I was try to solve Python program to create Fibonacci series using Lambda, but I unable to solve it

Question

So on internet they solve using:

from functools import reduce

fib = lambda n: reduce(lambda x, _: x+[x[-1]+x[-2]],range(n-2), [0, 1])

print(fib(5))

Fibonacci program

I don't understand what's the use of underscore _ and actually how this range(n-2) is working inside program

Answer 1

fib = lambda n: ... creates a one liner lambda function and assigns it to fib. Now, fib is a callable like a function e.g. fib(5).

reduce accepts 3 params: reduce(function, iterable, [, initializer]).

lambda x, _: x+[x[-1]+x[-2]] is your function. It concats current x sequence to a single-element sequence of sum of last and second last elements in the sequence x (see table below).

range(n-2) is iterable which generates a sequence of numbers [0,1,2] for n=5.

[0, 1] is the initial sequence to start with i.e. first 2 fixed numbers in fib sequence.

_ is used as second argument of function which is ignored. It is a convention to use _ for ignored argument in a functional program. The second argument receives one value from iterable for each run i.e. 0, 1, & 2.

Dry run for n=5

--------------------------------------------------------------
range(n-2)            x    _    x+[x[-1]+x[-2]]         output
--------------------------------------------------------------
         0        [0,1]    0        [0,1]+[0+1]        [0,1,1]
         1      [0,1,1]    1      [0,1,1]+[1+1]      [0,1,1,2]
         2    [0,1,1,2]    2    [0,1,1,2]+[1+2]    [0,1,1,2,3]

Note: [0,1] + [0+1] is list concatenation of [0,1] and [1]