how to define new optional formatting tag (like ordinal_compass for windDir)?

[Graham Eddy]

there is an existing optional formatting tag ‘ordinal_compass’ for data_type ‘windDir’
eg. where $current.windDir.raw = 45, $current.windDir.ordinal_compass = “NE"

i would like to create a new optional formatting tag ‘category’ for new xtype ‘aqi_pm2_51’ (air quality index derived PM2.5)
e.g. where $current.aqi_pm2_51.raw = 75, $current.aqi_pm2_51.category = “Moderate”

how do i create the new optional formatting tag?
so far all i have found is to extend weewx.units.ValueHelper - i guess a function called “catgeory” would be picked up to do the formatting. but this would require changing rather than extending weewx codebase.
have i missed something obvious?

[Tom Keffer]

I can't think of any elegant way to do it, but an inelegant way is to monkey patch ValueHelper.

import weewx.units

def category(vh):

  if vh.raw is None:

    result = "N/A"

  elif vh.raw < 50:

    result = "good"

  elif vh.raw < 100:

    result = "moderate"

  else:

    result = "bad"

  return result

weewx.units.ValueHelper.category = category

The ValueHelper class has been pretty stable for a long time, so you're unlikely to break things in an upgrade. But, there's no guarantees!

-tk

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[Graham Eddy]

thanks (i think).

if i use this - and i have yet to come up with a feasible alternative - i think i’ll bury it under a pile of leaves at the bottom of the file...

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[Tom Keffer]

That's wise. :-)

The alternative is a search list extension, dedicated to producing the proper string. Its use would look like

<p>The air quality is $aq_category($current.aqi_pm2_51.raw)</p>

That's not so bad.

-tk

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