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))
I don't understand what's the use of underscore _ and actually how this range(n-2) is working inside program
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]
_is just a variable name. It is commonly used to indicate that you are ignoring a value.