Why Python instead of PLine 
Are you applying the same percentage to all polyline segments or setting them separately?
Changing the length of polyline segments requires moving all but the first point, so a loop is needed. This uses Anemone.
otamesi_2024Jan11a.gh (24.7 KB)
P.S. The same thing can be achieved using Scale, on the PLine or on the points, using the start point as ‘C’ (Center) input.
otamesi_2024Jan11b.gh (28.6 KB)
P.P.S. Here is what happens using a Graph Mapper to apply different percentage length values to each segment of the polyline. Can’t do this with Scale. 
otamesi_2024Jan11c.gh (30.5 KB)