def median(numbers):
    numbers.sort() 
    if len(numbers) % 2:
        # if the list has an odd number of elements,
        # the median is the middle element
        middle_index = int(len(numbers)/2)
        return numbers[middle_index]
    else:
        # if the list has an even number of elements,
        # the median is the average of the middle two elements
        right_of_middle = len(numbers)//2 
        left_of_middle = right_of_middle - 1
        return (numbers[right_of_middle] + numbers[left_of_middle])/2

Examples of results:

>>> x=[5,10,15,20]
>>> median(x)
12.5
>>> x=[17,4,6,12]
>>> median(x)
9.0
>>> x=[13,6,8,14]
>>> median(x)
10.5

I have run this function and it works fine. At the beginning it was diffilcult to understand the results but finally I got it!.

However, I do not understand why only the first result is like it is intended to be. I mean the result is the average of the two middle numbers of the list.

I hope you understand I am learning on my own and sometimes it is not easy.

Your function works only in the first example because only the first list is sorted. Sort the other list or sort within the function.