# Registry¶

## What is it?¶

The registry (sncosmo.registry) is responsible for translating string identifiers to objects, for user convenience. For example, it is used in sncosmo.get_bandpass and sncosmo.get_source to return a Bandpass or sncosmo.Model object based on the name of the bandpass or model:

>>> sncosmo.get_bandpass('sdssi')
<Bandpass 'sdssi' at 0x28e7c90>


It is also used in methods like bandflux to give it the ability to accept either a Bandpass object or the name of a bandpass:

>>> model = sncosmo.Model(source='hsiao')
>>> model.bandflux('sdssg', 0.)  # works, thanks to registry.


Under the covers, the bandflux method retrieves the Bandpass corresponding to 'sdssg' by calling the sncosmo.get_bandpass function.

The registry is actually quite simple: it basically amounts to a dictionary and a few functions for accessing the dictionary. Most of the time, a user doesn’t need to know anything about the registry. However, it is useful if you want to add your own “built-ins” or change the name of existing ones.

## Using the registry to achieve custom “built-ins”¶

There are a small set of “built-in” models, bandpasses, and magnitude systems. But what if you want additional ones?

Create a file mydefs.py that registers all your custom definitions:

# contents of mydefs.py
import numpy as np
import sncosmo

wave = np.array([4000., 4200., 4400., 4600., 4800., 5000.])
trans = np.array([0., 1., 1., 1., 1., 0.])
band = sncosmo.Bandpass(wave, trans, name='tophatg')

sncosmo.registry.register(band)


Make sure mydefs.py is somewhere in your \$PYTHONPATH or the directory you are running your main script from. Now in your script import your definitions at the beginning:

>>> import sncosmo
>>> import mydefs
>>> # ... proceed as normal
>>> # you can now use 'tophatg' as a built-in


## Changing the name of built-ins¶

To change the name of the 'sdssg' band to 'SDSS_G':

# contents of mydefs.py
import sncosmo

band = sncosmo.get_bandpass('sdssg')
band.name = 'SDSS_G'
sncosmo.register(band)


## Large built-ins¶

What if your built-ins are really big or you have a lot of them? You might only want to load them as they are needed, rather than having to load everything into memory when you do import mydefs. You can use the sncosmo.registry.register_loader function. Suppose we have a bandpass that requires a huge data file (In reality it is unlikely that loading bandpasses would take a noticeable amount of time, but it might for models or spectra.):

# contents of mydefs.py
import sncosmo

# ...
# read data from filename, create a Bandpass object, "band"
# ...
return band

filename = 'path/to/datafile/for/huge_tophatg'

Now when you import mydefs the registry will know how to load the Bandpass named 'huge_tophatg' when it is needed. When loaded, it will be saved in memory so that subsequent operations don’t need to load it again.