How to implement readinto() method

Question

I would like to implement a class which inherits from RawIOBase. I'm struggling to implement the readinto method. Basically, I don't know how to alter the content of the bytearray object passed as a parameter.

I've tried following (naive) approach:

def readinto(self, b):
    data = self.read(len(b))
    b = data
    return len(data)

But this, as I suspect, will assign new bytearray object to local variable b and it does not change content of the original bytearray.

Answer 1

from the docs of RawIOBase.readinto:

Read bytes into a pre-allocated, writable bytes-like object b, and return the number of bytes read. If the object is in non-blocking mode and no bytes are available, None is returned.

its a bit confusing but you need to write into the bytes-like object b (not read)

Example Usage

import io

class MyIO(io.RawIOBase):
    def readinto(self, b):
        msg = b'hello'
        b[:len(msg)] = msg
        return len(msg)

i = MyIO()
buf = bytearray()
i.readinto(buf)
print(buf)

Take a look at CPython implementation of BytesIO.readinto. basically its does memcpy from the object's buffer to the function input buffer.