import io

class MyIO(io.RawIOBase):
    'Print size when invoke .read(size) function'
    
    def __init__(self, buffer):
        self.buffer = buffer
        self.posi = 0

    def readable(self):
        return True

    def read(self, size=-1):
        print('  MyIO.read(size), size:', size)
        
        if size < 0:
            return self.buffer
        
        end_size = min(self.posi+size, len(self.buffer))
        ret = self.buffer[self.posi:end_size]
        self.posi = end_size

        return ret

    def readinto(self, b):
        with memoryview(b) as view:
            data = self.read(len(view))
            view[:len(data)] = data
        return len(data)


b= b'a' * 100_000

print('br.read(50):')
br = io.BufferedReader(MyIO(b), buffer_size=32*1024)
dat = br.read(50)

print('br.read(-1):')
br = io.BufferedReader(MyIO(b), buffer_size=32*1024)
dat = br.read(-1)