How to solve index out of range error in python?

from sys import stdin

def arrayRotateCheck(arr, n):
    i=1
    
    while arr[i]>arr[i-1] and i<n-1:
        i+=1      
    if i==n-1:
        return 0
    else:
        return i

#Taking Input Using Fast I/O
def takeInput() :
    n = int(stdin.readline().rstrip())
    if n == 0:
        return list(), 0

    arr = list(map(int, stdin.readline().rstrip().split(" ")))
    return arr, n


#main
t = int(stdin.readline().rstrip())

while t > 0 :

    arr, n = takeInput()
    print(arrayRotateCheck(arr, n))

    t -= 1

This is a Python code finding array rotation index, the following line causing index out of range error. How to overcome this error.

while arr[i]>arr[i-1] and i<n-1:

you need to add a condition in arrayRotateCheck function, when the length of array is 1 then what will be the output of your program.

then here is code

def arrayRotateCheck(arr, n):
    i = 1
    if len(arr) == 1:
        return 0

    while arr[i] > arr[i - 1] and i < n - 1:
        i += 1
    if i == n - 1:
        return 0
    else:
        return i

Your test is backwards, so the second half of it doesn't save you the way it should:

while arr[i]>arr[i-1] and i<n-1:

is reading from and testing arr[i] before it's actually validated that i is less than n - 1. If you flip the test components so it doesn't read from arr until it's validated the index (and the inputs from the user aren't lies and n is never zero; as is, it's kind of a silly design, because n is the user's asserted number of inputs, but arr's length doesn't have to agree with that in any way), you won't go out of bounds:

while i < n-1 and arr[i] > arr[i-1]:

That short-circuits and doesn't even test arr[i] > arr[i-1] when i is greater than or equal to n - 1, exiting the loop immediately.

As a side-note, if you're accepting input a line at a time from the user, you don't need an import to gain access to sys.stdin, and you can simplify the code significantly, replacing each use of:

stdin.readline().rstrip()

with just:

input()

where input reads precisely one line from stdin, and strips the newline for you. I'd also recommend calling .split() instead of .split(" "); the latter will misbehave if the user accidentally includes additional spaces between elements, or uses tabs, or includes a leading space. The former will ignore all leading and trailing whitespace, and split on runs of any type of whitespace, so 1 2 3 will produce the same result as 1 2 3.