To simplify the units conversion interface -- between the data input and the sed/output (erg/s/cm2) -- a less programmatic (or python programming) interaction with the user is very welcome.

Context

Take for example the sdss catalog entry (assai/catalogs/sdss/): in the filters.py module, a set of equations define how to convert SDSS' nanomaggies to erg/s/cm2. It is a python module (working like plugin) where functions flux_density and wavelength must be defined -- catalogs/filters.py expect those functions to be defined as well as the filters dictionary

Problem

Too technical, too delicate. Besides the necessity of knowing the python language, the user is very much prone to errors; the structure in place is very specific.

Alternatives

Ideally, the user will have to define a flux-density expression in a simple pseudo-code. Something like it would be done in Mathematica, Maple, Latex, for example. If one simple equation is all it takes to convert the "whatever-units" to erg/s/cm2, great, but expression can be a system of equations also.

We cannot though accept freely equations/expressions given by the user and simply eval, this is clearly unsafe; We must apply some parsing and/or use a restricted framework. The following may help find the solution:

• SymPy
• PyParsing
• Ast (stdlib)
• and a discussion on the topic: https://stackoverflow.com/questions/11951701/safe-way-to-parse-user-supplied-mathematical-formula-in-python