Calling a class method

[vsoler]

I have the following script:

class TTT(object):
    def duplica(self):
        self.data *= 2
    def __init__(self, data):
        self.data = data
        TTT.duplica(self.data)
    def __str__(self):
        return str(self.data)

print
obj=TTT(7)
print obj

And I want 14 printed (twice 7)

I got the following error:
TypeError: unbound method duplica() must be called with TTT instance
as first argument (got int instance instead)

What am I doing wrong?