My function does not return anything

Works for me:

Python 3.7.3 (default, Mar 30 2019, 03:37:43) 
[Clang 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def fun(x,y):
...   return x+y
... 
>>> fun(1,2)
3
>>> 

Do you get any kind of error message?

I doesn't print anything, and there is no error message until I added "print(fun(1,2))" I did not write it on Shell

12 minutes ago, Lizwi said:

I doesn't print anything, and there is no error message until I added "print(fun(1,2))" I did not write it on Shell

Ah, OK. That is because the program itself didn't generate any output until you added the print. It would be up to the environment (eg shell) to print the result of the function.