Question

I’m not clear how to pose this question. If I did, I’d probably be a lot closer to a solution.. I need some insight into inheritance.

I want to make a custom subtype of float. But I want the instance of this subtype to re-evaluate its value before performing any of the normal float methods (__add__,__mul__, etc..). In this example it should multiply its value by the global FACTOR:

class FactorFloat(float):
    # I don't think I want to do this:
    ##    def __new__(self, value):
    ##        return float.__new__(self, value)
    def __init__(self, value):
        float.__init__(self, value)
    # Something important is missing..
    # I want to do something with global FACTOR
    # when any float method is called.

f = FactorFloat(3.)
FACTOR = 10.
print f   # 30.0
print f-1 # 29.0
FACTOR = 2.
print f   # 6.0
print f-1 # 5.0

This is a just a sanitized example that I think gets my point across. I’ll post a more complex “real” problem if necessary.

Answer 1

class FactorFloat(float):
    def _factor_scale(f):
        def wrapper(self, *args, **kwargs):
            scaled = float.__mul__(self, FACTOR)
            result = f(scaled, *args, **kwargs)
            # if you want to return FactorFloats when possible:
            if isinstance(result, float):
                result = type(self)(result/FACTOR)
            return result
        return wrapper

    def __repr__(self):
        return '%s(%s)' % (type(self).__name__, float.__repr__(self))

    __str__ = _factor_scale(float.__str__)
    __mul__ = _factor_scale(float.__mul__)
    __div__ = _factor_scale(float.__div__)
    __add__ = _factor_scale(float.__add__)
    __sub__ = _factor_scale(float.__sub__)


f = FactorFloat(3.)
FACTOR = 10.
print f   # 30.0
print f-1 # 29.0
FACTOR = 2.
print f   # 6.0
print f-1 # 5.0
print repr(f)

for:

30.0
29.0
6.0
5.0
FactorFloat(3.0)

EDIT:

In response to the question in the comment; making things slightly more general and automated, using a class decorator. I would not loop over dir(baseclass), but instead would explicitly list the methods I wished to wrap. In the example below, I list them in the class variable _scale_methods.

def wrap_scale_methods(cls):
    Base = cls.__base__
    def factor_scale(f):
        def wrapper(self, *args, **kwargs):
            scaled = Base.__mul__(self, FACTOR)
            result = f(scaled, *args, **kwargs)
            if isinstance(result, Base):
                result = type(self)(result/FACTOR)
            return result
        return wrapper
    for methodname in cls._scale_methods:
        setattr(cls, methodname, factor_scale(getattr(Base, methodname)))
    return cls

@wrap_scale_methods
class FactorComplex(complex):
    _scale_methods = '__str__ __mul__ __div__ __add__ __sub__'.split()
    def __repr__(self):
        return '%s(%s)' % (type(self).__name__, complex.__repr__(self)[1:-1])