如何生成包含带有 ruamel.yaml 的本地标签的 YAML？

Question

I'm using ruamel.yaml to generate a YAML file that will be read by Tavern , which requires the file to contain a list like this:

includes:
- !include vars.yaml

Attempting to use any of the usual approaches to dump the data as strings results in single quotes being added around the tags , which doesn't work when the YAML is ingested by the next tool.

How do I generate a YAML file that contains unquoted local tags , starting with data that is defined in a dictionary?

Answer 1

I was able to create a YAML file with the required format using the following approach, based on prior examples . My approach is more flexible because it allows the tag handle to be an instance property rather than a class property, so you don't need to define a different class for every tag handle.

import sys
from ruamel.yaml import YAML

yaml = YAML(typ='rt')

class TaggedString:    
    def __init__(self, handle, value):
        self.handle = handle
        self.value = value

    @classmethod
    def to_yaml(cls, representer, node):
        # I don't understand the arguments to the following function!
        return representer.represent_scalar(u'{.handle}'.format(node),
            u'{.value}'.format(node))

yaml.register_class(TaggedString)

data = {
    'includes': [
        TaggedString('!include', 'vars.yaml'),
        TaggedString('!exclude', 'dummy.yaml')
    ]
}

yaml.dump(data, sys.stdout)

Output:

includes:
- !include vars.yaml
- !exclude dummy.yaml

I am not sure if this is the best approach. I might be missing a simpler way to achieve the same result. Note that my goal is not to dump a Python class; I'm just doing that as a way to get the tag to be written correctly.

Answer 2

I am not sure if this is a better approach, but if you had tried to round-trip your required output, you would have seen that ruamel.yaml actually can preserve your tagged strings, without you having to do anything. Inspecting the Python datastructure, you'll notice that ruamel.yaml does this by creating a TaggedScalar (as you cannnot attach attributes to the built-in string type).

import sys
import ruamel.yaml

yaml_str = """\
includes:
- !include vars.yaml
- !exclude dummy.yaml
"""

yaml = ruamel.yaml.YAML()
data = yaml.load(yaml_str)
yaml.dump(data, sys.stdout)

incl = data['includes'][0]
print(type(incl))

which gives:

includes:
- !include vars.yaml
- !exclude dummy.yaml
<class 'ruamel.yaml.comments.TaggedScalar'>

After inspecting comments.py (and possible constructor.py ), you should be able to make ruamel.yaml 's internal data structure on the fly:

import sys
import ruamel.yaml
from ruamel.yaml.comments import TaggedScalar

def tagged_string(tag, val):
    # starting with ruamel.yaml>0.16.5 you can replace the following lines with:
    # return TaggedScalar(value=val, tag=tag)
    ret_val = TaggedScalar()
    ret_val.value = val
    ret_val.yaml_set_tag(tag)
    return ret_val

yaml = ruamel.yaml.YAML()
data = dict(includes=[tagged_string('!include', 'vars.yaml'), 
                      tagged_string('!include', 'vars.yaml'),
                     ])
yaml.dump(data, sys.stdout)

which also gives:

includes:
- !include vars.yaml
- !include vars.yaml