I have read that I can reveal the type of variables by using a function called reveal_type, but I can’t find how to use it or from where to import it.

I found out in the end how to use it: You should just put and use the reveal_type in the code, and run it with the mypy program. Then, it will log a message that look like this:

Revealed type is 'builtins.str*'

From the mypy documentation:

reveal_type is only understood by mypy and doesn’t exist in Python, if you try to run your program. You’ll have to remove any reveal_type calls before you can run your code. reveal_type is always available and you don’t need to import it.

For more reading: here.