After screen.blit, why is surf.fill not effective?

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There could be several reasons why surf.fill is not effective after screen.blit. One possibility is that the surface that is being filled (surf) is not the same as the surface that is being displayed on the screen (screen). In this case, filling the surf surface would have no effect on what is displayed on the screen.

Another possibility is that the blit function is overwriting the surface with the fill color, so the fill function is not visible. In this case, you would need to fill the surface after blitting it to the screen to see the fill color.

It's also possible that there is something else in the code that is interfering with surf.fill or preventing it from executing properly. Without seeing the code, it's difficult to diagnose the issue accurately.