Make sure to set GRAAL_PYTHONHOME to a valid Python location if you want to use its standard library.
Will work without it, but you won't have access to the standard library, e.g. import os will fail.
Then run:
./gradlew runClientThen, once in Minecraft, open the chat and type:
/run '<your command>'
For example:
/run '[1, 2, 3].count(2)'
State is stored throughout the entire Minecraft session, so you can do:
/run 'x = 5'
/run 'x * 2'
Same goes for imports, etc:
/run 'import math'
/run 'math.sqrt(16)'
Warning: by default, this runs with full permissions to the host, so you can do things like:
/run 'import os; os.remove("important_file.txt")'