<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_graph(self, ctx, bundle, extensions, caller=None):
 """ Run a graph and render the tag contents for each output """

        request = ctx.get('request')
        if request is None:
            request = get_current_request()
        if ':' in bundle:
            config_name, bundle = bundle.split(':')
        else:
            config_name = 'DEFAULT'
        webpack = request.webpack(config_name)
        assets = (caller(a) for a in webpack.get_bundle(bundle, extensions))
        return ''.join(assets)