You need to supplement the environment variables for the command during which you invoke python3 test.py as follows
$ LOL=HAHAHA python3 test.py
HAHAHA
$ env LOL=HAHAHA python3 test.py
HAHAHA
$ echo $LOL
<empty string>
or you can export the variable for the current session as:
$ export LOL=HAHAHA
$ python3 test.py
HAHAHA
$ echo $LOL
HAHAHA
Simply doing LOL=HAHAHA; python3 test.py doesn't work because that just sets the LOL=HAHAHA variable for the shell process.
Another thing to note, the first approach shown only sets the environment variable for that specific command. It does not set it in the environment. Doing it with the export instead, sets it for the environment. You can see the difference in the values of $LOL above