = ()
counts = {}
for kw in strings:
	counts.setdefault(kw,0)
	counts[kw] += 1

最后用 defaultdict 来实现

from collections import defaultdict
defaultdict类除了接受类型名称作为初始化函数的参数之外，还可以使用任何不带参数的可调用函数，到时该函数的返回结果作为默认值，这样使得默认值的取值更加灵活
def zero():
	return 0
counts = defaultdict(lambda:0)
counts = defaultdict(int)
for s in strings:
counts[s] += 1

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

class CountMissing(object):
	def __init__(self):
		self.added = 0
	def missing(self):
		self.added += 1
		return 0
counter = CountMissing()
result = defaultdict(counter.missing, current) # Method ref
for key, amount in increments:
	result[key] += amount
assert counter.added == 2

class bCountMissing(object):
	def __init__(self):
		self.added = 0
	def __call__(self):
		self.added += 1
		return 0
counter = bCountMissing()
assert callable(counter)
result = defaultdict(counter, current) # Method ref
for key, amount in increments:
	result[key] += amount
assert counter.added == 2