Python中的继承示例

Question

我不清楚如何提出这个问题.如果我这样做,我可能会更接近解决方案..我需要对继承有所了解.

我想制作一个自定义子类型float.但我想执行任何的正常浮动方法之前,这种亚型的实例,以重新评估其价值(__add__,__mul__,等).在这个例子中,它应该将其值乘以全局因子:

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

这是一个只是一个消毒的例子,我认为我的观点得到了解决.如有必要,我会发布更复杂的"真实"问题.

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)

对于:

30.0
29.0
6.0
5.0
FactorFloat(3.0)

编辑:

回应评论中的问题; 使用类装饰器使事情略微更加通用和自动化.我不会循环dir(baseclass),而是明确列出我希望包装的方法.在下面的示例中,我将它们列在类变量中_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])