Automatic field calculation

[dopplershift]

I have visions of being able to have certain fields automatically calculated on datasets. By providing a decorator for calculation functions (and utilizing unit handling), the calculations could specify its requirements, and a graph of relationships between the calculations could be generated. Using this graph, we could plot a course to calculate one variable from what's available.

[dopplershift]

As we add more calculations, like specific humidity, this becomes all the more interesting...

[dopplershift]

Ideas:

• Decorator to register functions for what they take and what they produce
• Strings like 'water_vapor_mixing_ratio' to identify parameters
• Possibly need id for functions (if we can't use their name) so that you can optionally specify one to use. For example, AMS glossary has RH as e/e_s, while WMO has r_v / r_vs; these are slightly different. Would be nice to have these available and selectable.
• Graph search to determine shortest path
• Driver function for performing calculation; take a dictionary (or maybe a Dataset) of stuff saying what I have and a request for what I want.
• Ability to override built-in calculations; essentially manually adding a node to the graph that could even be required to be in the final path
• Optional debugging/warnings that some passed in parameters/relations are not used to catch naming problems

Questions:

• Is it appropriate for things like EL, LCL, CAPE?
• What about dimensionality?

Use case:
I have specific humidity, temperature, and pressure; I want to get dewpoint without:

1. Creating a function to specifically enable this
2. Needing to discover for myself the easiest way to do this with the built-in primitive functions

This could help simplify, or at least prevent the explosion of, the calculations module as we build up functionality.