What is the difference between ! and !! in yaml?
opencv, python, yaml
Solution
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.
Problem
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?