Why do the Items in a Python ChainMap reverse order when converted to a Dictionary?-CodePudding

Why does converting a ChainMap to a dictionary reverse the order of the items?

Here is an example.

>>> d = [{'a': 1}, {'b': 2}, {'c': 3}]

>>> ChainMap(*d)
ChainMap({'a': 1}, {'b': 2}, {'c': 3})

>>> dict(ChainMap(*d))
{'c': 3, 'b': 2, 'a': 1}

I can write alternate code to combine the dictionaries that does not reverse.

But, I would like to understand why this reversal happens for ChainMap.

It seems it would be desirable to keep the order.

CodePudding user response：

ChainMap documentation mentions

Note, the iteration order of a ChainMap() is determined by scanning the mappings last to first.

The only mention of why ChainMap does this seems to be the following

This gives the same ordering as a series of dict.update() calls starting with the last mapping

Meanwhile, dict maintains insertion order since a few versions ago

Dictionaries preserve insertion order. Note that updating a key does not affect the order. Keys added after deletion are inserted at the end.

Changed in version 3.7: Dictionary order is guaranteed to be insertion order. This behavior was an implementation detail of CPython from 3.6.

So when you call dict(ChainMap(*d)), it iterates over the ChainMap i.e. reverse of the order of *d and that becomes insertion order for the new dict.