Overriding from_yaml to add custom YAML tag - yaml

Question

Is overriding from_yaml enough to register a tag from a class or is it necessary to use yaml.add_constructor(Class.yaml_tag, Class.from_yaml)? If I don't use te add_constructor method, my YAML tags are not recognized. Example of what I have:
import yaml
class Something(yaml.YAMLObject):
yaml_tag = u'!Something'
#classmethod
def from_yaml(cls,loader,node):
# Set attributes to None if not in file
values = loader.construct_mapping(node, deep=True)
attr = ['attr1','attr2']
result = {}
for val in attr:
try:
result[val] = values[val]
except KeyError:
result[val] = None
return cls(**result)
Is this enough for it to work? I'm confused with the use of from_yaml vs any other constructor you would register using the method I mentioned above. I suppose there's something fundamental I'm missing, since they say:
Subclassing YAMLObject is an easy way to define tags, constructors,
and representers for your classes. You only need to override the
yaml_tag attribute. If you want to define your custom constructor and
representer, redefine the from_yaml and to_yaml method
correspondingly.

Answer 1

There is indeed no need to register explicitly:
import yaml
class Something(yaml.YAMLObject):
yaml_tag = u'!Something'
def __init__(self, *args, **kw):
print('some_init', args, kw)
#classmethod
def from_yaml(cls,loader,node):
# Set attributes to None if not in file
values = loader.construct_mapping(node, deep=True)
attr = ['attr1','attr2']
result = {}
for val in attr:
try:
result[val] = values[val]
except KeyError:
result[val] = None
return cls(**result)
yaml_str = """\
test: !Something
attr1: 1
attr2: 2
"""
d = yaml.load(yaml_str)
which gives:
some_init () {'attr1': 1, 'attr2': 2}
But there is no need at all to use PyYAML's load() which is
documented to be unsafe. You can just use safe_load if you set the yaml_loader class attribute:
import yaml
class Something(yaml.YAMLObject):
yaml_tag = u'!Something'
yaml_loader = yaml.SafeLoader
def __init__(self, *args, **kw):
print('some_init', args, kw)
#classmethod
def from_yaml(cls,loader,node):
# Set attributes to None if not in file
values = loader.construct_mapping(node, deep=True)
attr = ['attr1','attr2']
result = {}
for val in attr:
try:
result[val] = values[val]
except KeyError:
result[val] = None
return cls(**result)
yaml_str = """\
test: !Something
attr1: 1
attr2: 2
"""
d = yaml.safe_load(yaml_str)
as this gives the same:
some_init () {'attr1': 1, 'attr2': 2}
(done both with Python 3.6 and Python 2.7)
The registering is done in the __init__() of the metaclass of yaml.YAMLObject:
class YAMLObjectMetaclass(type):
"""
The metaclass for YAMLObject.
"""
def __init__(cls, name, bases, kwds):
super(YAMLObjectMetaclass, cls).__init__(name, bases, kwds)
if 'yaml_tag' in kwds and kwds['yaml_tag'] is not None:
cls.yaml_loader.add_constructor(cls.yaml_tag, cls.from_yaml)
cls.yaml_dumper.add_representer(cls, cls.to_yaml)
So maybe you are somehow interfering with that initialisation in your full class definition. Try to start with a minimal implementation as I did, and add the functionality on your class that you need until things break.