mem32[] as default argument: should it work?

[cve2022]

I was debugging a problem with a function that read the global enable register of the PWMs in the Pico. I found the mem32[] call in the function default argument to be the culprit.

from machine import PWM, Pin, mem32
from micropython import const

REG_BASE = const(0x40050000)
GLOBAL_EN = const(REG_BASE + 0xA0)

def tst(en = mem32[GLOBAL_EN] & 0xFF):
    print("tst() input argument: " + hex(en))
    print("tst() read inside: " + hex(mem32[GLOBAL_EN] & 0xFF))

tst(0xA5)

pwm1 = PWM(Pin(0))
pwm1.freq(1000)
pwm1.duty_u16(1000)

tst()

pwm2 = PWM(Pin(14))
pwm2.freq(1000)
pwm2.duty_u16(1000)

tst()

Here the result of running it (either Thonny or importing on repl). mem32[] returns 0 when called as the default.

tst() input argument: 0xa5
tst() read inside: 0x0
tst() input argument: 0x0
tst() read inside: 0x1
tst() input argument: 0x0
tst() read inside: 0x81

Am I doing something wrong?

[robert-hh]

Argument defaults are evaluated only once at compilation/parsing time. That's written somewhere, and may be even Python standard.

[cve2022]

Hello, Robert, in hindsight it makes much sense.

Thank you.