You're seeing just the function fetch_access_token, go back to WeChat.Component module for more information.
Link to this function

fetch_access_token(appid, authorizer_appid, adapter_storage)

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A function helper to fetch authorizer application's access token from component application.

When apply it to hub, if no available access token from hub's storage, there will use refresh_token to refetch a new one, this function can be used for refresh function.