蟒蛇：强迫新式类

Question

我想这码"仅仅是工作"：

def main():
    c = Castable()
    print c/3
    print 2-c
    print c%7
    print c**2
    print "%s" % c
    print "%i" % c
    print "%f" % c

当然，简单的办法是写 int(c)/3, 但是我想启用一个简单的perl-ish语法结构的迷你的语言。

这是值得注意的是，如果我使用"旧式的"类(不继承的对象)我可以做这个很简单的定义 __coerce__ 方法，但是旧风格的课程都已过时，将删除在python3.

当我做同样的事情有一种新式类，我得到这个错误：

TypeError: unsupported operand type(s) for /: 'Castable' and 'int'

我相信这是通过设计的，但后我怎么能模拟的旧风格的 __coerce__ 行为有一种新式类？你可以找到我目前的解决方案之下，但它是很丑陋的，长篇大论.

这是相关的文件：(i think)

• 胁迫的规则
• 新式的特别方法查找

奖励点：

    print pow(c, 2, 100)

Answer 1

这一工作，并不总之后的几个方面的改进(道具@jchl)，但是仍然看来似乎应该是不必要的，特别是考虑到你得到这种免费的"老式"的课程。

我仍在寻找一个更好的答案。如果没有更好的方法，这似乎我像个回归的蟒蛇的语言。

def ops_list():
    "calculate the list of overloadable operators"
    #<type 'object'> has functions but no operations
    not_ops = dir(object)

    #calculate the list of operation names
    ops = set()
    for mytype in (int, float, str):
        for op in dir(mytype):
            if op.endswith("__") and op not in not_ops:
                ops.add(op)
    return sorted(ops)

class MetaCastable(type):
    __ops = ops_list()

    def __new__(mcs, name, bases, dict):
        #pass any undefined ops to self.__op__
        def add_op(op):
            if op in dict:
                return
            fn = lambda self, *args: self.__op__(op, args)
            fn.__name__ = op
            dict[op] = fn

        for op in mcs.__ops:
            add_op( op )
        return type.__new__(mcs, name, bases, dict)


class Castable(object):
    __metaclass__ = MetaCastable
    def __str__(self):
        print "str!"
        return "<Castable>"
    def __int__(self):
        print "int!"
        return 42
    def __float__(self):
        print "float!"
        return 2.718281828459045

    def __op__(self, op, args):
        try:
            other = args[0]
        except IndexError:
            other = None
        print "%s %s %s" % (self, op, other)
        self, other = coerce(self, other)
        return getattr(self, op)(*args)

    def __coerce__(self, other):
        print "coercing like %r!" % other
        if other is None: other = 0.0
        return (type(other)(self), other)

Answer 2

你需要定义 __div__ 如果你想要的 c/3 到工作。蟒蛇不会把你的目的向一些第一你。

Answer 3

class MetaCastable(type):
    __binary_ops = ( 
            'add', 'sub', 'mul', 'floordiv', 'mod', 'divmod', 'pow', 'lshift', 
            'rshift', 'and', 'xor', 'or', 'div', 'truediv',
    )

    __unary_ops = ( 'neg', 'pos', 'abs', 'invert', )

    def __new__(mcs, name, bases, dict):
        def make_binary_op(op):
            fn = lambda self, other: self.__op__(op, other)
            fn.__name__ = op
            return fn

        for opname in mcs.__binary_ops:
            for op in ( '__%s__', '__r%s__' ):
                op %= opname
                if op in dict:
                    continue
                dict[op] = make_binary_op(op)

        def make_unary_op(op):
            fn = lambda self: self.__op__(op, None)
            fn.__name__ = op
            return fn

        for opname in mcs.__unary_ops:
            op = '__%s__' % opname
            if op in dict:
                continue
            dict[op] = make_unary_op(op)

        return type.__new__(mcs, name, bases, dict)

class Castable(object):
    __metaclass__ = MetaCastable
    def __str__(self):
        print "str!"
        return "<Castable>"
    def __int__(self):
        print "int!"
        return 42
    def __float__(self):
        print "float!"
        return 2.718281828459045

    def __op__(self, op, other):
        if other is None:
            print "%s(%s)" % (op, self)
            self, other = coerce(self, 0.0)
            return getattr(self, op)()
        else:
            print "%s %s %s" % (self, op, other)
            self, other = coerce(self, other)
            return getattr(self, op)(other)

    def __coerce__(self, other):
        print "coercing like %r!" % other
        return (type(other)(self), other)

Answer 4

class Castable(object):
    def __div__(self, other):
        return 42 / other

Answer 5

新式类工作速度更快和更精确的比旧式的课程。因此，没有更昂贵的 __getattr__, __getattribute__ , __coerce__ 呼吁任何廉价的原因和在可疑订单。

旧风格的 __coerce__ 还有的问题，它被称为即使在已经超负载操作方法对于一些特殊的目的。它要求铸造平等常见的类型，并且仅限于某些二进制行动。想想所有其他方法和性质的一个int/浮/string和有关pow().由于所有这些限制 coerce 在PY3.这个问题的例子旨在以相当广泛的虚拟化。

与新风格的类别，它只是关于一个循环，提供了许多"类似"的方法与小代码，或路线，这些呼吁为一个虚拟的处理程序和随后其快速和精确的定义和子类化在正确和精确的方式。那不是一个"回归的蟒蛇的语言"。

但是我不会采用一元类所示的其他的答案只是这样一个循环或用于提供一个简单的基类种行为。这将是开裂的一个螺帽用大锤。

---

这里的一个例子帮助的虚拟化的"变体":

def Virtual(*methods):
    """Build a (new style) base or mixin class, which routes method or
    operator calls to one __virtualmeth__ and attribute lookups to
    __virtualget__ and __virtualset__ optionally.

    *methods (strings, classes): Providing method names to be routed
    """
    class VirtualBase(object):  
        def __virtualmeth__(self, methname, *args, **kw):
            raise NotImplementedError
    def _mkmeth(methname, thing):
        if not callable(thing):
            prop = property(lambda self:self.__virtualget__(methname),
                            lambda self, v:self.__virtualset__(methname, v))
            return prop
        def _meth(self, *args, **kw):
            return self.__virtualmeth__(methname, *args, **kw)
        _meth.__name__ = methname
        return _meth
    for m in methods:
        for name, thing in (isinstance(m, str) and
                            {m:lambda:None} or m.__dict__).items():
            if name not in ('__new__', '__init__', '__setattr__', ##'__cmp__',
                            '__getattribute__', '__doc__', ):   ##'__getattr__', 
                setattr(VirtualBase, name, _mkmeth(name, thing))
    return VirtualBase

和这里一个例子使用情况：一个Anaphor!(PY2和PY3):

import operator
class Anaphor(Virtual(int, float, str)):   
    """remember a sub-expression comfortably:

    A = Anaphor()      # at least per thread / TLS
    if re.search(...) >> A:
        print(A.groups(), +A)
    if A(x % 7) != 0:
        print(A, 1 + A, A < 3.0, A.real, '%.2f' % A, +A)
    """
    value = 0
    def __virtualmeth__(self, methname, *args, **kw):
        try: r = getattr(self.value, methname)(*args, **kw)
        except AttributeError:
            return getattr(operator, methname)(self.value, *args, **kw)
        if r is NotImplemented: # simple type -> coerce
            try: tcommon = type(self.value + args[0])    # PY2 coerce
            except: return NotImplemented
            return getattr(tcommon(self.value), methname)(*args, **kw)
        return r
    def __call__(self, value):   
        self.value = value
        return value
    __lshift__ = __rrshift__ = __call__     # A << x;  x >> A
    def __pos__(self):                      # real = +A
        return self.value
    def __getattr__(self, name):
        return getattr(self.value, name)
    def __repr__(self):
        return '<Anaphor:%r>' % self.value

无缝它还处理3-arg opertor pow() :-) :

>>> A = Anaphor()
>>> x = 1
>>> if x + 11 >> A:
...     print repr(A), A, +A, 'y' * A, 3.0 < A, pow(A, 2, 100)
...     
<Anaphor:12> 12 12 yyyyyyyyyyyy True 44