Find out why this script doesn't work (discount calculator)

discount_calculator.py

# -*- coding: utf-8 -*-                                                         
"""                                                                             
Why calculator doesn't work and how to fix it?                                  
                                                                                
                                                                                
I expect to see:                                                                
                                                                                
$ python /tmp/discount_calculator.py                                            
950.0                                                                           
                                                                                
                                                                                
Instead I've got:                                                               
                                                                                
$ python /tmp/discount_calculator.py                                            
Traceback (most recent call last):                                              
  File "/tmp/discount_calculator.py", line 18, in <module>                      
    print calculator(5)                                                         
  File "/tmp/discount_calculator.py", line 11, in _calculator                   
    base_price *= (100 - discount_in_percent) / 100.0                           
UnboundLocalError: local variable 'base_price' referenced before assignment     
"""                                                                             
                                                                                
def discount_calculator(base_price):                                            
                                                                                
    def _calculator(discount_in_percent):                                       
        base_price *= (100 - discount_in_percent) / 100.0                       
        return base_price                                                       
                                                                                
    return _calculator                                                          
                                                                                
if __name__ == '__main__':                                                      
    calculator = discount_calculator(1000)                                      
    print calculator(5)