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


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