python redefined __setattr__

现在在定义新的 python 数据结构 Tree，其目标是希望其在 python 中的操作非常简单，象 builtins 的变量那样，例如 root.branch1.branch11() 能返回这个节点的值。

现在，在对一个树的节点赋值时，如：

root.branch1.branch11 = value
或：
root.branch1.branch12 = Tree(value)

对于这个节点的名字要有一点限制(也就是前面讨论的要避免名字空间冲突的问题)，就是对于一个 Tree instance 已经保留使用的 name，如 method 名：__repr__, __getitem__, __setitem__，以及作为节点值的 __node_value 和作为键值索引的 __node_items。这时，调用的方法是 Tree.__setattr__，为了达到保留字的目的，最初的做法是这样的：

class Tree:
    _("""A Tree instance can only contains sub Tree instances as its attributes""")
    # (1)
    __used_names = None
    __used_names = Tree.__dict__.copy()
    __used_names.update(object.__dict__.copy())
    __used_names = used_names.keys()
    def __init__(self, value=None, **kwargs):
        self.__node_value = value
        # __node_value should always be Non-Tree
        self.__node_items = {}
        for k, v in kwargs.items(): self.__setattr__(k, v)
    ...
    def __setattr__(self, attr_name, value):
        _("""The operation 'root.br1.br2 = value' works on root.br1 rather than root.br1.br2""")
        if attr_name in self.__used_names:
            raise TreeExc(_("Attribute name '%s' is reserved" % attr_name))
        try:
            existed = getattr(self, attr_name)
            # self.attribute exists
            if isinstance(value, Tree):
                subtree = value
                if not subtree._Tree__node_value:
                    subtree._Tree__node_value = existed._Tree__node_value
                setattr(self, attr_name, subtree)
                # replace directly after the tree __node_value has been reserved
            else:
                setattr(self, '%s._Tree__node_value' % attr_name, value)
        except AttributeError:
        # if self.attribute does not exists, assign it a EMPTY node
            if isinstance(value, Tree):
                subtree = value
                setattr(self, attr_name, subtree)
            else:
                setattr(self, attr_name, Tree(value))

然后运行：

sh$ python -c "import tree; t = tree.Tree(1, data='head')"
Traceback (most recent call last):
  File "", line 1, in ?
  File "tree.py", line 16, in ?
    class Tree:
  File "tree.py", line 20, in Tree
    __used_names = Tree.__dict__.copy()
NameError: name 'Tree' is not defined

用 self 也是一样的报告没有 difined 的错误。这是很好理解的，在 Tree 完成所有的 attributes 的 assign 之前，它当然是没有定义的。所以这种办法不可行。

于是重新定义：

class Tree:
    _("""A Tree instance can only contains sub Tree instances as its attributes""")
    # (1)
    # __used_names = None
    # __used_names = Tree.__dict__.copy()
    # __used_names.update(object.__dict__.copy())
    # __used_names = used_names.keys()
    # ------------------------------------------------------------------------
    def __init__(self, value=None, **kwargs):
        # (2)
        self.__used_names = None
        self.__used_names = Tree.__dict__.copy()
        self.__used_names.update(object.__dict__)
        self.__used_names = self.__used_names.keys()
        self.__node_value = value
        # __node_value should always be Non-Tree
        self.__node_items = {}
        for k, v in kwargs.items(): self.__setattr__(k, v)
    def __getattr__(self, attr_name): pass
    def __setattr__(self, attr_name, value):
        _("""The operation 'root.br1.br2 = value' works on root.br1 rather than root.br1.br2""")
        if attr_name in self.__used_names:
            raise TreeExc(_("Attribute name '%s' is reserved" % attr_name))
        try:
            existed = getattr(self, attr_name)
            # self.attribute exists
            if isinstance(value, Tree):
                subtree = value
                if not subtree._Tree__node_value:
                    subtree._Tree__node_value = existed._Tree__node_value
                setattr(self, attr_name, subtree)
                # replace directly after the tree __node_value has been reserved
            else:
                setattr(self, '%s._Tree__node_value' % attr_name, value)
        except AttributeError:
        # if self.attribute does not exists, assign it a EMPTY node
            if isinstance(value, Tree):
                subtree = value
                setattr(self, attr_name, subtree)
            else:
                setattr(self, attr_name, Tree(value))

运行结果：

sh$ python -c "import tree; t = tree.Tree(1, data='head')"
Traceback (most recent call last):
  File "", line 1, in ?
  File "tree.py", line 30, in __init__
    self.__used_names = None
  File "tree.py", line 55, in __setattr__
    if attr_name in self.__used_names:
TypeError: iterable argument required

可以看到，__setattr__ 被调用了，说明当重新定义了 __setattr__ 之后，不仅仅是外部调用如 root.branch1.branch11 = value 这样的情况会收到影响，而且自身的使用如 self.attribute = value 也会受到影响，都会把要操作的对象当作一个 Tree Node 来处理。同时这也说明接下来的两个语句：

self.__node_value = value
# __node_value should always be Non-Tree
self.__node_items = {}

也是有问题的。实际上，这会导致赋值语句的死循环，最后在运行时可能会报这样的错误：

......
  File "tree.py", line 67, in __setattr__
    setattr(self, '%s._Tree__node_value' % attr_name, value)
  File "tree.py", line 67, in __setattr__
    setattr(self, '%s._Tree__node_value' % attr_name, value)
  File "tree.py", line 67, in __setattr__
    setattr(self, '%s._Tree__node_value' % attr_name, value)
  File "tree.py", line 67, in __setattr__
    setattr(self, '%s._Tree__node_value' % attr_name, value)
  File "tree.py", line 67, in __setattr__
    setattr(self, '%s._Tree__node_value' % attr_name, value)
  File "tree.py", line 67, in __setattr__
    setattr(self, '%s._Tree__node_value' % attr_name, value)
  File "tree.py", line 67, in __setattr__
    setattr(self, '%s._Tree__node_value' % attr_name, value)
  File "tree.py", line 67, in __setattr__
    setattr(self, '%s._Tree__node_value' % attr_name, value)
  File "tree.py", line 48, in __setattr__
    _("""The operation 'root.br1.br2 = value' works on root.br1 rather than root.br1.br2""")
  File "/usr/lib/python2.3/gettext.py", line 472, in gettext
    return dgettext(_current_domain, message)
  File "/usr/lib/python2.3/gettext.py", line 454, in dgettext
    t = translation(domain, _localedirs.get(domain, None))
  File "/usr/lib/python2.3/gettext.py", line 403, in translation
    mofiles = find(domain, localedir, languages, all=1)
  File "/usr/lib/python2.3/gettext.py", line 366, in find
    val = os.environ.get(envar)
  File "/usr/lib/python2.3/UserDict.py", line 51, in get
    if not self.has_key(key):
RuntimeError: maximum recursion depth exceeded

那么如果使用内置的 setattr() 函数呢？将上面的(2)改成(3)：

        # (3)
        setattr(self, '__used_names', None)
        setattr(self, '__used_names', Tree.__dict__.copy())
        self.__used_names.update(object.__dict__)
        setattr(self, '__used_names', self.__used_names.keys())

运行：

sh$ python -c "import tree; t = tree.Tree(1, data='head')"
Traceback (most recent call last):
  File "", line 1, in ?
  File "tree.py", line 35, in __init__
    setattr(self, '__used_names', None)
  File "tree.py", line 55, in __setattr__
    if attr_name in self.__used_names:
TypeError: iterable argument required

同样的错误，说明 setattr(self, attr_name, value) 与 self.attribute = value 一样收到 self.__setattr__() 的影响。

可以考虑的另一种办法是在每个 __init__ 中增加相应的初始化语句：

class Tree:
    _("""A Tree instance can only contains sub Tree instances as its attributes""")
    # (1)
    # __used_names = None
    # __used_names = Tree.__dict__.copy()
    # __used_names.update(object.__dict__.copy())
    # __used_names = used_names.keys()
    # ------------------------------------------------------------------------
    # __used_names = object.__dict__.copy().keys() + [
    #   '__used_names', '__node_value', '__node_items',
    #   '__getattr__', '__setitem__', '__getitem__', '__call__',
    #   'tree', '__traverse__'   ]
    def __init__(self, value=None, **kwargs):
        # (2)
        # self.__used_names = None
        # self.__used_names = Tree.__dict__.copy()
        # self.__used_names.update(object.__dict__)
        # self.__used_names = self.__used_names.keys()
        # (3)
        # setattr(self, '__used_names', None)
        # setattr(self, '__used_names', Tree.__dict__.copy())
        # self.__used_names.update(object.__dict__)
        # setattr(self, '__used_names', self.__used_names.keys())
        # --------------------------------------------------------------------
        # self.__node_value = value
        self.__dict__['__node_value'] = value
        # __node_value should always be Non-Tree
        # self.__node_items = {}
        self.__dict__['__node_items'] = {}
        for k, v in kwargs.items(): self.__setattr__(k, v)
    def __setattr__(self, attr_name, value):
        _("""The operation 'root.br1.br2 = value' works on root.br1 rather than root.br1.br2""")
        # (4)
        __used_names = Tree.__dict__.copy()
        print __used_names
        __used_names.update(object.__dict__.copy())
        __used_names = __used_names.keys() + ['__node_value', '__node_items']
        if attr_name in __used_names:
        # --------------------------------------------------------------------
        # if attr_name in self.__used_names:
            raise TreeExc(_("Attribute name '%s' is reserved" % attr_name))
        try:
            existed = getattr(self, attr_name)
            # self.attribute exists
            if isinstance(value, Tree):
                subtree = value
                if not subtree._Tree__node_value:
                    subtree._Tree__node_value = existed._Tree__node_value
                setattr(self, attr_name, subtree)
                # replace directly after the tree __node_value has been reserved
            else:
                setattr(self, '%s._Tree__node_value' % attr_name, value)
        except AttributeError:
        # if self.attribute does not exists, assign it a EMPTY node
            if isinstance(value, Tree):
                subtree = value
                # setattr(self, attr_name, subtree)
                self.__dict__[attr_name] = subtree
            else:
                # setattr(self, attr_name, Tree(value))
                self.__dict__[attr_name] = Tree(value)

可见，在重定义了 __setattr__ 之后，赋值只能使用 self.__dict__ 来完成了。

这样做的潜在的问题是开销可能会比较大，因为每次创建 Tree instance 的时候到要重复一遍。还是回到定义 Tree class static variable 的思路上来的话，最终是这样定义的：

class Tree:
    _("""A Tree instance can only contains sub Tree instances as its attributes""")
    # (1)
    # __used_names = None
    # __used_names = Tree.__dict__.copy()
    # __used_names.update(object.__dict__.copy())
    # __used_names = used_names.keys()
    # ------------------------------------------------------------------------
    __used_names = object.__dict__.copy().keys() + [
        '__used_names', '__node_value', '__node_items',
        '__getattr__', '__setitem__', '__getitem__', '__call__',
        'tree', '__traverse__'   ]
    def __init__(self, value=None, **kwargs):
        # (2)
        # self.__used_names = None
        # self.__used_names = Tree.__dict__.copy()
        # self.__used_names.update(object.__dict__)
        # self.__used_names = self.__used_names.keys()
        # (3)
        # setattr(self, '__used_names', None)
        # setattr(self, '__used_names', Tree.__dict__.copy())
        # self.__used_names.update(object.__dict__)
        # setattr(self, '__used_names', self.__used_names.keys())
        # --------------------------------------------------------------------
        # self.__node_value = value
        self.__dict__['__node_value'] = value
        # __node_value should always be Non-Tree
        # self.__node_items = {}
        self.__dict__['__node_items'] = {}
        for k, v in kwargs.items(): self.__setattr__(k, v)
    def __setattr__(self, attr_name, value):
        _("""The operation 'root.br1.br2 = value' works on root.br1 rather than root.br1.br2""")
        # (4)
        # __used_names = Tree.__dict__.copy()
        # __used_names.update(object.__dict__.copy())
        # __used_names = __used_names.keys() + ['__node_value', '__node_items']
        # if attr_name in __used_names:
        # --------------------------------------------------------------------
        if attr_name in self.__used_names:
            raise TreeExc(_("Attribute name '%s' is reserved" % attr_name))
        try:
            existed = getattr(self, attr_name)
            # self.attribute exists
            if isinstance(value, Tree):
                subtree = value
                if not subtree._Tree__node_value:
                    subtree._Tree__node_value = existed._Tree__node_value
                # setattr(self, attr_name, subtree)
                self.__dict__[attr_name] = subtree
                # replace directly after the tree __node_value has been reserved
            else:
                # setattr(self, '%s._Tree__node_value' % attr_name, value)
                self.__dict__[attr_name]._Tree__node_value = value
                # self.__dict__[attr_name] = Tree(value)
        except AttributeError:
        # if self.attribute does not exists, assign it a EMPTY node
            if isinstance(value, Tree):
                subtree = value
                # setattr(self, attr_name, subtree)
                self.__dict__[attr_name] = subtree
            else:
                # setattr(self, attr_name, Tree(value))
                self.__dict__[attr_name] = Tree(value)