why cannot assign to function call

[scsoce]

I have a function return a reference, and want to assign to the 
reference, simply like this:
 >>def f(a)
          return a
     b = 0
    * f( b ) = 1*
but the last line will be refused as "can't assign to function call".
In my thought , the assignment is very nature,  but  why the interpreter 
refused to do that ?

thks