MicroPython four-digit display on a Pico, multiplexed

# Four digits from one 74HC595: the chip (DS GP18, ST_CP GP20, SH_CP
# GP21) sets the segments, and GP17, GP16, GP15 and GP14 each switch on
# one digit's common anode. Lighting one digit at a time, a millisecond
# each, is fast enough for the eye to see all four: 0123.
from machine import Pin
import time
from hc595 import HC595

DIGITS = [0xC0, 0xF9, 0xA4, 0xB0, 0x99, 0x92, 0x82, 0xF8, 0x80, 0x90]

chip = HC595(ds=18, stcp=20, shcp=21)
commons = [Pin(n, Pin.OUT, value=0) for n in (17, 16, 15, 14)]
print("showing 0123")

while True:
    for place, common in enumerate(commons):
        chip.write(0xFF, msb_first=False)      # blank, so nothing ghosts
        common.value(1)
        chip.write(DIGITS[place], msb_first=False)
        time.sleep_ms(1)
        common.value(0)

# hc595.py: a 74HC595 shift register on three pins.
# Each bit goes out on DS and is clocked in on a rising SH_CP; a rising
# ST_CP then copies all eight to the outputs Q0 to Q7 at once.
from machine import Pin

class HC595:
    def __init__(self, ds, stcp, shcp, oe=None):
        self.ds = Pin(ds, Pin.OUT, value=0)
        self.stcp = Pin(stcp, Pin.OUT, value=0)
        self.shcp = Pin(shcp, Pin.OUT, value=0)
        if oe is not None:
            self.oe = Pin(oe, Pin.OUT, value=0)    # OE low: the outputs are on

    def shift(self, byte, msb_first=True):
        for i in range(8):
            bit = 7 - i if msb_first else i
            self.ds.value((byte >> bit) & 1)
            self.shcp.value(1)
            self.shcp.value(0)

    def latch(self):
        self.stcp.value(1)
        self.stcp.value(0)

    def write(self, byte, msb_first=True):
        self.shift(byte, msb_first)
        self.latch()