How can you output a varying argument in Python?

How can you output a varying argument in Python?

I am writing a function in Python.
So lets say I have 3 arguments in this function:
Function (arg1, arg2, arg3)
arg1 = arg2 * arg3
When the function is called, it is as such:
function(arg1=var1, arg2=var2, arg3=var3)
Now the thing is, these variables, (var1, var2, var3) vary in the case of
this function being called. So they could be var4, var5, var6, etc.
Given that these variables vary, what would be the syntax to make this
function change a varying argument?
ie. arg1 = arg2 * arg3 changes arg1. Because arg1 = var1.
What I need is somehow for arg1 to stand in as an actual proxy to var1 so
that not only is arg1 changed but var1 is changed also.
This is evidently not doable with simply with arg1=var1
I assume this could also be done with other function commands or
structure. How does not really concern me I just need to output a change
into a varying argument.