A more flexible fix for custom tag constructors

[ingydotnet]

For #266 and #271. Replaces #273.

The yaml_loader class variable can be a list of loaders.
The YAMLObject helper class targets all the loader classes except SafeLoader.

Here's a test script that works. Uncomment the comments to support SafeLoader:

    #!/usr/bin/env python
    import yaml

    class Monster(yaml.YAMLObject):
        yaml_tag = u'!Monster'

        # yaml_loader = yaml.YAMLObject.yaml_loader
        # yaml_loader.append(yaml.SafeLoader)

        def __init__(self, name, hp, ac, attacks):
            self.name = name
            self.hp = hp
            self.ac = ac
            self.attacks = attacks
        def __repr__(self):
            return "%s(name=%r, hp=%r, ac=%r, attacks=%r)" % (
                self.__class__.__name__, self.name, self.hp, self.ac, self.attacks)

    input = """
    --- !Monster
    name: Cave spider
    hp: [2,6]    # 2d6
    ac: 16
    attacks: [BITE, HURT]
    """

    print(yaml.load(input))
    print(yaml.load(input, Loader=yaml.Loader))

    print(yaml.full_load(input))
    print(yaml.load(input, Loader=yaml.FullLoader))

    print(yaml.unsafe_load(input))
    print(yaml.load(input, Loader=yaml.UnsafeLoader))

    # print(yaml.safe_load(input))
    # print(yaml.load(input, Loader=yaml.SafeLoader))

Here is output:

$ PYTHONPATH=lib3 python3 test.py
test.py:27: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
  print(yaml.load(input))
Monster(name='Cave spider', hp=[2, 6], ac=16, attacks=['BITE', 'HURT'])
Monster(name='Cave spider', hp=[2, 6], ac=16, attacks=['BITE', 'HURT'])
Monster(name='Cave spider', hp=[2, 6], ac=16, attacks=['BITE', 'HURT'])
Monster(name='Cave spider', hp=[2, 6], ac=16, attacks=['BITE', 'HURT'])
Monster(name='Cave spider', hp=[2, 6], ac=16, attacks=['BITE', 'HURT'])
Monster(name='Cave spider', hp=[2, 6], ac=16, attacks=['BITE', 'HURT'])

Works with Python 2 as well.

[kgutwin]

I like this solution. Were you also planning to address the convenience methods issue as raised in #271?

Also, while this isn't really in the scope for this PR, I noticed during my review that the convenience methods scan(), parse(), compose() and compose_all() still reference Loader rather than FullLoader as their default Loader. For consistency, it may make sense to update those to behave similarly to load() and load_all().

[ingydotnet]

@kgutwin Thanks for your time.

I think #274 is meant to address the convenience methods. You should review and comment there.

[perlpunk]

This could actually work. The only thing it would break is cls.yaml_loader in from_yaml(). But since from_yaml() gets a loader instance, probably nobody would use the class attribute.
For the record, just add this to the script:

    @classmethod
    def from_yaml(cls, loader, node):

        # do something with cls.yaml_loader

        data = loader.construct_mapping(node)
        print(data['name'])
        a, b = map(int, data['hp'].split('d'))
        return cls(data['name'], [a, b], data['ac'], data['attacks'])

and change the input like this:

input = """
--- !Monster
name: Cave spider
#hp: [2,6]    # 2d6
hp: 2d6
ac: 16
attacks: [BITE, HURT]
"""

The only thing I'm not sure about is if people would want to have their objects added to three loaders.

I would not add SafeLoader. Things should only be added to SafeLoader explicitly.

[perlpunk]

For #266 and #271.

I think it does not address #271

[perlpunk]

To be really consistent, yaml.add_constructor should also add to three loaders.

[kgutwin]

The only thing I'm not sure about is if people would want to have their objects added to three loaders.

If my experience is anything of a "typical user", I'd say the vast majority of people don't care about the internal structure of PyYAML Loaders and would just like the module to work 😄 That's why I think this fix is a good strategy - it solves the broadest set of cases, and for users who want to be more fine-grained about their use of Loaders, it's easy for them to set the yaml_loader class attribute.

[perlpunk]

I created #287 which adds the same kind of logic to yaml.add_constructor