Is there a way to define local symbolic variables inside functions that do not overwrite global ones?

edit

Consider the behaviour of the following function:

k, c = var('k c')

q = k * c
show(q)

def dummy():
    q = 4

dummy()
show(q)

Before and after the function runs, q stores kc.

But now consider the function

k, c = var('k c')

q = k * c
show(q)

def dummy():
    q = var('q')

dummy()
show(q)

But now, after the function returns, the value of q has changed to q.

How can I write a "safe" function such that any dummy variable I create inside does not overwrite a global variable? reset() and restore() both result in an undefined error.