Read from :stream and write to :target until :maxread or EOF.

def copy_file(stream, target, maxread=-1, buffer_size=2*16):
    ''' Read from :stream and write to :target until :maxread or EOF. '''
    size, read = 0, stream.read
    while 1:
        to_read = buffer_size if maxread < 0 else min(buffer_size, maxread-size)
        part = read(to_read)
        if not part:
            return size
        target.write(part)
        size += len(part)