python - leave optional parameters out conditionally -

consider function:

def foo(x, y=1):   return x*x + y*y 

now want call foo this:

if param_y == none:   z = foo(3) else:   z=foo(3,param_y) 

where param_y determined somewhere before. want simplify this, such there 1 foo-call in code, since if have multiple of such optional parameters, i'd enormous (and ugly!!) if-else clause, can do:

z = foo(3, param_y if param_y != none else 1) 

but requires caller know 1 default value of y, find bit ugly well. there alternative syntax this, like:

z =  foo(3, if param_y != none: param_y) 

can't update foo function?

def foo(x, y):     if y none:         y = 1     return x*x + y*y 

and call everytime :

z=foo(3,param_y)