Question

I have two list of marker objects:

class Marker():
    def __init__(self,marker_number, marker_data):
        self.marker_number = marker_number
        self.marker_data = marker_data

and what I want to do is this:

>>> existing_markers = [Marker(marker_number=1, marker_data= 'a'),
                     Marker(marker_number=2, marker_data= 'b'),
                     Marker(marker_number=3, marker_data= 'c'),
                     Marker(marker_number=4, marker_data= 'd'),
                     Marker(marker_number=5, marker_data= 'e'),]

>>> new_markers = [  Marker(marker_number=1, marker_data= 'aa'),
                     Marker(marker_number=3, marker_data= 'bb'),
                     Marker(marker_number=5,  marker_data= 'cc'),]

>>> interlace_markers(existing_markers, new_markers)

[Marker(marker_number=1, marker_data= 'a'),
Marker(marker_number=2, marker_data= 'aa'),
Marker(marker_number=3, marker_data= 'b'),
Marker(marker_number=4, marker_data= 'c'),
Marker(marker_number=5, marker_data= 'bb'),
Marker(marker_number=6, marker_data= 'd'),
Marker(marker_number=7, marker_data= 'e'),
Marker(marker_number=8,  marker_data= 'cc')]

So that when I interlace the markers, markers with the same number in the new_markers list as the original markers list appear after the original marker, but the numbering updates to maintain a numbered sequence. This is my current approach:

def interlace_markers(current_markers_list, new_markers_list):

    interlaced_markers = []
    #interlace the markers
    for existing_marker in current_markers_list:
        interlaced_markers.append(existing_marker)
        for new_marker in new_markers_list:
            if new_marker.marker_number== existing_marker.marker_number:
                interlaced_markers.append(new_marker)

    #reset the sequence
    sequence_index = 1
    for marker in interlaced_markers:
         marker.marker_number= sequence_index
         sequence_index += 1

    return interlaced_markers

I have two problems here:

1. What is the most pythonic way to do this
2. The original list will likely be large ~10k, and created from an
   external file, but the new list will be small ~100-300 long, is
   there a more efficient way of doing this rather than using lists?

Answer 1

If I understand you properly, I think this is most easily accomplished with simple sorting:

interlace_markers = existing_markers + new_markers
interlace_markers.sort(key = lambda x: x.marker_number)

Now if you want to change the numbering, you can do that in a simple loop:

for i,marker in enumerate(interlace_markers):
    marker.marker_number = i+1 #indexing is 1 based instead of zero based

Note that this works because python’s sorting is stable which means that the order isn’t changed if it doesn’t have to be.

This leads to an O(n) + O(nlogn) = O(nlogn) algorithm which isn’t too bad.