python 命名元祖之实现 -- python元编程

    命名元祖比起元祖写代码清晰明了很多，很想知道内部怎么实现，内部有没有增加内存，是否影响性能之类，看手册的时候，

发现有个参数可以打印出代码，惊奇的发现，其实很简单，就是用到了描述符/property而已，为什么自己就想不到呢？看样子

还没有把这些玩意真正融汇入心中；贴手册中的例子：

Point = namedtuple('Point', "x y", verbose=True)  
class Point(tuple):
        'Point(x, y)' 
        __slots__ = () 
        _fields = ('x', 'y') 

        def __new__(_cls, x, y):
            'Create new instance of Point(x, y)'
            return _tuple.__new__(_cls, (x, y)) 

        @classmethod
        def _make(cls, iterable, new=tuple.__new__, len=len):
            'Make a new Point object from a sequence or iterable'
            result = new(cls, iterable)
            if len(result) != 2:
                raise TypeError('Expected 2 arguments, got %d' % len(result))
            return result 

        def __repr__(self):
            'Return a nicely formatted representation string'
            return 'Point(x=%r, y=%r)' % self 

        def _asdict(self):
            'Return a new OrderedDict which maps field names to their values'
            return OrderedDict(zip(self._fields, self)) 

        def _replace(_self, **kwds):
            'Return a new Point object replacing specified fields with new values'
            result = _self._make(map(kwds.pop, ('x', 'y'), _self))
            if kwds:
                raise ValueError('Got unexpected field names: %r' % kwds.keys())
            return result 

        def __getnewargs__(self):
            'Return self as a plain tuple.  Used by copy and pickle.'
            return tuple(self) 

        x = _property(_itemgetter(0), doc='Alias for field number 0')
        y = _property(_itemgetter(1), doc='Alias for field number 1')

    看到了么？其实namedtuple就是一个普通的类而已，他继承了tuple，关键的地方在于他把属性映射到了index上面，这一映射就是

通过描述符/property来完成的，原理其实很简单；关键是没意识往这方面想；

    另外，非常重要的一点是，这个类完全是动态创建的，即“元编程”搞出来的东东，我想如果熟悉python的元编程，应该比较简单；

干脆自己尝试写一个吧：

def my_namedtuple(name, attrs):
        if isinstance(attrs, str):
                attrs = attrs.split()

        import string
        attrs_join = string.join(attrs, ',')
        newfns = "def newfn(cls, %s): return tuple.__new__(cls, (%s))"%(attrs_join, attrs_join)
        exec(newfns)

        adict = {'__slots__':(), '_fields':tuple(attrs), '__doc__':'%s(%s)'%(name, attrs_join), '__new__':newfn}

        import operator as op
        for i,e in enumerate(attrs):
                adict[e] = property(fget=op.itemgetter(i))

        return type(name, (tuple,), adict)

Angle = my_namedtuple("Angle", "x y")

print dir(Angle)

angle_test = Angle(1, 15)
print angle_test
print "x,y=", angle_test.x, angle_test.y
print "0,1=", angle_test[0], angle_test[1]

Sincos = my_namedtuple("Sincos", "m n q")
sincos = Sincos(5, 9, 2)
print sincos

输出如下

['__add__', '__class__', ..., '__slots__', '__str__', '__subclasshook__', '_fields', 'count', 'index', 'x', 'y']
(1, 15)
x,y= 1 15
0,1= 1 15
(5, 9, 2)