MicroPython 4x4 keypad on an ESP32, on an ESP32-DevKitC V4

# Matrix keypad: print each key as it is pressed. Rows on GPIO 14, 27,
# 26, 25; columns on 13, 21, 22, 23. keypad.py is in its own tab.
from keypad import Keypad
import time

pad = Keypad([14, 27, 26, 25], [13, 21, 22, 23])
print("Press a key")

while True:
    key = pad.key()
    if key:
        print("Key:", key)
    time.sleep_ms(10)

# keypad.py: a 4x4 matrix keypad. Each row is pulled low in turn and the
# columns, held high by pull-ups, show which key joins them to that row.
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=1) for n in rows]
        self.cols = [Pin(n, Pin.IN, Pin.PULL_UP) for n in cols]
        self.down = None

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

    def key(self):
        """The key just pressed, once per press, or None."""
        held = self._held()
        if held == self.down:
            return None
        self.down = held
        if held is not None:
            time.sleep_ms(10)           # let the contact settle
        return held