MicroPython 74HC595 flowing light on a Pico

# A flowing light through a 74HC595: three pins (DS GP18, ST_CP GP20,
# SH_CP GP21) set eight LEDs of the bar graph. One lit bit walks across
# the eight outputs one way, then back. hc595.py is in the next tab.
# GP19 is the chip's OE, held low so the outputs are on.
import time
from hc595 import HC595

chip = HC595(ds=18, stcp=20, shcp=21, oe=19)

while True:
    for i in range(8):
        chip.write(0x01 << i)
        time.sleep_ms(300)
    for i in range(6, 0, -1):
        chip.write(0x01 << i)
        time.sleep_ms(300)
    print("sweep")

# 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()