repr, str, int, float magic...

class FooError(Exception):
    pass

class Foo(object):

    def __init__(self, data):
        self.data = data
    def __repr__(self):
        return "%s(%s)" % (self.__class__.__name__, self.data)

    def __str__(self):
        return str(self.data)

    def __int__(self):
        try:
            return int(self.data)
        except ValueError:
            raise FooError("ValueError")

    def __float__(self):
        try:
            return float(self.data)
        except ValueError:
            raise FooError("ValueError")


foo = Foo(42)
bar = Foo("bar")

print "s: %s | r: %r | %d | f: %f" % (foo, foo, foo, foo)
str(foo) # %s calls __str__
repr(foo) #  %r calls __repr__
float(foo) #  %f calls __float__
int(foo) #  %d calls __int__


str(bar) # %s calls __str__
repr(bar) #  %r calls __repr__

# Will raise an error
float(bar) #  %f calls __float__
int(bar) #  %d calls __int__