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

[cl at isbd.net]

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.


> or even
> 
> for row in sorted(
>         map(operator.attrgetter("ain", "Description"), adc.cfg.values())):
>     print(*row)
> 

-- 
Chris Green
·