unexpected try-except drop through

[krismc]

Reading an rpi pico uart with
start=ticks_ms()
while ticks_ms()-start<100:
if uart.any(): return uart.read().decode()
print('timeout', ticks_ms()-start); return uart.read().decode()

it works OK, but if I put the last line inside a try except

start='ticks_ms()
while ticks_ms()-start<100:
if uart.any(): return uart.read().decode()
try:
print('timeout', ticks_ms()-start); return uart.read().decode()
except Exception as e: sys.print_exception(e)

it occasionally drops through into the timeout print without the while timeout loop having expired, typically after 0 to 2ms. I'm struggling to explain this. Any ideas?

[ricksorensen]

Could it be the wraparound on time.ticks_ms()? There us a time function to help avoid this:

start=ticks_ms()
while time.ticks_diff(ticks_ms(),start<100):
     dostuff()

https://docs.micropython.org/en/latest/library/time.html#time.ticks_diff

[GitHubsSilverBullet]

furthermore, there's a ticks_add() function, allowing to add or subtract an initial value.
target = ticks_add(ticks_ms(), 100)

Not to mention the inherent flaw:

from machine import UART
from time import ticks_ms, ticks_diff, ticks_add
from sys import print_exception

def anything():
    timeout_at = ticks_add(ticks_ms(), 500)
    while ticks_diff((now:=ticks_ms()), timeout_at) < 0:
        if uart.any():
            return uart.read().decode() # there's something to decode
    try:
        print('timeout', now)
        return uart.read().decode()     # reading without checking, there might NOT be anything to decode,
    except Exception as e:              # thus throwing an…
        print_exception(e)              # …AttributeError

uart = UART(0)
print(anything())

[peterhinch]

Why not use the UART's built-in timeout?

from machine import UART
from time import ticks_ms, ticks_diff

def anything():
    start = ticks_ms()
    res = uart.read()
    if res is None:  # Timed out waiting for 1st or subsequent chars
        print(f"Timeout at {ticks_diff(ticks_ms(), start)}ms")
    else:
        return res.decode()

uart = UART(0, timeout=500, timeout_char=500)
print(anything())