'setdefault'dict方法的用例 [英] Use cases for the 'setdefault' dict method

问题描述

在Python 2.5中添加 collections.defaultdict 大大减少了对 dict 的 setdefault 方法。这个问题是为了我们的集体教育：

The addition of collections.defaultdict in Python 2.5 greatly reduced the need for dict's setdefault method. This question is for our collective education:

1. 什么是 setdefault 今天在Python 2.6 / 2.7？


2. setdefault 的受欢迎的用例取代了 collections.defaultdict ？

1. What is setdefault still useful for, today in Python 2.6/2.7?
2. What popular use cases of setdefault were superseded with collections.defaultdict?

推荐答案

你可以说 defaultdict在填写dict 和 setdefault 之前设置默认值对于在填充或填充之后设置默认值很有用dict 。

You could say defaultdict is useful for settings defaults before filling the dict and setdefault is useful for setting defaults while or after filling the dict.

可能是最常见的用例：分组项目（未排序的数据，否则使用 itertools.groupby ）

Probably the most common use case: Grouping items (in unsorted data, else use itertools.groupby)

# really verbose
new = {}
for (key, value) in data:
    if key in new:
        new[key].append( value )
    else:
        new[key] = [value]


# easy with setdefault
new = {}
for (key, value) in data:
    group = new.setdefault(key, []) # key might exist already
    group.append( value )


# even simpler with defaultdict 
new = defaultdict(list)
for (key, value) in data:
    new[key].append( value ) # all keys have a default already

有时你想确保具体创建一个dict后存在键。在这种情况下， defaultdict 不起作用，因为它仅在显式访问上创建密钥。认为您使用HTTP-ish与许多标题 - 一些是可选的，但您需要默认值：

Sometimes you want to make sure that specific keys exist after creating a dict. defaultdict doesn't work in this case, because it only creates keys on explicit access. Think you use something HTTP-ish with many headers -- some are optional, but you want defaults for them:

headers = parse_headers( msg ) # parse the message, get a dict
# now add all the optional headers
for headername, defaultvalue in optional_headers:
    headers.setdefault( headername, defaultvalue )

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