imo

  defaultdict(callback,{**a,**b}) 

is more readable than a | b without knowing that a or b are defaultdicts and having to reason about which default callback will be used

It’s also really expensive. This new operator works with any Mapping type without needing silly hacks.

Note that a | b is already a disaster if you try to use subclasses of set. In python 2.7, iirc it would return a set of a's type but without calling the constructor. In python 3 it seems to return a set (not the type of a or the type of b).

Not sure about the set operator, but this operator is meant to handle subclasses better than the status quo.

Wow, one more point for python 3.

Just override ‘__and__’ in your whatever class to replace default return.

Pretty explicit in my book.

the easy to forget justification is surprising to me. especially when most modern languages have the concept of unpacking, rest, spread or etc.

making the trick work with other mapping types and making it faster is totally understandable though.

It's pretty obvious with the context of other languages, but wildly outside the norm for Python. I rarely see dict unpacking outside function signatures.