Disagree.

In the first instance, the original code is readable and tells me exactly what's what. In your example, you're sacrificing readability for being clever.

Clear code(even if verbose) is better than being clever.

Using a very common utility in the standard library is to avoid reinventing the wheel is not "clean code"?

defaultdict is ubiquitous in modern python, and is far from a complicated concept to grasp.

I think code clarity is subjective. I find the second easier to read because I have to look at less code. When I read code, I instinctively take it apart and see how it fits together, so I have no problem with the second approach. Whereas the first approach is twice as long so it takes me roughly twice as long to read.

The 2nd version is the most idiomatic.

Interesting! Thanks for the responses. I'm not python native and haven't worked as extensively with python as some of you here.

That said, I'll change my mind here and agree on using std library, but I'd still have separate 'key' assignment here for more clarity.

I would keep the explicit key= assignment since it's more than just a single literal but otherwise the second version is more idiomatic and readable.