Pythonic way to merge two overlapping lists, preserving order

Question

Alright, so I have two lists, as such:

• They can and will have overlapping items, for example, [1, 2, 3, 4, 5], [4, 5, 6, 7].
• The

Answer 1

First of all and for clarity, you can replace your while loop with a for loop:

def merge(master, addition):
    for n in xrange(1, len(master)):
        if master[-n:] == addition[:n]:
            return master + addition[n:]
    return master + addition

Then, you don't have to compare all possible slices, but only those for which master's slice starts with the first element of addition:

def merge(master, addition):
    indices = [len(master) - i for i, x in enumerate(master) if x == addition[0]]
    for n in indices:
        if master[-n:] == addition[:n]:
            return master + addition[n:]
    return master + addition

So instead of comparing slices like this:

1234123141234
            3579
           3579
          3579
         3579
        3579
       3579
      3579
     3579
    3579
   3579
  3579
 3579
3579

you are only doing these comparisons:

1234123141234
  |   |    |
  |   |    3579
  |   3579
  3579

How much this will speed up your program depends on the nature of your data: the fewer repeated elements your lists have, the better.

You could also generate a list of indices for addition so its own slices always end with master's last element, further restricting the number of comparisons.