Keepin constants, configuration values, etc. in Python - dedicated module or what?

[Dave Angel]

cl at isbd.net Wrote in message:
> I am developing some code which runs on a (remote from me most of the
> time) Beaglebone Black single board computer.  It reads various items
> of data (voltages, currents, temperatures, etc.) using both a 1-wire
> bus system and the Beaglebone's ADC inputs.  The values are stored
> at hourly intervals into a database which is regularly rsync'ed
> across to my home system where I use the values for monitoring etc.
> 
> Associated with each value are various parameters, i.e. for each ADC
> input of the Beaglebone there is the ADC input number, a short name
> for the value, a conversion factor and a description.  Similarly for
> the 1-wire inputs there is the 1-wire filesystem filename, a short
> name for the data and a longer description.
> 
> I am puzzling where and how to keep these configuration values. My
> current design has them in dedicated tables in the database but this
> is rather clumsy in many ways as there's an overhead reading them
> every time the program needs them and changing them isn't particularly
> convenient at the Beaglebone doesn't have a GUI so it has to be done
> using SQL from the command line.

It is very useful for the database to be self contained and self
 describing.  Among other things it means that you can easily take
 a snapshot at any time,  and that snapshot is not tied to any
 specific version of the code. As for changing with sql, you can
 always add a configuration utility when using sql becomes a
 nuisance,  or introduces errors. 

The key thing is to manage change.  You need to decide which
 things are conceivably going to change and how you'll manage both
 the change itself and the inevitable split between prechange data
 and post. One kind of change might be fixing the spelling of
 battery.  No biggie,  just let new accesses get the new one. 
 Another kind might be the addition of a new instance of an adc
 converter.  Not a problem,  as long as you don't reuse an old
 name. And presumably you never remove an old name from the
 config.

More troublesome is adding a new kind of data. You have to decide
 whether it's worth generalizing the code to anticipate the
 change,  or just admit that the code will grow at that point and
 that old code won't deal with the new data. 

> 
> Does it make sense to add them to the modules which handle the reading
> of the inputs?  I already have modules defining classes called Adc and
> OneWire, is it a reasonable approach to add the configuration to these
> as, probably, dictionaries?  Modifying it would be pretty simple -
> just edit the python source (one of the easiest things for me to do on
> the Beaglebone as my sole access is via ssh/command line).
> 

Nope, read it from the db. You still might be loading it to a cfg
 variable, however. 

> Thus I'd have something like (apologies for any syntax errors):-
> 
> cfg = { "LeisureVolts": ["AIN0", 0.061256, "Leisure Battery Voltage"],
>         "StarterVolts": ["AIN1", 0.060943, "Starter Battery Voltage"],
>         "LeisureAmps1": ["AIN2", 0.423122, "Leisure Battery Current"}
> 
> (It might be better to makes those lists dictionaries, but it shows
> the idea)

Actually it's probably better to use named tuples. All those lists
 are apparently of identical length with identical meaning of a
 given element offset.  But if named tuple isn't flexible,enough, 
 you probably need a new class.

> 
> Are there any better ways of doing this?  E.g. some sort of standard
> configuration file format that Python knows about?  I would actually
> quite like to keep the configuration data separate from the code as it
> would simplify using the data at the 'home' end of things as I'd just
> need to copy the configuration file across.  This was why the database
> approach appealed at first as all I need to do is copy the database
> and everything is in there.
> 
> I'm not really expecting quick/glib answers, just some ideas and
> comments on the various ways of doing this and their advantages and
> disadvantages.

Main next question is whether you can restart each system when you
 add to the configuration.  


-- 
DaveA