1.

While loop version:

def linear_search(lst, value):
    """ (list, object) -> int

    Return the index of the last occurrence of value in lst, or return
    -1 if value is not in lst.

    >>> linear_search([2, 5, 1, -3], 5)
    1
    >>> linear_search([2, 4, 2], 2)
    2
    >>> linear_search([2, 5, 1, -3], 4)
    -1
    >>> linear_search([], 5)
    -1
    """ 

    i = len(lst) - 1  # The index of the next item in lst to examine.

    # Keep going until we reach the end of lst or until we find value.
    while i != -1 and lst[i] != value:
        i = i - 1

    # If we fell off the end of the list, we didn't find value.
    if i == -1:
        return -1
    else:
        return i

For loop version:

def linear_search(lst, value):
    """ (list, object) -> int

    Return the index of the last occurrence of value in lst, or return
    -1 if value is not in lst.

    >>> linear_search([2, 5, 1, -3], 5)
    1
    >>> linear_search([2, 4, 2], 2)
    2
    >>> linear_search([2, 5, 1, -3], 4)
    -1
    >>> linear_search([], 5)
    -1
    """ 

    # The first index is included, the second is not, and the third is the
    # increment.
    for i in range(len(lst) - 1, -1, -1):
        if lst[i] == value:
            return i

    return -1

Sentinal version:

def linear_search(lst, value):
    """ (list, object) -> int

    Return the index of the last occurrence of value in lst, or return
    -1 if value is not in lst.

    >>> linear_search([2, 5, 1, -3], 5)
    1
    >>> linear_search([2, 4, 2], 2)
    2
    >>> linear_search([2, 5, 1, -3], 4)
    -1
    >>> linear_search([], 5)
    -1
    """ 

    # Add the sentinel at the beginning.
    lst.insert(0, value)

    i = len(lst) - 1

    # Keep going until we find value.
    while lst[i] != value:
        i = i - 1

    # Remove the sentinel.
    lst.pop(0)

    # If we reached the beginning of the list we didn't find value.
    if i == 0:
        return -1
    else:
        # When we inserted, we shifted everything one to the right. Subtract 1
        # to account for that.
        return i - 1