How to easily create a single-page Web Application?

What is the easiest way to make a one-page web application, where there will be two input text, in which the variables a, b are entered and one button for accessing the python script to display the image at the received URL

def get_pic(a,b):
    *magic*
    return *pic url*

I've tried Django, but since I'm a beginner, I didn't understand how to assign a python function call to a button. Maybe there are ways as simple as possible and without unnecessary troubles, I need an elementary interface as in the attached picture

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P.S. Before that, I was engaged in creating desktop applications in PyQt and it was much easier there, you just drag-n-drop the necessary buttons, text blocks in the editor and then bind functions to them in Python, but with web applications, as I understand it, it will not work