[Tkinter] What is the subscript error?

[buran]

when txt_2_search is not present in the dict keys, dict.get() will return default value of None. None[3] raise the exception you see.

Then there is one more problem in that line

if unit[3] == "fundamentals" or "supplementary quantities" or "derived quantities":

it will not work as you expect. see https://python-forum.io/Thread-Multiple-...or-keyword