How to show a dictionary sorted on a value within its data?

[cl at isbd.net]

Joel Goldstick <joel.goldstick at gmail.com> wrote:
> On Wed, Oct 1, 2014 at 6:45 AM,  <cl at isbd.net> wrote:
> > Peter Otten <__peter__ at web.de> wrote:
> >> cl at isbd.net wrote:
> >>
> >> > I have a dictionary as follows:-
> >> >
> >> > {
> >> > u'StarterAmps1': Row(id=4, ain=u'AIN3', name=u'StarterAmps1',
> >> > conv=6834.374834509803, Description=u'Starter Amps'), u'LeisureVolts':
> >> > Row(id=1, ain=u'AIN0', name=u'LeisureVolts', conv=29.01374215995874,
> >> > Description=u'Leisure Volts'), u'RudderPos': Row(id=6, ain=u'AIN5',
> >> > name=u'RudderPos', conv=0.028125, Description=u'Rudder Position'), u'xx':
> >> > Row(id=7, ain=u'AIN6', name=u'xx', conv=0.028125, Description=u''),
> >> > u'LeisureAmps1': Row(id=3, ain=u'AIN2', name=u'LeisureAmps1',
> >> > conv=32.727273081945, Description=u'Leisure Amps'), u'StarterVolts':
> >> > Row(id=2, ain=u'AIN1', name=u'StarterVolts', conv=28.94469628911757,
> >> > Description=u'Starter Volts') }
> >> >
> >> > I want to output a menu to a user comprising some parts of the
> >> > dictionary (ain and Description) sorted by ain.
> >> >
> >> > Is there some incantation of sorted() that will do what I want?  I
> >> > can't quite fathom out the 'key=' parameter needed to sort it by the
> >> > tuple item.  Maybe I need a cmp= ?
> >> >
> >> > E.g. I want to do something like:-
> >> >
> >> >     for meas in sorted(adc.cfg, key=???):
> >> >         print(adc.cfg[meas].ain, adc.cfg[meas].Description)
> >> >
> >> > What's needed in the ???
> >>
> >> for meas in sorted(adc.cfg, key=lambda key: adc.cfg[key].ain):
> >>     print(adc.cfg[meas].ain, adc.cfg[meas].Description)
> >>
> > Brilliant, worked perfectly, thank you.  The bit I didn't understamd was
> > that 'lambda' bit, but I just looked it up and I'm a bit clearer now.
> >
> >> or simpler
> >>
> >> for row in sorted(adc.cfg.values(), key=operator.attrgetter("ain"))
> >>     print(row.ain, row.Description)
> >>
> > I tried this, I got:-
> >
> >     Traceback (most recent call last):
> >       File "/home/chris/bin/calibrate.py", line 24, in <module>
> >           for meas in sorted(adc.cfg.values,
> >           key=operator.attrgetter("ain")):
> >           NameError: name 'operator' is not defined
> > I must admit that it's the bits like 'operator' in the parameters that
> > I can't really understand where they come from.
> >
> 
> operator is a module. See
> https://docs.python.org/2/library/operator.html#module-operator

Ah, OK, as it sort of might be a keyword I hadn't thought of that. :-)

-- 
Chris Green
·