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This is not a well written article. The Python documentation (https://docs.python.org/3/library/collections.html#collectio...) is clearer.

TLDR from the docs: ChainMap groups multiple dicts or other mappings together to create a single, updateable view.

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> It is often much faster than creating a new dictionary and running multiple update() calls.

This is the important detail. This is much faster than how you'd solve it pre-3.3.

I found myself wondering why this required a new class in the stdlib until that line.

It would be nicer if Python just optimized the old way.

Semantically they’re not the same. In the ‘old way’ adding a key to one of the original dicts does not change the new dict. Whereas the new key will appear in the ChainMap.

The author mentions right at the top of the blog that they are going through a tough time, and they put substantially more effort into this post than you did with your comment.

> Be kind.

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