Jinja keys

[redhog]

Closes #85

API for operations writers:

    def evaluate_expression(self, config_path, default_expression=None, default_value=None, **context):
        """Evaluates a jinja2 expression specified in the operation
        config (or a default expression if not found) against a given
        context.

        config_path is itself a jinja2 expression, evaluated within
        the context of the operation config (self.config) and one
        additional variable: config, which is bound to the entire
        config file.

        Evaluating config_path should yield a jinja2 expression, or
        alternatively a list of expressions (if so, a list of values
        is returned).

        If no expression is found and no default expression is
        provided, or the expression evaluates to Undefined,
        default_value is returned.

        Example:

        Assuming
        
          config_path = "input.title_keys"
          self.config = {"input": {"title_keys": ["title", "categories.0.title"]}}
          context = {"input": {"title": "Hello", "categories": [{"title": "world"}]}}

        this function will return ["Hello", "world"].

        """
    def evaluate_template(self, config_path, default_template=None, **context):
        """Renders a jinja2 template specified in the operation config
        (or a default template if not found) against a given context.

        config_path is a jinja2 expression, evaluated within
        the context of the operation config (self.config) and one
        additional variable: config, which is bound to the entire
        config file.
        """