templated-yaml.py

# Alternative to None that is used to mark that something was optional and the
# slot in the container should be deleted. This is how we support optional
# properties
deleteMarker = {}

def renderString(string, params):
    # Could also use pystache
    for k,v in params.items():
        string = string.replace('{{' + k + '}}', v)
    return string

def render(template, params):
    if template is deleteMarker:
        return template
    elif isinstance(template, str):
        return renderString(template, params)
    elif isinstance(template, dict):
        if '$if' in template:
            if eval(template['$if']):
                return render(template.get('then', deleteMarker), params)
            else:
                return render(template.get('else', deleteMarker), params)
        if '$eval' in template:
            return eval(template['$eval'])
        retval = {}
        for key, value in template.items():
            key = render(key, values)
            value = render(value, values)
            if value is not deleteMarker:
                retval[key] = value
        return retval
    elif isinstance(template, list):
        retval = []
        for i in template:
            i = render(i, values)
            # Allow for optional list entries
            if i is not deleteMarker:
                retval.append(i)
        return retval
    raise ValueError('Unsupported template: %s' % template)

key being that {"$if": "False", then: "not-happening"} evaluates to deleteMarker
so {prop1: "value", optionalProperty: {"$if": "False", then: "not-happening"}} will render to {prop1: "value"}