What is a difference between an `if` block and `on_click` for a button?

[dataprofessor]

Hi @mkaut

Thanks for the question. In essence, Streamlit reruns the app upon widget interaction (e.g. triggered by on_click or on_change). To preserve variables from losing their values during the rerun, one can use Session state for that. In your example, your preserving the variable values through the use of session state. So in spite of the app reruns (as noticed from the balloons going off in your app), the count value was added to session state and thus is preserved.

There’s a similar thread here:

Explaining run logic with buttons

Summary New to streamlit here and enjoying it. I’m hoping someone could explain button logic for me, and the differences between using callbacks vs the if button_pressed idioms. In short, under my beginner reading of the tutorial, after a button press the entire script is re-run, and so I expected the two idioms to behave differently. However, if I implement a counter using the two different idioms, I find that the if button_pressed idiom leaves any references to the counter variable before th…

Also, in the Docs, there is a short mention of this here (Main concepts - Streamlit Docs):

Then every time a user interacts with a widget, Streamlit simply reruns your script from top to bottom, assigning the current state of the widget to your variable in the process.

Hope this helps!

Best regards,
Chanin