Flip Every Alternate Row in 2-D array- Python

I am a beginner to programming and was trying how to learn to flip every alternate row in my 2-D array(python)

For Example:

     `Input
    a=[[1,2,3,4,5],
    [10,9,8,7,6],
    [11,12,13,14,15],
    [20,19,18,17,16],
    [21,22,23,24,25],
    [30,29,28,27,26]]'

    Output
    a_f=[[1,2,3,4,5],
    [6,7,8,9,10],
    [11,12,13,14,15],
    [16,17,18,19,20],
    [21,22,23,24,25],
    [26,27,28,29,30]] ` 

I tried using flip function from python doc strings but it reversed the whole array. Also is it possible that a code can flip every alternate row without me needing to specify how many rows are there every time.

I suggest you start using NumPy when working with arrays. Here is a small script that performs your task. It uses enumerate to index the rows, modulus (i % 2 == 1) to distinguish between odd and even rows and use the numpy indexing [::-1] to flip the rows:

import numpy as np

def flip_even_rows(a):
    for i, row in enumerate(a):
        if i % 2 == 1:
            a[i] = row[::-1]
    return a

a = [[1,2,3,4,5],
     [10,9,8,7,6],
     [11,12,13,14,15],
     [20,19,18,17,16],
     [21,22,23,24,25],
     [30,29,28,27,26]]

a = np.array(a)

print(flip_even_rows(a))