Question

I’ve tried following the instructions here, which led me to this code:

import yaml
class Step(yaml.YAMLObject):

    yaml_tag = "!step"

    def __init__(self, *args, **kwargs):
        raise Exception("Intentionally.")

yaml.load("""
--- !step
    foo: bar
    ham: 42
""")

Expected behaviour: I get an exception. But what I observe is, that my YAML markup results in a Step instance and I’m able to work with it, access methods, attributes (like foo in the code above) and so on. Reading the documentation, I cannot find my mistake since it suggests that the constructor is called with all the key-value-pairs as keyword arguments.

Basically the example in the doc works, but not because of the constructor’s implementation, but because of the fact that the key-value-pairs (properties of the Monster) are used to fill the object’s dict.

Anyone here knows about that?

I’m working with python3 but did a quick evaluation in python2 and observed the same.

edit

What I wanted to do: To stay in the linked example (documentation), if the Monsters name starts with a B, double the value of ac.

Answer 1

From the documentation:

yaml.YAMLObject uses metaclass magic to register a constructor, which transforms a YAML node to a class instance, and a representer, which serializes a class instance to a YAML node.

Internally, the default constructor registered by yaml.YAMLObject will call YourClass.__new__ then set the fields on your class by using instance.__dict__. See this method for more detail.

Depending on what you want to do, you could either put some logic in Step.__new__ (but you won’t be getting any of the fields in **kwargs, or register a custom constructor.