why this work:

def function1():                                                                                                             
       a = 10                                                                                                                    
       def function2():
          print a
       function2()

but this not:

def function1():
    a = 10
    def function2():
        print a
        a -= 1
        if a>0:
           function2()
    function2()

error:

UnboundLocalError: local variable 'a' referenced before assignment