The docs lead one to believe that slicing a memoryview should be allocation free, but this seems not to be the case.

This arose in this forum thread. The user has a timer ISR which uses SPI to read first the number of bytes to read, followed by a subsequent SPI read of the payload. I suggested a memoryview, but this does not work as the slice operation causes an allocation.

I replicated the issue with this sample which can be pasted to a Pyboard:

from machine import SPI
from pyb import Timer
import micropython
import time
micropython.alloc_emergency_exception_buf(100)

s = SPI(1)
buf = bytearray(20)
mv = memoryview(buf)
tim = Timer(1)

def cb(t):
    #s.readinto(mv)  # Works
    s.readinto(mv[:5])  # Memory error

tim.init(freq=10, callback=cb)
while True:
    print(buf)
    time.sleep(1)