How do I reverse a sublist in a list in place?

Question

I\'m supposed to create a function, which input is a list and two numbers, the function reverses the sublist which its place is indicated by the two numbers. for example this is

Answer 1

lst[::-1] is the idiomatic way to reverse a list in Python, The following show how and that it was in-place:

>>> lst = [1, 2, 3, 4, 5]
>>> id(lst)
12229328
>>> lst[:] = lst[::-1]
>>> lst
[5, 4, 3, 2, 1]
>>> id(lst)
12229328

Answer 2

Two methods in-place and constant memory:

def reverse_swap(arr, start=None, end=None):
    """
    Swap two edge pointers until meeting in the center.
    """

    if start is None:
        start = 0
    if end is None:
        end = len(arr)

    i = start
    j = end - 1
    while i < j:
        arr[i], arr[j] = arr[j], arr[i]
        i += 1
        j -= 1

def reverse_slice(arr, start=None, end=None):
    """
    Use python slice assignment but use a generator on the right-hand-side
    instead of slice notation to prevent allocating another list.
    """

    if start is None:
        start = 0
    if end is None:
        end = len(arr)

    arr[start:end] = (arr[i] for i in range(end - 1, start - 1, -1))

Answer 3

def reverse_sublist(lst,start,end):
    lst[start:end] = lst[start:end][::-1]
    return lst

Answer 4

... I'm not sure is it's in place.

...

lst[start:end]=sublist

Yes, it's in place. lst is never rebound, only its object mutated.

Answer 5

Partial reverse with no temporary list (replace range with xrange if you use Python 2):

def partial_reverse(list_, from_, to):
    for i in range(0, int((to - from_)/2)):
        (list_[from_+i], list_[to-i]) = (list_[to-i], list_[from_+i])

list_ = [1, 2, 3, 4, 5, 6, 7, 8]
partial_reverse(list_, 3, 7)
print(list_)

Answer 6

Not sure if you have a similar problem as mine, but i needed to reverse a list in place.

The only piece I was missing was [:]

exStr = "String"

def change(var):
  var[:] = var[::-1] # This line here

print(exStr) #"String"
change(exStr)
print(exStr) #"gnirtS"