[chimera-dev] Get value of 'measure distance' command

Hi, I need to calculate the minimum distance between 2 residues but It seems that I can't find the correct way to use the 'measure distance' command in python script. I tried: import Measure temp = Measure.measure_command('distance', 'distance :628.a :20.b') but it gave me the error: "No target atoms or surface" Can you please precise the correct syntax of this command? Thank you! Thu