Problems with fitMCMC (with proposed fixes)

[jfrostburke]

Hi, I’m a grad student attempting to use snpy to fit some SNe Ia data. Specifically I’m attempting to use the fitMCMC method to enforce priors. I have two corrections which I believe should be made:

1. In the documentation for fitMCMC (https://users.obs.carnegiescience.edu/cburns/SNooPyDocs/html/fitting_MCMC.html), the first fit is run using

s.fit(Rv=2.0)

This correctly fixes the Rv parameter. However, the MCMC fit is run using

s.fitMCMC(bands=['u','B','V','g','r','i','Y','J','H'], R_V="N,2.3,0.9")

R_V is not a parameter, Rv is. I believe passing this extra argument does nothing (e.g. if I pass foobar=’N,2.3,0.9’ then the fit will run without errors, but obviously that’s not a parameter).

I believe that R_V should be changed to Rv in the MCMCfit documentation.

2. However, making sure parameter names match introduces an error. Here’s the partial traceback when I try to enforce a prior on EBVhost:

emcee: Exception while calling your likelihood function:
  params: [3.25815455e+01 1.55605255e+00 5.81841998e+04]
  args: ({'varlist': ['DM', 'dm15', 'EBVhost', 'Tmax'], 'fitflux': True, 'free': ['DM', 'dm15', 'Tmax'], 'DM': {'fixed': False, 'index': 0, 'prior_type': 'model'}, 'dm15': {'fixed': False, 'index': 1, 'prior_type': 'model'}, 'EBVhost': {'value': 'U,0,1', 'fixed': True}, . . .
. . .
"/Users/jamisonfrost/miniconda3/envs/earlysne/lib/python3.7/site-packages/snpy/model.py", line 473, in __call__
    temp = temp + self.Robs[band]*self.EBVhost + R*self.parent.EBVgal
TypeError: can't multiply sequence by non-int of type 'numpy.float64'

The parameter is getting fixed as a string, instead of translated into the desired prior. In order for the built-in priors (uniform, exponential, and normal) to be instantiated, I believe this line should be changed:

snpy/snpy/snemcee.py

Line 65 in f8c6ca8

if type(args[var]) is bytes:

Currently it’s checking for an arg with type bytes, and I think this should be changed to str. Passing the argument as a byte string doesn’t work, I get ValueError: I don't understand the prior code b'U,0,1' (since it checks if st[0] in [‘U’, ‘G’, ‘E’], and b’U,0,1’[0] is apparently 85). I’ve tested it and making the change to str allows priors to be enforced as expected.

For completeness, I’m running this on python 3.7.9, snpy version 2.5.3.