Negative indexing/selecting working and not working

[Andrzej_Andrzej]

Can you post the code that makes a list named "perc" that contains 40 elements.

I have posted it previously, but here you are:

perc = (source["value"] / source["value"].sum()) * 100

perc[-2]

I tried to change perc to dataframe but could not get it to work. I would like to admit that I am learning Python, so sometimes
even basic subjects are a struggle to me. I apologize for those basic questions. What I want to understand is why this perc[-2] is working inside a function and why does it error when is written separately, meaning not inside a function.
The code I provided in my first post works perfectly, I just want to understand what is happening in it and why I got those errors when I started to experiment with that code.
I hope this clarifies it a bit.