Parameters linking issues

Gammapy version 1.0

Bug description The current way of linking parameters (par1 = par2) works only if par2.name is contained in the default_parameters.names list of the model to which par1 belongs. This generates several issues.

For example, I think this should not fail:

On the other hand, this should fail (since the linked parameters don’t have the same units):

Fixing this second issue is quite easy:

Instead for the first example it’s much more tricky. Perhaps something like a LinkedParameter class is needed

[luca-giunti]

OK thanks, we can indeed explore this kind of possibilities.

An alternative solution, that does not require re-thinking too much of the current modeling architecture, is based on a new LinkedParameter class that:

• takes care of updating the parameter value when needed
• checks that units are consistent
• adapts the parameter name to be consistent with the model.default_parameters.names list

As a big bonus, such class can easily be generalized to also accept an operator argument to perform arithmetic operation between parameters, which I think is a nice feature that we don’t currently support. In this case the identity would be just a special case, one could do complex links such as index2 = index1 + 1 or E_break = constant / B**2. Here is an example of this: https://github.com/luca-giunti/share/blob/master/LinkedParameter_Generalized.ipynb

[maxnoe]

@luca-giunti I didn’t mean it as “this should already work” but as the proposed API for how it should work.