File-stream emulation with Python buffer API.

bytewriter.py

import io

class ByteWriter(io.RawIOBase):
    """
    Emulates file-writing by pushing the content into a underyling buffer.
    Underlying buffer stores the data contents in the RAM, dumps them when required.
    
    >>> from bytewriter import ByteWriter
    >>> b = ByteWriter()
    >>> b.write(b"Hello World")
    >>> bytes(b)
    b'Hello World'
    """
    _rawbuf = bytes()
    
    def read(self, n=-1):
        raise OSError()

    def readable(self):
        return False

    def writable(self):
        return True

    def write(self, _o):
        self._rawbuf+= _o

    def writelines(self, lines, crlf= False):
        LINE_CHAR = '\r\n' if crlf else '\n'
        for line in lines:
            self.write(line + LINE_CHAR)

    def _flush(self):
        self._rawbuf = bytes()

    def __bytes__(self):
        return self._rawbuf
        self._flush()

This is shouldn't be used as BytesIO is a inbuilt CPython implementation which is way faster than this.