Clever

Nice :)

Yeah, this needs memoization. Lots of repeated calculations. Super inefficient.

Coming to see other solutions after solving a kata is one of my favorite moments because of things like this, I always learn something new.

Why recursive lambda?
It is generaly not a good practice to try to make the code as short as possible but to make it as readable as possible...
For example if you look at my solution, it is basicaly the same as yours but a lot more readable.
(Just a tip :))

I'm surprised this didn't time out

# 1/n! * (n! + (n-1)! + (n-2)! ......1!)      # can be factored down to:

Very clever

# n!/n! + (n-1)!/n! + (n-2)!/n! .....1!/n!    # write out the factorial e.g. 5 x 4 x 3 ... and you'll see it better

#  s(n) =  1   +    1/n    +  1/n(n-1) ...1/n!       # 1/n plays the largest role. In fact, after n=15, 
                                              # the 1st 6 decimals aren't even affected - trial and error
# s(n-1) = 1 + 1/(n - 1) + ... + 1/(n-1)!

# A recursive formula indicates s(n-1)/n + 1 = 1 + 1/n + 1/(n-1)n + ... + 1/n! =s(n)

incredible solution you wrote!

Not straightforward, but smart!

to truncate to 6 decimal places

can one explain how the main formula (1 / n!) * (1! + 2! + 3! + ... + n!) been simplified to the above solution (in a simple for loop). Also please recommend me where I can learn the tricks to simply maths to code in efficient way.

I liked this solution, it is for sure one of the most pythonics and cleaners i saw.

I think this is the first time I've ever seen a recursive lambda. Really cool, thanks.

Maybe because it's truncating, not rounding?