[Tutor] Don't understand this class/constructor call syntax

[dave]

Hello,

I'm trying to work on GNU Radio and having trouble understanding some of the
Python code.  I have a C/C++ coding background.  I'm looking at the
ieee802.15.4 code found on CGRAN.  It's about 4 years old and runs but doesn't
function anymore so I'm trying to fully understand it to fix it.

In one file (a file in src/example called cc2420_txtest.py) I have the
following line from a constructor for:

class transmit_path(gr.top_block)
...
...
...
        self.packet_transmitter = ieee802_15_4_pkt.ieee802_15_4_mod_pkts(self,
spb=self._spb, msgq_limit=2)



Now in the src/python directory for this project I have ieee802_15_4pkt.py
which has the following class:



class ieee802_15_4_mod_pkts(gr.hier_block2):
    """
    IEEE 802.15.4 modulator that is a GNU Radio source.
    Send packets by calling send_pkt
    """
    def __init__(self, pad_for_usrp=True, *args, **kwargs):[/code]



What I don't understand is the call to the constructor and the constructor
definition.  Since it's using a number of advanced features, I'm having
trouble looking it all up in documentation.

What does it mean to call with spb=self._spb?  In the example file, spb is set
= to 2 and so is self._spb.  Is it a sort of pass by reference like C while
also assigning a value? Why the  ** on kwargs then? as if it is a matrix

(and does anyone have any idea what kwargs are (as opposed to args)?)

I'm uncertain about the first argument, but I guess it must be the
transmit_path object passed in place of the usually implicit self...  I'm just
not sure how Python figures out that it's not pad_for_usrp... magic I guess!


Thanks for your help,
Dave