Osmosis/Polygon Filter File Python Parsing

Function to convert a textual POLY file to Shapely MultiPolygon.

from shapely.geometry import MultiPolygon, Polygon

def parse_poly(lines):
    """ Parse an Osmosis polygon filter file.

        Accept a sequence of lines from a polygon file, return a shapely.geometry.MultiPolygon object.

        http://wiki.openstreetmap.org/wiki/Osmosis/Polygon_Filter_File_Format
    """
    in_ring = False
    coords = []
    
    for (index, line) in enumerate(lines):
        if index == 0:
            # first line is junk.
            continue
        
        elif index == 1:
            # second line is the first polygon ring.
            coords.append([[], []])
            ring = coords[-1][0]
            in_ring = True
        
        elif in_ring and line.strip() == 'END':
            # we are at the end of a ring, perhaps with more to come.
            in_ring = False
    
        elif in_ring:
            # we are in a ring and picking up new coordinates.
            ring.append(list(map(float, line.split())))
    
        elif not in_ring and line.strip() == 'END':
            # we are at the end of the whole polygon.
            break
    
        elif not in_ring and line.startswith('!'):
            # we are at the start of a polygon part hole.
            coords[-1][1].append([])
            ring = coords[-1][1][-1]
            in_ring = True
    
        elif not in_ring:
            # we are at the start of a polygon part.
            coords.append([[], []])
            ring = coords[-1][0]
            in_ring = True
    
    return MultiPolygon(coords)