Pythonic way to copy list to a sip.voidptr from QSharedMemory

Question

I am using PyQT5 and the QSharedMemory class. I am creating a shared memory which can hold up 6 1-byte elements. To copy these elments in the shared memory array I am looping over the elments from the python list like the following snippet:

f = shared_mem.data()
k = f.asarray()
memtocopy = [0,1,2,3,4,5]
for i in range(0,len(memtocopy)):
    k[i]  = memtocopy[i]
shared_mem.unlock()

Which seems very unpythonic and boilerplate-code like. I am wondering if there is a more suitable way of achieving the same result?

When using

k[:] = memtocopy

or:

k[:] = np.asarray(memtocopy,np.uint8)

It will fail with the error message:

TypeError: can only assign another array of unsigned char to the slice

The whole test code for reproducing looks like the following:

from PyQt5 import QtCore 

# Create shared memory and attach it
shared_mem = QtCore.QSharedMemory()
shared_mem.setNativeKey("test")
shared_mem.create(4*6)
shared_mem.attach()


# Fill in 
shared_mem.lock()
f = shared_mem.data()
k = f.asarray()

memtocopy = [0,1,2,3,4,5]

# Loop in question
for i in range(0,len(memtocopy)):
    k[i]  = memtocopy[i]

shared_mem.unlock()



# Read out
shared_mem.lock()
f1 = shared_mem.data()
k1 = f1.asarray()
shared_mem.unlock()

# Test results
if k1[0] == memtocopy[0]:
    print("success!")
else:
    print("fail!")

Answer 1

Here's a simpler approach using struct and memoryview that reads and writes the data with a couple of one-liners :

import struct
from PyQt5 import QtCore

shared_mem = QtCore.QSharedMemory()
shared_mem.setNativeKey("test")
shared_mem.create(4*6)
shared_mem.attach()

memtocopy = [0,1,2,3,4,5]

try:

    # Fill in 
    shared_mem.lock()
    shared_mem.data()[:] = memoryview(struct.pack('=6i', *memtocopy))
    shared_mem.unlock()

    # Read out
    shared_mem.lock()
    # python3
    k = memoryview(shared_mem.data()).cast('i')
    # python2
    # k = struct.unpack('=6i', memoryview(shared_mem.data()))
    shared_mem.unlock()

    if k[3] == memtocopy[3]:
        print("success!")
    else:
        print("fail!")

finally:

    shared_mem.detach()