switch to the spare cdn address when the cdn resource of the unpkg fails

My current solution is as below:

class CustomDash(dash.Dash):

    def interpolate_index(self, **kwargs):
        scripts = kwargs.pop('scripts')

        external_scripts = re.findall(
            '(<script src="http.*?"></script>)', scripts)

        for external_script in external_scripts:
            scripts = scripts.replace(
                external_script,
                '''<script src="{}" onerror='this.remove(); let fallbackScript = document.createElement("script"); fallbackScript.src = "{}"; document.querySelector("footer").appendChild(fallbackScript);'></script>'''.format(
                    re.findall('"(.*?)"', external_script)[0]
                    .replace('https://unpkg.com/', 'https://unpk.com/'), # Manually simulated unpkg resource loading failure error
                    re.findall('"(.*?)"', external_script)[0]
                    .replace('https://unpkg.com/', 'https://npm.elemecdn.com/') # Switch to standby address
                )
            )

        return super(CustomDash, self).interpolate_index(scripts=scripts, **kwargs)

but it meets the Uncaught ReferenceError: DashRenderer is not defined error:

is there a stable way to solve this issue🤔?

Start with the CustomDash.interpolate_index override shown in the issue and trace how Dash assembles external scripts and DashRenderer. Reproduce the unpkg failure and the resulting “DashRenderer is not defined” error, then verify that a stable fallback loads all required resources without that error.