A function in Python is a first class object. In the code below, f is defined
inside of gen_datapath_join_callback, and you get a new f. Since a function is
just an object, gen_datapath_join_callback then adds a new property to f -- the
cb property. Inside f, this property is accessed.
I didn't write this piece of code, but it can actually be done a bit more
simply. I would write it like this:
def gen_datapath_join_callback(handler):
def f(event):
attrs = {}
attrs[core.N_BUFFERS] = event.n_buffers
attrs[core.N_TABLES] = event.n_tables
attrs[core.CAPABILITES] = event.capabilities
attrs[core.ACTIONS] = event.actions
attrs[core.PORTS] = event.ports
for i in range(0, len(attrs[core.PORTS])):
port = attrs[core.PORTS][i]
config = port['config']
state = port['state']
attrs[core.PORTS][i]['link'] = (state &
openflow.OFPPS_LINK_DOWN) == 0
attrs[core.PORTS][i]['enabled'] = (config &
openflow.OFPPC_PORT_DOWN) == 0
attrs[core.PORTS][i]['flood'] = (config &
openflow.OFPPC_NO_FLOOD) == 0
ret = handler(event.datapath_id, attrs)
if ret == None:
return CONTINUE
return ret
return f
I don't know how much that explains. The big point is that a function is an
object like any other: it has properties, it can be passed around and
returned, and there can be multiple instances of it.
As for how f is invoked...
register_for_datapath_join() takes a callback function (handler) that is
supposed to get called on datapath join.
Rather than actually have the given handler be the callback function,
gen_datapath_join_callback() is used to generate a wrapper function. The
callback that gen_datapath_join_callback() creates (f) does some stuff (e.g.,
stocking the attrs dictionary), and then calls handler.
So the NOX event system calls f (via pyrt's Python_event_manager), and f calls
the user-provided handler function.
I hope that sheds at least a little light.
-- Murphy
On Nov 2, 2011, at 7:03 AM, hzy wrote:
> Hi all:
> when I look at the code in nox/src/nox/lib/util.py written in python. I was
> confused about some codes there. Because I am newbie to python.
>
> def gen_datapath_join_callback(handler):
> def f(event):
> attrs = {}
> attrs[core.N_BUFFERS] = event.n_buffers
> attrs[core.N_TABLES] = event.n_tables
> attrs[core.CAPABILITES] = event.capabilities
> attrs[core.ACTIONS] = event.actions
> attrs[core.PORTS] = event.ports
> for i in range(0, len(attrs[core.PORTS])):
> port = attrs[core.PORTS][i]
> config = port['config']
> state = port['state']
> attrs[core.PORTS][i]['link'] = (state &
> openflow.OFPPS_LINK_DOWN) == 0
> attrs[core.PORTS][i]['enabled'] = (config &
> openflow.OFPPC_PORT_DOWN) == 0
> attrs[core.PORTS][i]['flood'] = (config &
> openflow.OFPPC_NO_FLOOD) == 0
>
> ret = f.cb(event.datapath_id, attrs)
> if ret == None:
> return CONTINUE
> return ret
> f.cb = handler
> return f
>
> Python can define a function in a function really confused me. I do not know
> how the function f(event) is invoked. and what the f.cb mean.
> Any ideas?
> Thanks forward.
> hzy
> _______________________________________________
> nox-dev mailing list
> nox-dev@noxrepo.org
> http://noxrepo.org/mailman/listinfo/nox-dev
_______________________________________________
nox-dev mailing list
nox-dev@noxrepo.org
http://noxrepo.org/mailman/listinfo/nox-dev
