Python question

GNU Radio
1

Hi,

I know this isn’t really a gnuradio question, but a half-hour on Google
and browsing the Python site didn’t find me the answer. What does ** in
Python do? Not in the exponent context (2 ** 4 = 16). In particular, I’m
looking at receive_path.py in the digital examples folder, the following
code:

    # Get demod_kwargs
    demod_kwargs = \
        self._demod_class.extract_kwargs_from_options(options)

    ...

    # receiver
    self.packet_receiver = \
        demod_pkts(fg,
                        self._demod_class(fg, **demod_kwargs),
                        access_code=None,
                        callback=self._rx_callback,
                        threshold=-1)

Is this some crazy C that SWIG does? Thanks,

-Dan

2

On Thu, Jan 04, 2007 at 08:55:56PM -0800, Dan H. wrote:

Hi,

I know this isn’t really a gnuradio question, but a half-hour on Google
and browsing the Python site didn’t find me the answer. What does ** in
Python do? Not in the exponent context (2 ** 4 = 16). In particular, I’m
looking at receive_path.py in the digital examples folder, the following
code:

As Tom said it allows dictionaries to be used for passing argument
lists. Details on both * and ** can be found in the Python Language
Reference here: http://docs.python.org/ref/function.html

Eric

3

and browsing the Python site didn’t find me the answer. What does ** in
# receiver
self.packet_receiver =
demod_pkts(fg,
self._demod_class(fg, **demod_kwargs),
access_code=None,
callback=self._rx_callback,
threshold=-1)

Is this some crazy C that SWIG does? Thanks,

Nope, just a fun feature Python provides. It allows you to pass a
dictionary
object as the arguments to a function.

Say we have a function prototype:
def foo(a, b, c):
We normally call this r = foo(<value_a>, <value_b>, <value_c>), but we
could
just as easily call this as r = foo(b=<value_b>, c=<value_c>,
a=<value_a>).

What we’re doing in the receive_path is setting up a dictionary that
would
look like (in the above example) di = {“a”: <value_a>, “b”: <value_b>,
“c”:
<value_c>} and calling foo as ret = foo(**di).

It allows us to create different modulators that have different
arguments
they require. The “self._demod_class.extract_kwargs_from_otpions”
formats
the dictionary for the specific modulation class being used, and that’s
then
passed to the modulator object. Makes for nice, flexible code.

Tom