I understand that you would like to download your source code using the command. I have looked at our documentation and found that the command, appcfg.py, is deprecated and that the Google Cloud SDK does not support downloading the source code.
There is an alternative as mentioned here where you can view the code, then manually copy and paste it to your local machine. “As of October 2019 you can simply go to --> App Engine --> Services and in the tool dropdown select 'source' and the source code is there”
I would like to investigate further. To do this, can you tell me what kind of App Engine you are using?
I know this is not ideal but it is a workaround but this is something you can try if in the meantime.
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