To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: How to show a dictionary sorted on a value within its data?
Date: Wed, 01 Oct 2014 13:12:37 +0200
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cl@isbd.net wrote:

> Peter Otten <__peter__@web.de> wrote:
>> cl@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.

`lambda` is just a fancy way to define a function inline. With a normal 
`def` it would be

def get_ain(key):
    return adc.cfg[key].ain

or meas in sorted(adc.cfg, key=get_ain):
   print(adc.cfg[meas].ain, adc.cfg[meas].Description)

> 
>> 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.

As Joel says, operator is a module. To use it add

import operator

at the beginning of your module. To check if you've really understood the 
lambda you can try to replace the operator.attrgetter("ain") in

>> for row in sorted(adc.cfg.values(), key=operator.attrgetter("ain")):

with 

for row in sorted(adc.cfg.values(), key=lambda ...):

>> or even
>> 
>> for row in sorted(
>>         map(operator.attrgetter("ain", "Description"),
>>         adc.cfg.values())):
>>     print(*row)