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?
当前回答
文档和解释几乎是不言自明的:
http://docs.python.org/library/collections.html#collections.defaultdict
类型函数(int/str等)作为参数传递，用于初始化dict中没有的任何给定键的默认值。
其他回答
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。
文档和解释几乎是不言自明的:
http://docs.python.org/library/collections.html#collections.defaultdict
类型函数(int/str等)作为参数传递，用于初始化dict中没有的任何给定键的默认值。
Usually, a Python dictionary throws a KeyError if you try to get an item with a key that is not currently in the dictionary. The defaultdict in contrast will simply create any items that you try to access (provided of course they do not exist yet). To create such a "default" item, it calls the function object that you pass to the constructor (more precisely, it's an arbitrary "callable" object, which includes function and type objects). For the first example, default items are created using int(), which will return the integer object 0. For the second example, default items are created using list(), which returns a new empty list object.
如果没有defaultdict，您可能可以为不可见的键分配新值，但不能修改它。例如:
import collections
d = collections.defaultdict(int)
for i in range(10):
d[i] += i
print(d)
# Output: defaultdict(<class 'int'>, {0: 0, 1: 1, 2: 2, 3: 3, 4: 4, 5: 5, 6: 6, 7: 7, 8: 8, 9: 9})
import collections
d = {}
for i in range(10):
d[i] += i
print(d)
# Output: Traceback (most recent call last): File "python", line 4, in <module> KeyError: 0
简而言之:
Defaultdict (int) -参数int表示值将是int类型。
Defaultdict (list) -参数列表指示值将是列表类型。
