How does reduce function work?

Question

As far as I understand, the reduce function takes a list l and a function f. Then, it calls the function f on first two elements of th

Answer 1

From the Python reduce documentation,

reduce(function, sequence) returns a single value constructed by calling the (binary) function on the first two items of the sequence, then on the result and the next item, and so on.

So, stepping through. It computes reduce_func of the first two elements, reduce_func(1, 3) = 1! * 3! = 6. Then, it computes reduce_func of the result and the next item: reduce_func(6, 1) = 6! * 1! = 720.

You missed that, when the result of the first reduce_func call is passed as input to the second, it's factorialized before the multiplication.