Dude! It's the first time I've seen using lambda inside a class instead of a function!
You managed to create the smallest and, somehow, easiest answer to analyse of us all!!!

Sir, you got yourself a huge fan in me!

Best practice and Clever! You got it!

same problem there. I get manage the mathematic solution who doesn't need to calcul the factorial and who have only one loop to get a sum requested for the solution. All tests are green, but attempt said server time ran out... i can not imagine what can be different to pass the server time obstruction. On my computer, there is no significative delay as long as it is very simple.
In fact, i got an answer show me that my solution was too slow... i do simplification for remove the sum part, but again... too slow.

In cases when you misunderstand the task and implement the wrong algorithm.

I gotta say... not satisfied with this test.
I managed to get it done, but after many hours.
When I ran several of valid answers, which ran on a very good time on my pc, I got the error 'no data was written to STDOUT or STDERR'.

Also, several answers of mine were rejected because the end result was float.

However, when I checked some of the valid answers, I very saw similar answers to mine, and answers whose end result was float.

My python version, on my Jupyter Notebook:
3.7.1 (default, Dec 10 2018, 22:54:23) [MSC v.1915 64 bit (AMD64)]