asfyaml/validators.py

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import strictyaml
import strictyaml.exceptions


class EmptyValue(strictyaml.ScalarValidator):
    """Legacy YAML null type that supports the tilde null marker not considered proper by strictyaml:
    a: null
    a: ~  (also null)
    """

    def validate_scalar(self, chunk):
        val = chunk.contents
        if val.lower() not in ("null", "~"):
            chunk.expecting_but_found(f"when expecting a 'null', got '{val}' instead.")
        else:
            return self.empty(chunk)

    def empty(self, chunk):
        return None

    def to_yaml(self, data):
        if data is None:
            return "null"
        raise strictyaml.exceptions.YAMLSerializationError("expected None, got '{}'")