我已经阅读了python文档中的示例,但仍然不明白这个方法是什么意思。有人能帮帮我吗?下面是python文档中的两个例子
>>> from collections import defaultdict
>>> s = 'mississippi'
>>> d = defaultdict(int)
>>> for k in s:
... d[k] += 1
...
>>> d.items()
[('i', 4), ('p', 2), ('s', 4), ('m', 1)]
and
>>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
>>> d = defaultdict(list)
>>> for k, v in s:
... d[k].append(v)
...
>>> d.items()
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]
参数int和list是干什么用的?
标准字典包含setdefault()方法,用于检索值并在值不存在时建立默认值。相反,defaultdict允许调用者在容器初始化时预先指定默认值。
import collections
def default_factory():
return 'default value'
d = collections.defaultdict(default_factory, foo='bar')
print 'd:', d
print 'foo =>', d['foo']
print 'bar =>', d['bar']
只要所有键都具有相同的默认值,这种方法就可以很好地工作。如果默认值是用于聚合或累积值的类型,例如列表、集合甚至int,那么它会特别有用。标准库文档包含了以这种方式使用defaultdict的几个示例。
$ python collections_defaultdict.py
d: defaultdict(<function default_factory at 0x100468c80>, {'foo': 'bar'})
foo => bar
bar => default value
字典是一种方便的方式来存储数据,以便以后按名称(键)检索。键必须是唯一的、不可变的对象,通常是字符串。字典中的值可以是任何东西。对于许多应用程序,值是简单类型,如整数和字符串。
It gets more interesting when the values in a dictionary are collections (lists, dicts, etc.) In this case, the value (an empty list or dict) must be initialized the first time a given key is used. While this is relatively easy to do manually, the defaultdict type automates and simplifies these kinds of operations.
A defaultdict works exactly like a normal dict, but it is initialized with a function (“default factory”) that takes no arguments and provides the default value for a nonexistent key.
defaultdict永远不会引发KeyError。任何不存在的键都将获得默认工厂返回的值。
from collections import defaultdict
ice_cream = defaultdict(lambda: 'Vanilla')
ice_cream['Sarah'] = 'Chunky Monkey'
ice_cream['Abdul'] = 'Butter Pecan'
print(ice_cream['Sarah'])
>>>Chunky Monkey
print(ice_cream['Joe'])
>>>Vanilla
下面是另一个关于如何使用defaultdict的例子,我们可以降低复杂性
from collections import defaultdict
# Time complexity O(n^2)
def delete_nth_naive(array, n):
ans = []
for num in array:
if ans.count(num) < n:
ans.append(num)
return ans
# Time Complexity O(n), using hash tables.
def delete_nth(array,n):
result = []
counts = defaultdict(int)
for i in array:
if counts[i] < n:
result.append(i)
counts[i] += 1
return result
x = [1,2,3,1,2,1,2,3]
print(delete_nth(x, n=2))
print(delete_nth_naive(x, n=2))
总之,当您需要字典,并且每个元素的值都应该以默认值开始时,请使用defaultdict。