Python and Function parameters?

Hi, Lets say I have a function called cake(a,b=1, c=2,d=3) From what I understand b,c,d are optional parameters, and if nothing is given when cake() is called they will default to 1,2,3 respectively. What I dont understand is how do we specify when calling the function what parameter we are referring to when we leave one or more out? for example cake(1,2,3) could refer to cake(a,b,c), cake(a,c,d), cake(a,b,d) etc. Thanks