def __isPercent__(obj):
    """Check if object is an percent instance."""
    if not isinstance(obj, Percent):
        raise AttributeError, 'percent expected, %s found' % type(obj)        
# END __isPercent__
     
def __isNatural__(obj):
    """Check if object is an positive integer instance."""
    if not isinstance(obj, int):
        raise AttributeError, 'integer expected, %s found' % type(obj)  
    elif isinstance(obj, int) and obj < 0:
        raise AttributeError, 'natural expected'
# END __isInteger__
 
##################################################
 
class Percent(float):
     
    def __init__(self, value=-1):
        """Create a percentage.
        @param value: value of the percentage
        @type value: integer or float between 0 and 100"""
        if value < 0 or value > 100:
            raise ValueError, 'incorrect value %s' % str(value)
        float.__init__(self, round(value, 2))
    # END __init__
 
    def __add__(self, other=None):
        """Add a percentage to another.
        @param other: percentage to add
        @type other: instance of the Percent class"""
        __isPercent__(other)
        return Percent(float.__add__(self, other))  
    # END __add__
     
    def __sub__(self, other=None):
        """Substract a percentage from another.
        @param other: percentage to substract
        @type other: instance of the Percent class"""
        __isPercent__(other)
        return Percent(float.__sub__(self, other))  
    # END __sub__
     
    def __mul__(self, other=None):
        """Multiply a percentage by an int.
        @param other: factor
        @type other: integer"""
        __isNatural__(other)
        return Percent(float.__mul__(self, other))  
    # END __mul__
     
    def __floordiv__(self, other=None):
        """Divide a percentage by an int.
        @param other: denominator
        @type other: integer"""
        __isNatural__(other)
        return Percent(float.__floordiv__(self, other))  
    # END __floordiv__
     
    def __coerce__(self, other=None):
        """Handling different operand types is left to the operation."""
        return (self, other)
    # END __coerce__
     
    def getUnitar(self):
        """Return the fractional value of the percentage."""
        return round(self / 100, 2)
    # END getUnitar
 
# END Percent