[Ns-bugs] [Bug 127] Add trace callback type information for Python bindings

[code@nsnam.ece.gatech.edu]

http://www.nsnam.org/bugzilla/show_bug.cgi?id=127





--- Comment #5 from Gustavo J. A. M. Carneiro <gjcarneiro at gmail.com>  2009-01-09 18:00:56 EDT ---
(In reply to comment #4)
> (In reply to comment #3)
> > This is still an issue AFAIK.  Not P1 because Python bindings will be merged
> > after ns 3.1.
> > 
> 
> Gustavo, can you clarify the status of this?  My understanding is that:
> 
> 1) if a callback is exported explicitly at the API, it is possible to register
> a python method with it, thereby enabling people to write python code for ns-3
> 
> 2) but due to this bug, we just can't hook them through the trace hooks
> obtained through the trace path
> 
> If I am correct, are there any examples (even a small sample program for
> samples/ directory) of how to do this?  Someone here at UW would like to try
> writing some Python ns-3 code.
> 

The problem is that 1) in Python we cannot introspect the signature of a
"callable" (i.e. function or method, or class instance with a __call__ method),
and 2) NS-3 also does not provide an API that says what is the expected
callback type for a certain trace path, but it eventually checks in runtime
whether supplied callbacks are of the correct type.  Although the
Config::Connect API accepts a generic callback type (CallbackBase) as
parameter, it later matches it against one of the concrete callback types
(Callback<R,T1,T2,...>).  So the Python bindings have to supply a concrete
callback wrapper based on a Python callable for which I cannot obtain signature
information.

I can think of two possible and contrasting ways to move forward:

 1- Add a new NS-3 API to give back callback type(s) regarding trace path.  The
Python bindings could work things out and the Python API would turn out to be
simple:


   def my_callback(packet):
      print packet

   ns3.Config.Connect("/NodeList/*/....", my_callback)


 2- Force the Python programmer to tell the NS-3 Python bindings what is the
callback signature via a decorator design pattern, e.g.:

   def my_callback(packet):
      print packet

   ns3.Config.Connect("/NodeList/*/....",
ns3.Callback__lt__Ptr__lt__Packet__gt____gt__(my_callback))


Obviously I would prefer option 1 because it is much simpler to the NS-3
programmer.  I wonder if there isn't some Match API already in the works that
could be used as basis for this?...


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