I don't understand what a YAML tag is

Question

I get it on some level, but I have yet to see an example that didn\'t bring up more questions than answers.

http://rhnh.net/2011/01/31/yaml-tutorial

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Answer 1

I don't know a lot about YAML but I'll give it a shot:

Tags are used to denote types. A tag is declared using ! as you have seen from the "refcard" there. The %TAG directive is kind of like declaring a shortcut to a tag.

I'll demonstrate with PyYaml. PyYaml can parse the secondary tag of !!python/object: as an actual python object. The double exclamation mark is a substitution in itself, short for !tag:yaml.org,2002:, which turns the whole expression into !tag:yaml.org,2002:python/object:. This expression is a little unwieldy to be typing out every time we want to create an object, so we give it an alias using the %TAG directive:

%TAG !py! tag:yaml.org,2002:python/object:            # declares the tag alias
---
- !py!__main__.MyClass                                # creates an instance of MyClass

- !!python/object:__main__.MyClass                    # equivalent with no alias

- !<tag:yaml.org,2002:python/object:__main__.MyClass> # equivalent using primary tag

Nodes are parsed by their default type if you have no tag annotations. The following are equivalent:

- 1: Alex
- !!int "1": !!str "Alex"

Here is a complete Python program using PyYaml demonstrating tag usage:

import yaml

class Entity:
    def __init__(self, idNum, components):
        self.id = idNum
        self.components = components
    def __repr__(self):
         return "%s(id=%r, components=%r)" % (
             self.__class__.__name__, self.id, self.components)

class Component:
    def __init__(self, name):
        self.name = name
    def __repr__(self):
        return "%s(name=%r)" % (
            self.__class__.__name__, self.name)

text = """
%TAG !py! tag:yaml.org,2002:python/object:__main__.
---
- !py!Component &transform
  name: Transform

- !!python/object:__main__.Component &render
  name: Render

- !<tag:yaml.org,2002:python/object:__main__.Entity>
  id: 123
  components: [*transform, *render]

- !<tag:yaml.org,2002:int> "3"
"""

result = yaml.load(text)

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