collections. What is the purpose of chainmap- What is the purpose of collections. ChainMap?

problem ：

In Python 3.3 a ChainMap class was added to the collections module: stay Python 3.3 in , One ChainMap Class is added to collections Module ：

A ChainMap class is provided for quickly linking a number of mappings so they can be treated as a single unit. Provides a ChainMap class , Used to quickly link multiple mappings , In order to treat them as a unit .It is often much faster than creating a new dictionary and running multiple update() calls. It is usually more than creating a new dictionary and running multiple update（） Call much faster .

Example: example ：

>>> from collections import ChainMap>>> x = {'a': 1, 'b': 2}>>> y = {'b': 10, 'c': 11}>>> z = ChainMap(y, x)>>> for k, v in z.items():        print(k, v)a 1c 11b 10

It was motivated by this issue and made public by this one (no PEP was created). It is motivated This problem , And through public This one （ No PEP establish ）.

As far as I understand, it is an alternative to having an extra dictionary and maintaining it with update() s. as far as I am concerned , It's an alternative , There is an extra dictionary , And use update() s Maintain it .

The questions are: The problem is ：

• What use cases does ChainMap cover?ChainMap What use cases are covered ？
• Are there any real world examples of ChainMap ?ChainMap Is there any real-world example ？
• Is it used in third-party libraries that switched to python3? Is it used to switch to python3 Third party library ？

Bonus question: is there a way to use it on Python2.x? Additional questions ： Is there any way Python2.x Use it on ？

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I've heard about it in Transforming Code into Beautiful, Idiomatic Python PyCon talk by Raymond Hettinger and I'd like to add it to my toolkit, but I lack in understanding when should I use it. I am here Raymond Hettinger Of “ Transforming Code into Beautiful, Idiomatic Python PyCon Transforming Code into Beautiful, Idiomatic Python Heard of it , I want to add it to my toolkit , But I don't know when to use it .

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Solution ：

Reference resources ： https://stackoom.com/en/question/1a9a4