selectors.py 文件源码

def _can_allocate(struct):
    """ Checks that select structs can be allocated by the underlying
    operating system, not just advertised by the select module. We don't
    check select() because we'll be hopeful that most platforms that
    don't have it available will not advertise it. (ie: GAE) """
    try:
        # select.poll() objects won't fail until used.
        if struct == 'poll':
            p = select.poll()
            p.poll(0)

        # All others will fail on allocation.
        else:
            getattr(select, struct)().close()
        return True
    except (OSError, AttributeError) as e:
        return False


# Choose the best implementation, roughly:
# kqueue == epoll > poll > select. Devpoll not supported. (See above)
# select() also can't accept a FD > FD_SETSIZE (usually around 1024)