robot.py

from gpiozero import LineSensor, Robot

robot = Robot(left=(9, 10), right=(7, 8))
line = LineSensor(25)

speed = 0.6

while True:
    robot.forward(speed)
    line.wait_for_no_line()
    robot.left(speed)
    line.wait_for_line()

robot2.py

from gpiozero import LineSensor, Robot

robot = Robot(left=(9, 10), right=(7, 8))
line = LineSensor(25)

robot.source_delay = 0.5
speed = 0.6

def look_for_line():
    while True:
        yield (1, 0.5)  # left motor full speed, right motor half speed
        yield (0.5, 1)  # left motor half speed, right motor full speed

while True:
    robot.forward(speed)
    line.wait_for_no_line()  # go forward until the line is lost
    robot.source = look_for_line()  # alternate between slight left and slight right
    line.wait_for_line()  # until the line is found
    robot.source = None  # unset the source to stop looking left and right