“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 方法的需求.这个问题是给我们集体教育的:

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. 今天在 Python 2.6/2.7 中 setdefault 仍有什么用?
2. collections.defaultdict 取代了哪些 setdefault 的流行用例?

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 What popular use cases of setdefault were superseded with collections.defaultdict?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 在填充 dict 和 setdefault<之前对设置默认值很有用/code> 可用于填充字典时或之后设置默认值.

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(在未排序的数据中，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 
from collections import defaultdict
new = defaultdict(list)
for (key, value) in data:
    new[key].append( value ) # all keys have a default already

有时您想确保在创建字典后特定键存在.defaultdict 在这种情况下不起作用，因为它只在显式访问时创建密钥.认为您使用带有许多标头的 HTTP-ish —— 有些是可选的，但您想要它们的默认值:

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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