How to show a dictionary sorted on a value within its data?
Peter Otten
__peter__ at web.de
Wed Oct 1 07:12:37 EDT 2014
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.
`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)
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