Python: Skip a block of code (calculation) when certain condition is met without using "if" statements

I have this small block of code which I am trying to write better way since this one has a bunch of "if" statements. This is the small code of some big project. The problem is this: as the code runs, function "f", "g", or/and "k" can return None or numerical data. Whenever None value is returned, rest of the calculation has to get skipped since mathematical operations (which happens in those functions) cannot be done. I tried to rewrite the code using TRY/CATCH method, but couldn't make it work. I am trying to avoid "if" statements and rewrite concise way. I appreciate the help.

---

def f(output):
    #some code which computes output which be None or numerical
    return [output*1,2]
def g(Y):
    #some code which computes Y which be None or numerical
    return Y*3
def k(output):
  #some code which computes output which be None or numerical
  return output*4
def foutput():
  #some code which computes "value" which be None or numerical 
  value=2.0
  return 1.0*value


#####START
#some code
output=foutput()

if output is not None:
    print 'S1'
    [output,A]=f(output)
    if output is not None:
        print 'S2'
        [a,b,c,Y]=[1,2,3,k(output)]
        if Y is not None:
            print 'S3'
            A=g(Y)
        else:
            [Q,A,output]=[None,None,None]
    else:
        [Q,A,output]=[None,None,None]
else:
    [Q,A,output]=[None,None,None]