Question

I would like to allocate a large bytearray, but also divide it up into a number of smaller bytearray-like objects. When the smaller objects are modified, they should modify the underlying bytearray.

I’ve tried both of the following strategies.

from pprint import pprint as pp
b = bytearray(10)
c = b[5:]
c = bytearray(1 for d in c)
pp(c)
pp(b)

and

from pprint import pprint as pp
b = bytearray(10)
c = b[5:]
for i in range(len(c)):
  c[i] = 1
pp(c)
pp(b)

They both output

bytearray(b'\x01\x01\x01\x01\x01')
bytearray(b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')

My aim would be to have a similar piece of code, which outputs

bytearray(b'\x01\x01\x01\x01\x01')
bytearray(b'\x00\x00\x00\x00\x00\x01\x01\x01\x01\x01')

That is, when c is updated, b is also updated in the appropriate position.

Answer 1

memoryview is close to what you want

>>> b = bytearray(10)
>>> c = memoryview(b)[5:]
>>> c[:] = bytearray(1 for d in c)
>>> b
bytearray(b'\x00\x00\x00\x00\x00\x01\x01\x01\x01\x01')

you have to use c.tobytes() or pass c to bytearray to see it’s contents

>>> c.tobytes()
'\x01\x01\x01\x01\x01'
>>> bytearray(c)
bytearray(b'\x01\x01\x01\x01\x01')