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Accessing a base class primitive in python

I am trying to get a class from a python primitive, float, to print another line of a string while printing it out.

How do I access the underlying data from the derived class when I do this?

Here's a simplified example of what I'm trying to do:

class efloat(float):
    def __repr__(self):
        return "here my number: %s" % str(WHAT CAN I PUT HERE???)

Ok, thanks guys! I think I figured it out now. Here's a ready-made class for anyone interested:

import math

class efloat(float):
    """efloat(x) -> floating point number with engineering representation when printed
       Convert a string or a number to a floating point number, if possible.
       When asked to render itself for printing (via str() or print) it is normalized
       to engineering style notation at powers of 10 in multiples of 3
       (for micro, milli, kilo, mega, giga, etc.)        
    """

    def _exponent(self):   
        if self == 0.0:
           ret = 0
        else:
           ret = math.floor(math.log10(abs(self)))
        return ret

    def _mantissa(self):
        return self/math.pow(10, self._exponent())

    def _asEng(self):
        shift = self._exponent() % 3

        retval = "%3.12ge%+d" % (self._mantissa()*math.pow(10, shift), self._exponent() - shift)
        return retval

    def __str__(self):
        return self._asEng()

    def __repr__(self):
        return str(self)

    def __add__(self, x):
        return efloat(float.__add__(self, float(x)))

    def __radd__(self, x):
        return efloat(float.__add__(self, float(x)))

    def __mul__(self, x):
        return efloat(float.__mul__(self, float(x)))

    def __rmul__(self, x):
        return efloat(float.__mul__(self, float(x)))

    def __sub__(self, x):
        return efloat(float.__sub__(self, float(x)))

    def __rsub__(self, x):
        return efloat(float.__rsub__(self, float(x)))

    def __div__(self, x):
        return efloat(float.__div__(self, float(x)))

    def __rdiv__(self, x):
        return efloat(float.__rdiv__(self, float(x)))

    def __truediv__(self, x):
        return efloat(float.__truediv__(self, float(x)))

    def __rtruediv__(self, x):
        return efloat(float.__rtruediv__(self, float(x)))

    def __pow__(self, x):
        return efloat(float.__pow__(self, float(x)))

    def __rpow__(self, x):
        return efloat(float.__rpow__(self, float(x)))

    def __divmod__(self, x):
        return efloat(float.__divmod__(self, float(x)))

    def __neg__(self):
        return efloat(float.__neg__(self))

    def __floordiv__(self, x):
        return efloat(float.__floordiv__(self, float(x)))
+2


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2 answers


If you don't override __str__

this will still refer to the underlying method, so:

class efloat(float):
    def __repr__(self):
        return "here my number: %s" % self


will work. More generally, you can use self+0

, self*1

or any other identifier manipulation that you have not explicitly overridden; if you turned them all over, worst case, float.__add__(self, 0)

or the like.

+4


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You can call methods of the base class by accessing them from the base class to get an unbound method and call them yourself:



class myfloat(float):
    def __str__(self):
        return "My float is " + float.__str__(self)

print(myfloat(4.5))
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