README.md

Demo difference in behaviors between pyyaml 3 and 4 when using custom tags and safe loading. Also check against a pending Ingy change.

Run tox to see it in action. The different versions will do different things. Which is right?

The key difference is that in PyYAML 3 we see ConstructorErrors from safe_load and in PyYAML 4 they are from danger mode.

custom.py

import yaml

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

    def __eq__(self, other):
        try:
            return math.isnan(other)
        except TypeError:
            return False

yaml.add_constructor(u'!IsNAN', lambda loader, node: NanChecker())

def test():
    safe_load = yaml.safe_load

    try:
        danger_load = yaml.danger_load
    except AttributeError:
        try:
            danger_load = yaml.python_load
        except AttributeError:
            print('danger_load not found, using load')
            danger_load = yaml.load

    test_strings = [
            "foo: !IsNAN",
            "foo: !NanChecker {}",
            "foo: !!python/object:__main__.NanChecker {}",
    ]

    print("#### danger load is: %s" % danger_load)
    print("#### safe load is: %s" % safe_load)
    for test_string in test_strings:
        print("#### TRYING: %s" % test_string)
        try:
            safe_tag_data = safe_load(test_string)
        except Exception as err:
            safe_tag_data = err.__class__.__name__
        try:
            danger_tag_data = danger_load(test_string)
        except Exception as err:
            danger_tag_data = err.__class__.__name__
        print('safe %s' % safe_tag_data)
        print('danger %s' % danger_tag_data)
    

if __name__ == '__main__':
    test()

tox.ini

[tox]
envlist = yIngy,y4,y3
basepython = python3
skipsdist = True


[testenv:yIngy]
deps = git+https://github.com/yaml/pyyaml@3dc3f5f0fc41db52e894f65cc4eec9470ffdf725
commands = python custom.py

[testenv:y4]
deps = PyYAML>=4.0
commands = python custom.py

[testenv:y3]
deps = PyYAML<4.0
commands = python custom.py