Bubble sort and merge sort step by step

Start at the left. Compare each pair of neighbours and swap them if they are in the wrong order. One sweep along the list is a pass.
After pass 1 the largest item has 'bubbled' to the end; after pass 2 the two largest are in place, and so on.
The algorithm stops after a pass in which no swaps happen. That final pass is needed to prove the list is sorted.

A flag variable records whether any swap happened. The while loop keeps making passes until a pass makes no swaps.
numbers = [7, 3, 9, 2, 5]
swapped = True
while swapped:
swapped = False
for i in range(len(numbers) - 1):
if numbers[i] > numbers[i + 1]:
temp = numbers[i]
numbers[i] = numbers[i + 1]
numbers[i + 1] = temp
swapped = True
print(numbers)
Output: [2, 3, 5, 7, 9]
With n items, each pass makes n - 1 comparisons. Bubble sort is simple and needs almost no extra memory, but on a large, jumbled list it makes a very large number of comparisons.

Divide: split the list in half again and again until every list has one item (a list of one item is already sorted).
Merge: join pairs of lists. Compare the front items of the two lists, move the smaller one into the new list, and repeat until both are empty.

Merge sort is much faster on large lists, because each merge stage handles every item once and there are only a few stages. It needs extra memory for the smaller lists and is harder to code.
Bubble sort is easy to code and uses very little memory. It is fine for small lists, or lists that are almost sorted, where it may finish after one or two passes.