binary search first occurance

def binary_search(sorted_list, value):
    left = 0
    right = len(sorted_list) - 1
    while left <= right:
        mid = right + left // 2
        if sorted_list[mid] > value:
            right = mid - 1
        elif sorted_list[mid] < value:
            left = mid +1
        else:
# found value but is it the first occurence
            chk = mid
            while sorted_list[chk] == value:
                mid = chk
                if chk > 0:
                    chk -= 1
                else:
                    return mid
            return mid
# loop ends return `False`
    return -1

list_of_numbers = [1, 2, 3, 4, 4, 4, 4, 4, 5, 6, 7, 8, 9]
print("list of numbers is {}".format(list_of_numbers))

search_number = 4
print("Number to find is {}".format(search_number))

item_at = binary_search(list_of_numbers, search_number)
# Display result of search
if item_at < 0:
    print("{} is not in the list of numbers".format(search_number))
else:
    print("The first time {} is in the list is at posn {}".format(search_number, item_at))