How to implement math formulas in Kivy + Python?

Question

Python

Hello.
The problem is that mathematical formulas (without using the math library) are only considered when the application is run on the computer. If the application is imported to the phone, then the variables are recorded, but the formula is not considered.
I use only simple characters * and /
I write code in a .py file, I use .kv only for describing graphics and styles.
Math is written as a separate function in a python file