What is the difference between ! and !! in yaml?

Question

I'm trying to load a YAML that looks like this:

dist: !!opencv-matrix
   rows: 380
   cols: 380
   dt: f
   data: [ 0., 0., -1.88644529e+18, 2.45423365e+00, 11698176.,
       2.03862047e+00, -8.85501460e+10, 2.54738545e+00, 1.71208843e+20,
       ...
       2.44447327e+00 ]

The loading code is just:

import yaml
y = yaml.load(s)

where s is the YAML loaded into a string.

I get this error:

yaml.constructor.ConstructorError: could not determine a constructor for the tag 'tag:yaml.org,2002:opencv-matrix'
  in "<string>", line 382, column 7:
    dist: !!opencv-matrix

This is fair enough, so I add the constructor for that tag:

def opencv_matrix(loader, node):
    mapping = loader.construct_mapping(node)
    mat = np.array(mapping["data"])
    mat.resize(mapping["rows"], mapping["cols"])
    return mat

yaml.add_constructor(u"!!opencv-matrix", opencv_matrix)
y = yaml.load(s)

I still get the error. However, if I replace !!opencv_matrix with !opencv_matrix, then everything works.

What's going on here?

Answer 1

Secondary tags like !!opencv-matrix are actually shorthand for tag:yaml.org,2002:opencv-matrix (mentioned in the reference card). It looks like PyYAML's add_constructor method doesn't correctly handle this shorthand notation.

This may be a bug, depending on how secondary tags are interpreted (see second part below). I've submitted a bug report here, and hopefully it will be addressed.

Primary tags like !opencv-matrix are defined explicitly, and loading seem to work without any problems in PyYAML.

It works for me if you replace !!opencv-matrix with tag:yaml.org,2002:opencv-matrix in the add_constructor call.

---

As for the original question, AFAIK primary tags (!) are for user-defined types, while secondary tags (!!) are meant to represent standard language-independent types defined here (hence the long and fancy format).

If this is an OpenCV-generated file, then maybe it would probably be simpler if these tags were changed to primary tags in the application.