How to use io.BytesIO in Python to write to an existing buffer?

According to How the write(), read() and getvalue() methods of Python io.BytesIO work? , it seems io.BytesIO will copy initial bytes provided.

I have a buffer, and I want pickle to directly dump object in that buffer, without any copy. So I tried to use f = io.BytesIO(buffer). However, after f.write, my buffer is not modified.

Here is an example:

a = bytearray(1024)
import io
b = io.BytesIO(memoryview(a))
b.write(b"1234")
a[:4] # a does not change

What I want, is to make io.BytesIO directly writes to my buffer.

My ultimate goal is:

a = bytearray(1024)
import io, pickle
with io.BytesIO(memoryview(a)) as f:
    obj = [1, 2, 3]
    pickle.dump(obj, f)
# a should hold the pickled data of obj

>Solution :

IIUC you can try to make your own IO class:

import io
import pickle


class MyIO(io.IOBase):
    def __init__(self, buf):
        self.buf = memoryview(buf)
        self.position = 0

    def write(self, b):
        start = self.position
        n = len(b)
        end = start + n
        self.buf[start: end] = b
        self.position = end


a = bytearray(1024)
with MyIO(a) as f:
    obj = [1, 2, 3]
    pickle.dump(obj, f)

print(a)

Prints:

bytearray(b'\x80\x04\x95\x0b\x00\x00\x00\x00\x00\x00\x00]\x94(K\x01K\x02K\x03e.\x00\x00\x00  ...