Asynchronous method calls

Simon McVittie simon.mcvittie at
Wed May 9 05:55:00 PDT 2007

On Wed, 09 May 2007 at 12:02:46 +0200, Luigi Paioro wrote:
> Suppose I have an object which exposes a method myMethod(arg1, arg2, 
> ...). Now I want to call it 3 times asynchronously with different 
> arguments each time:
> call 1: myMethod(1, 2, 3, ...)
> call 2: myMethod(6, 2, 4, ...)
> call 3: myMethod(8, 10, 3, ...)
> Using reply_handler and error_handler I easily link the responses to a 
> suitable callback function. As a result I can have 3 callbacks which are 
> in a different time order respect the calls:
> response 1: callback of call 2
> response 2: callback of call 3
> response 3: callback of call 1
> The problem is that I cannot recognize each response to which original 
> call are related (and sometime it can be important). I could do it 
> passing a call ID as an argument and getting it from the response, but 
> I'm wondering whether there could be another inner mechanism in DBus 
> itself or not.

There is in fact a message ID internally, but dbus-python doesn't expose it
(unless you're using dbus.lowlevel and building messages yourself - but
don't do that if you can help it).

In dbus-python you can easily construct a new callback for each call, using
nested functions or lambdas, which is why I don't provide a "user_data"
argument like dbus-glib would.

For instance, suppose your service exports Add(i, i) -> i. You currently

def print_reply(sum):
    print "something + something = %d" % sum

def add_and_print_reply():
    my_iface.Add(1, 2, reply_handler=print_reply, error_handler=explode)
    my_iface.Add(3, 4, reply_handler=print_reply, error_handler=explode)

You could change this to:

def print_reply(a, b, sum):
    print "%d + %d = %d" % sum

def add_and_print_reply():
    def rh(sum):
        print_reply(1, 2, sum)
    my_iface.Add(1, 2, reply_handler=rh, error_handler=explode)
    def rh(sum):
        print_reply(3, 4, sum)
    my_iface.Add(3, 4, reply_handler=rh, error_handler=explode)

If you expect this to be a frequent use case I could add a mechanism to
get the original arguments passed to the reply handler, but I suspect
that's needless complexity.


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