Sort a list of objects based on the index of the object's property from another list

Question

I have a tuple, RELAY_PINS that holds GPIO pin numbers in the order that the relays are installed. RELAY_PINS is immutable, and its ordering does not change, while the order that the devices are defined in changes frequently.

The MRE:

from random import shuffle, randint

class Device:
    def __init__(self, pin_number):
        self.pin_number = pin_number

    def __str__(self):
        return str(self.pin_number)

RELAY_PINS = ( 14, 15, 18, 23, 24, 25, 1, 12, 16, 20, 21, 26, 19, 13, 6, 5 )

def MRE():
    devices = [ Device(pin) for pin in RELAY_PINS ]

    # the ordering for the list of devices should be considered random for the sake of this question
    shuffle(devices)
    return devices

My solution "works", but frankly, it's embarrassing:

def main():
    devices = MRE()
    pin_map = { pin_number : index for index, pin_number in enumerate(RELAY_PINS) }

    ordered_devices = [ None for _ in range(len(RELAY_PINS)) ]

    for device in devices:
        index = pin_map[device.pin_number]
        ordered_devices[index] = device

    return [ dev for dev in ordered_devices if dev is not None ]

I know there is a better solution, but I can't quite wrap my head around it.

What is the pythonic solution to this problem?

Answer 1

You can use sorted with a key function:

from random import randint, shuffle


class Device:

  def __init__(self, pin_number: int):
    self.pin_number = pin_number

  def __str__(self) -> str:
    return str(self.pin_number)

  def __repr__(self) -> str:
    return f'Device(pin_number={self.pin_number})'


RELAY_PINS: tuple[int, ...] = (14, 15, 18, 23, 24, 25, 1, 12, 16, 20, 21, 26,
                               19, 13, 6, 5)


def MRE() -> None:
  devices = [Device(pin) for pin in RELAY_PINS]
  devices.pop(randint(0, len(RELAY_PINS) - 1))
  shuffle(devices)
  return devices


def main() -> None:
  devices = MRE()
  ordered_devices = sorted(devices, key=lambda d: RELAY_PINS.index(d.pin_number))
  print(ordered_devices)


if __name__ == '__main__':
  main()

Example Output (device with pin_number=12 randomly popped):

[Device(pin_number=14), Device(pin_number=15), Device(pin_number=18), Device(pin_number=23), Device(pin_number=24), Device(pin_number=25), Device(pin_number=1), Device(pin_number=16), Device(pin_number=20), Device(pin_number=21), Device(pin_number=26), Device(pin_number=19), Device(pin_number=13), Device(pin_number=6), Device(pin_number=5)]