The ole Cape II

from smbus2 import SMBus
from time import sleep

# relevant registers
MODE1       = 0x00
MODE2       = 0x01
LED         = 0x06
ALL_LED     = 0xFA
PRE_SCALE   = 0xFE

def pwm_reg( output ):
    if output == 'all':
        return ALL_LED
    else:
        assert output in range(0, 16)
        return LED + 4 * output

class Pca9685:
    def __init__(self, bus, addr):
        self.addr = 0b10000000 | addr
        self.bus = bus
        self.write_reg(MODE1, 1 << 5)  # initialize MODE1 register
        sleep(500e-6)  # wait 500us to allow oscillator to power up

    def read_reg(self, reg):
        assert reg in range(0, 256)
        return self.bus.read_byte_data(self.addr, reg)

    def write_reg(self, reg, value):
        assert reg in range(0, 256)
        return self.bus.write_byte_data(self.addr, reg, value)

    def read_regs(self, reg, count):
        assert reg in range(0, 256)
        assert count in range(1, 257-reg)
        return self.bus.read_i2c_block_data(self.addr, reg, count)

    def write_regs(self, reg, values):
        assert reg in range(0, 256)
        return self.bus.write_i2c_block_data(self.addr, reg, values)

    def get_pwm(self, output):
        [on_l, on_h, off_l, off_h] = self.read_regs(pwm_reg(output), 4)
        on = on_l | on_h << 8
        off = off_l | off_h << 8
        return (off, on)

    def set_pwm(self, output, off, on=0):
        assert on in range(0, 4097)
        assert off in range(0, 4097)
        assert on <= off
        on_l = on & 0xff
        on_h = on >> 8
        off_l = off & 0xff
        off_h = off >> 8
        self.write_regs(pwm_reg(output), [on_l, on_h, off_l, off_h])