MicroPython keypad door lock on a Pico: servo and buzzer

# A keypad door lock: every key beeps the buzzer (GP15, through an
# S8050). After four keys, 1234 swings the servo bolt on GP0 open for a
# second; anything else gets a long beep.
from machine import Pin
import time
from keypad import Keypad
from servo import Servo

CODE = "1234"
keys = Keypad(rows=(13, 12, 11, 10), cols=(9, 8, 7, 6))
buzzer = Pin(15, Pin.OUT, value=0)
bolt = Servo(0)
bolt.angle(0)

def beep(ms):
    buzzer.value(1)
    time.sleep_ms(ms)
    buzzer.value(0)

typed = ""
print("Type the code")
while True:
    key = keys.scan()
    if key:
        beep(50)
        typed += key
        print("typed", typed)
        if len(typed) == 4:
            if typed == CODE:
                print("open")
                bolt.angle(90)
                time.sleep(1)
                bolt.angle(0)
            else:
                print("wrong code")
                beep(1000)
            typed = ""
    time.sleep_ms(20)

# keypad.py: a 4x4 matrix keypad.
# Each row is driven high in turn; a held key joins its row to its column,
# so that column reads high. The columns are pulled down when nothing is held.
from machine import Pin
import time

KEYS = ("123A", "456B", "789C", "*0#D")

class Keypad:
    def __init__(self, rows, cols):
        self.rows = [Pin(n, Pin.OUT, value=0) for n in rows]
        self.cols = [Pin(n, Pin.IN, Pin.PULL_DOWN) for n in cols]
        self.last = None

    def held(self):
        for r, row in enumerate(self.rows):
            row.value(1)
            time.sleep_us(50)
            for c, col in enumerate(self.cols):
                if col.value():
                    row.value(0)
                    return KEYS[r][c]
            row.value(0)
        return None

    def scan(self):
        """The key just pressed, once per press, or None."""
        key = self.held()
        pressed = key if key != self.last else None
        self.last = key
        return pressed

# servo.py: a hobby servo on a PWM pin.
# The servo wants a pulse every 20 ms (50 Hz); the pulse's width picks the
# angle: 0.5 ms is 0 degrees and 2.5 ms is 180 degrees.
from machine import Pin, PWM

class Servo:
    def __init__(self, pin):
        self.pwm = PWM(Pin(pin))
        self.pwm.freq(50)

    def angle(self, degrees):
        degrees = max(0, min(180, degrees))
        self.pwm.duty_ns(500_000 + int(degrees * 2_000_000 / 180))