Hi, I recently tried Streamlit deployment to App Engine and wrote an article.
Let me link it here though this topic is already resolved as it has some additional info not written in this thread:
  
      

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How to deploy Streamlit apps to Google App Engine 15

  Things to know   There are some things to know when you deploy Streamlit apps to App...


  

  
    
    
  

  


I think it answers all the questions here and also contains some more tips.

Here is a summary. Please read the article for the details.

the App must be deployed in the flexible (not standard) environment, with a Dockerfile

Why is a container necessary?


As Randy (@randyzwitch ) answered, the flexible environment is necessary for WebSockets.
And the flexible environment is container-based, so “container is necessary” in that sense.
However, a custom runtime with original Dockerfile is not necessary. You can use pre-defined Python3 runtime.

Is there an official Streamlit tutorial for launching the “hello world” streamlit app on App Engine?

The article contains a small but complete example though it’s unofficial.