YAML Constructors

All YAML constructors must inherit from the scs.yaml.SCSYamlTagConstructor class. SCS adds the the following attributes to a loader that’s passed to the construct() method:

.resave (bool): If during parsing you edit some part of the JSON, set this to True inside your constructor, to indicate the parsed data should be saved back to the file after loading.
.filepath (pathlib.Path): Use this inside your constructor to for example parse relative paths.

If you set .resave = True, the parsed data is saved back to the file, meaning any additional whitespace or comments from the original file will be lost

Below is a simple example of a custom YAML constructor:

mymodule.py

from scs.yaml import SCSYamlTagConstructor

class AddOwnPathConstructor(SCSYamlTagConstructor):
    """
    A YAML tag that is replaced by the path of the file itself

    Attributes:
      resave: Whether to save the generated value back to the file
    """
    # Note that self.tag can also be made dynamic, by setting it as input to
    # the init function
    tag = '!set-own-path'

    def __init__(self, *args, resave: bool, **kwargs):
        self.resave = resave
        super().__init__(*args, **kwargs)

    def construct(self, loader: Loader, node: Node) -> Any:
        # If resave is true, replace tag with value on first load.
        # Note that the 'resave' option removes any comments and additional
        # whitespace from the YAML file
        if self.resave:
          # Prevent setting it to false when self.resave=False, because another
          # constructor may have set it to True already...
          loader.resave = True
        return loader.filepath.as_posix()

To use this YAML constructor, add the following to your scs-configuration.yaml file:

extensions:
  constructors:
    - name: mymodule.AddOwnPathConstructor
      options:
        resave: false