[Solved] Mergesort Python implementation

The mergesort function you call doesn’t modify its argument. Rather, it returns a new sorted list.

A simple fix would be:

def mergesort(data):
    if len(data) < 2:
        return data              # Fix1
    left = data[:len(data)//2]
    print(left)
    right = data[len(data)//2:]
    print(right)
    print("left only now")
    left = mergesort(left)       # Fix2
    print("right now")
    right = mergesort(right)     # Fix3
    return merge(left,right)

data = mergesort([1,20, 30, 25, 8, 7, 9])