Tap a piece
Build the file in that order. File name idea: calc.py.
Two numbers
Ask for two numbers and + or −. Print the answer.
That is enough. Multiply and divide can wait. A calculator that only adds is still a calculator you wrote.
If it breaks
A blank answer or a crash often means you added words instead of numbers. Read the last line of the error.
Next step
Ask for a task. Save it into a text file.