How to fix ESP32 MicroPython 'ValueError: pin can only be input'

Problem:

You are trying to initialize an ESP32 pin in MicroPython using

import machine
machine.Pin(34, machine.Pin.OUT)

but you see an error message like

>>> machine.Pin(34, machine.Pin.OUT)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: pin can only be input

Solution

On the ESP32, pins with numbers >= 34 are input-only pins!

You need to use other pins < 34 if you need output capability!

For reference, see the relevant MicroPython source code section:

// configure mode
if (args[ARG_mode].u_obj != mp_const_none) {
    mp_int_t pin_io_mode = mp_obj_get_int(args[ARG_mode].u_obj);
    if (self->id >= 34 && (pin_io_mode & GPIO_MODE_DEF_OUTPUT)) {
        mp_raise_ValueError("pin can only be input");
    } else {
        gpio_set_direction(self->id, pin_io_mode);
    }
}