Python Pass by assignment but value is not retained

Question

I am having this issue in a large code base so I created a small reproducer below:

Here, I have the following two functions, func2() and func3()

def func2(a,b):
     a = 'new-value'
     b = b+1
     return a, b

def func4():
     x,y = 'old-value',3
     func2(x,y)
     print(x,y)

The output I am getting is:

>>> func4()
old-value 3

I thought Python is pass-by assignment and so the func4() should overwrite x and y values by new-value and 4

What am I misunderstanding? Also, how can I change the above code to make x, y have new-value and 4

Any help is greatly appreciated!.