import os


def remove_first_part_of_filenames(file_folder, size_str):

    for path, subdirs, files in os.walk(file_folder):

        for name in files:

            print("filename: %s" % name[len(size_str):])

            os.rename(os.path.join(path,name), os.path.join(path, name[len(size_str):]))


file_folder = "C:\\Users\\gmasetti\\Dropbox\\ifremer\\renamed\\ascii"
size_str = "survey_supporter_2016-260_"

remove_first_part_of_filenames(file_folder=file_folder, size_str=size_str)