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import sysv_ipc
# Create a shared memory segment and write the (English) alphabet to the shared memory.
mem = sysv_ipc.SharedMemory(None, sysv_ipc.IPC_CREX, size=sysv_ipc.PAGE_SIZE)
ASCII_A = 0x61
alphabet = ''.join([chr(ASCII_A + i) for i in range(26)])
alphabet = bytes(alphabet, 'ASCII')
mem.write(alphabet)
# Create a bytearray from the SharedMemory.
ba = bytearray(mem)
# bytearray instances have "most of the usual methods of mutable sequences", such as replace.
# https://docs.python.org/3/library/functions.html#func-bytearray
ba = ba.replace(b'c', b'x')
assert(ba[:4] == b'abxd')
# Unlike a memoryview (see below), changes to the bytearray do *not* affect the underlying
# SharedMemory -- the bytearray is a copy.
assert(mem.read(4) == b'abcd')
# Reset the memory to contain the alphabet unmodified.
mem.write(alphabet)
# Create a memoryview from the SharedMemory.
mv = memoryview(mem)
# This memoryview has format = 'B', itemsize = 1, shape = (sysv_ipc.PAGE_SIZE, ), ndim = 1,
# strides = (1, ), and is read/write.
# This shows that you can take slices of a memoryview
assert([chr(c) for c in mv[3:6]] == ['d', 'e', 'f'])
# This shows that you can write to the memoryview.
mv[4] = ord('x')
assert([chr(c) for c in mv[3:6]] == ['d', 'x', 'f'])
# Changes to the underlying segment are reflected in the memoryview
mem.write(b'xxx')
assert([chr(c) for c in mv[:6]] == ['x', 'x', 'x', 'd', 'x', 'f'])
mem.detach()
mem.remove()
print('Done!')
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