Traditionally on Google App Engine Standard Environment, at the moment, it seems there is no option to download the Source Code from App Engine. As you already know, the Source Diagnose button on the App Engine application version would redirect to the Stackdriver Debugger which allows viewing the source code and performing debugging-like analysis. You could copy and paste the contents to your local file. It is not recommended to rely on Google App Engine as a source control[2].
[1]https://stackoverflow.com/questions/47498821/google-cloud-datastore-automatic-indexing/47499884#answer-47499884 [2]https://issuetracker.google.com/77891431#comment2 -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/e2637a36-ddba-484f-b5b6-7a65f4ec3882%40googlegroups.com.
