Having title_quantiles=None when quantiles is passed without having exactly 3 elements raises a ValueError about corner.py HOT 4 OPEN

The main problem is backward compatibility: any code that ran smoothly before v2.2.2 now breaks, even if the quantiles value it provides is still valid from what can be read in the documentation.
From what I see this is an actual issue due to the former behaviour of show_titles and quantiles which did not actually reflect what was written in the documentation (from what I understand and see from older versions of the code, while quantiles was supposed to impact what was shown in the title, the quantiles were actually hardwired to [0.16, 0.5, 0.84]). You can take a look at issue #193 which mentions the corresponding issue.

Anyway, I submitted as a suggestion pull request #244, hopefully it restores backward compatibility while maintaining the expected behaviour of title_quantiles when provided (and I took care of clarifying in the docstrings the role of each argument, the description of show_titles seemed obsolete).

from corner.py.

As a regular corner user this error makes complete sense to me. Can you tell why this behaviour is not desired?

from corner.py.

Well I have not specified it explicitly in the issue description above, but the problem is that quantiles is not an argument that requires mandatorily 3 elements as title_quantiles, here is what the documentation says:

quantiles (iterable) – A list of fractional quantiles to show on the 1-D histograms as vertical dashed lines.

Thus passing a list with arbitrary length should not raise any exception (and it also means that upgrading to corner==2.2.2 breaks codes that were working fine with previous versions).

To clarify even more, here is the expected behaviour if you run the example code above with corner=2.2.1:

import corner
import numpy as np
print (corner.__version__)

ndim, nsamples = 2, 10000
np.random.seed(42)
samples = np.random.randn(ndim * nsamples).reshape([nsamples, ndim])
figure = corner.corner(samples, quantiles=[0.16, 0.84], 
                       show_titles=True, title_quantiles=None)

2.2.1

from corner.py.

I really dont see any problem with this. title_quantiles and quantiles are different yet same. And I guess the code changes in 2.2.2 is useful. title_quantiles of course needs 3 values, else how will you show the mean and std? Morever if your quantiles is already a 3-vector which is used to draw the dashed lines then just passing show_titles=True is more than sufficient. In case you only pass 2-vector to quantiles, the the title_quantiles needs to passed which should be a 3-vector. One thing that can be done is to default to 1-sigma values if the title_quantiles is not passed (which internally is None)

from corner.py.