Popups are used as an interactive means to inform user of some action. It is available in python, however it needs to be available in Streamlit framework.
Steps to reproduce
There could be more than one way to bring this functionality in Streamlit. One approach could be create a component which is designed to generate popup post an event.
Another approach could be to display the popup in streamlit originated from an underlying python library. That is Streamlit collects the info from python code and throws to user in form of popup, take input from popup and pass on to python code as well. This approach has been used by ZOHO creator in their framework and many similar frameworks.
Popup requirement has been in discussion for past several years, it would certainly give lot of power to Streamlit in designing interactive applications.