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const.html
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<html>
<head>
<title>c:\const.py.html</title>
<meta name="Generator" content="Vim/7.0">
<meta http-equiv="content-type" content="text/html; charset=GBK">
</head>
<body bgcolor="#000000" text="#cccccc">
<pre>
<font color="#80a0ff"># -*- coding: utf-8 -*-</font>
<span style="background-color: #000000"><font color="#cccccc">'''</font></span>
<font color="#ffa0a0">感觉楼主的这篇和上次用 python 实现 define 的那篇帖子，想说的都是</font>
<font color="#ffa0a0">一个东西，就是静态语言中的 const，第一次初始化后不能修改的东西。</font>
<font color="#ffa0a0">说起来，python 对象中其实是有这样的东西，就是 imutable object 。</font>
<font color="#ffa0a0">不过常量针对的是名字而非对象，所以在 python 中常量的准确定义应该</font>
<font color="#ffa0a0">是：在第一次绑定后不能重新绑定其他对象的名字。</font>
<font color="#ffa0a0">遗憾的是 python 中没有这样的东西。</font>
<font color="#ffa0a0">其实和类型检查、访问控制等东西一样，静态语言中常量是通过编译器在</font>
<font color="#ffa0a0">编译时进行检查，而 python 就算实现那也只能是在运行时进行计算，势</font>
<font color="#ffa0a0">必损耗性能，我想这也是 python 中没有这样的东西的原因。</font>
<font color="#ffa0a0">但是正如 python 中的访问控制是通过对名字的约定来做的一样，其实常</font>
<font color="#ffa0a0">量也比较适合这样做。</font>
<font color="#ffa0a0">如果实在要用动态语言模拟 const，那么关键在于对名字的绑定进行控制。</font>
<font color="#ffa0a0">下面总结一下各种做法：</font>
<span style="background-color: #000000"><font color="#cccccc">'''</font></span>
<font color="#ffff00">def</font> <font color="#40ffff">a_const_value</font>():
<span style="background-color: #000000"><font color="#cccccc">'''</font></span>
<font color="#ffa0a0"> 方法1是通过使用函数替代对名字的直接访问，好像是比较傻的方法。</font>
<font color="#ffa0a0"> 不过 ruby 中函数调用可以省略括号就有点像了</font>
<font color="#ffa0a0"> >>> a_const_value()</font>
<font color="#ffa0a0"> 'const'</font>
<font color="#ffa0a0"> </font><span style="background-color: #000000"><font color="#cccccc">'''</font></span>
<font color="#ffff00">return</font> <span style="background-color: #000000"><font color="#cccccc">'</font></span><font color="#ffa0a0">const</font><span style="background-color: #000000"><font color="#cccccc">'</font></span>
<font color="#ffff00">class</font> <font color="#40ffff">Temp</font>(object):
<span style="background-color: #000000"><font color="#cccccc">'''</font></span>
<font color="#ffa0a0"> class 中通过 property 可以做得更漂亮：</font>
<font color="#ffa0a0"> >>> t = Temp()</font>
<font color="#ffa0a0"> >>> t.a_const_value</font>
<font color="#ffa0a0"> 'const'</font>
<font color="#ffa0a0"> >>> t.a_const_value = 'another value'</font>
<font color="#ffa0a0"> Traceback (most recent call last):</font>
<font color="#ffa0a0"> ...</font>
<font color="#ffa0a0"> AttributeError: can't set attribute</font>
<font color="#ffa0a0"> </font><span style="background-color: #000000"><font color="#cccccc">'''</font></span>
<font color="#ff80ff">@</font><font color="#40ffff">property</font>
<font color="#ffff00">def</font> <font color="#40ffff">a_const_value</font>(self):
<font color="#ffff00">return</font> <span style="background-color: #000000"><font color="#cccccc">'</font></span><font color="#ffa0a0">const</font><span style="background-color: #000000"><font color="#cccccc">'</font></span>
<font color="#ffff00">class</font> <font color="#40ffff">ConstError</font>(Exception):
<font color="#ffff00">pass</font>
<font color="#ffff00">class</font> <font color="#40ffff">Consts</font>(object):
<span style="background-color: #000000"><font color="#cccccc">'''</font></span>
<font color="#ffa0a0"> 方法2是将常量名字放入一个 class 中统一进行管理：</font>
<font color="#ffa0a0"> >>> consts = Consts()</font>
<font color="#ffa0a0"> >>> consts.a = 2</font>
<font color="#ffa0a0"> >>> consts.a</font>
<font color="#ffa0a0"> 2</font>
<font color="#ffa0a0"> >>> consts.a = 3</font>
<font color="#ffa0a0"> Traceback (most recent call last):</font>
<font color="#ffa0a0"> ...</font>
<font color="#ffa0a0"> ConstError: can't rebind const name</font>
<font color="#ffa0a0"> 不过需要注意的是，仍然可以通过 __dict__ 直接访问常量：</font>
<font color="#ffa0a0"> >>> consts.__dict__['a'] = 3</font>
<font color="#ffa0a0"> >>> consts.a</font>
<font color="#ffa0a0"> 3</font>
<font color="#ffa0a0"> </font><span style="background-color: #000000"><font color="#cccccc">'''</font></span>
<font color="#ffff00">def</font> <font color="#40ffff">__setattr__</font>(self, name, value):
<font color="#ffff00">if</font> name <font color="#ffff00">in</font> self.__dict__:
<font color="#ffff00">raise</font> ConstError, <span style="background-color: #000000"><font color="#cccccc">'</font></span><font color="#ffa0a0">can</font><font color="#ffa500">\'</font><font color="#ffa0a0">t rebind const name</font><span style="background-color: #000000"><font color="#cccccc">'</font></span>
<font color="#ffff00">else</font>:
self.__dict__[name] = value
<font color="#ffff00">class</font> <font color="#40ffff">ConstBase</font>(object):
<span style="background-color: #000000"><font color="#cccccc">'''</font></span>
<font color="#ffa0a0"> 或者让 class 自己指定那些是常量：</font>
<font color="#ffa0a0"> >>> class Temp(ConstBase):</font>
<font color="#ffa0a0"> ... __consts__ = {'a':None, 'b':2}</font>
<font color="#ffa0a0"> ... def __init__(self, a):</font>
<font color="#ffa0a0"> ... self.a = a</font>
<font color="#ffa0a0"> ...</font>
<font color="#ffa0a0"> >>> t = Temp(2)</font>
<font color="#ffa0a0"> >>> t.a</font>
<font color="#ffa0a0"> 2</font>
<font color="#ffa0a0"> >>> t.b</font>
<font color="#ffa0a0"> 2</font>
<font color="#ffa0a0"> >>> t.a = 3</font>
<font color="#ffa0a0"> Traceback (most recent call last):</font>
<font color="#ffa0a0"> ...</font>
<font color="#ffa0a0"> ConstError: can't rebind const name</font>
<font color="#ffa0a0"> >>> t.b = 3</font>
<font color="#ffa0a0"> Traceback (most recent call last):</font>
<font color="#ffa0a0"> ...</font>
<font color="#ffa0a0"> ConstError: can't rebind const name</font>
<font color="#ffa0a0"> >>> t.c = 5</font>
<font color="#ffa0a0"> >>> t.c</font>
<font color="#ffa0a0"> 5</font>
<font color="#ffa0a0"> 使用这种方式，也可以直接通过 __dict__ 对常量进行修改：</font>
<font color="#ffa0a0"> >>> t.__dict__['a']= 3</font>
<font color="#ffa0a0"> >>> t.a</font>
<font color="#ffa0a0"> 3</font>
<font color="#ffa0a0"> </font><span style="background-color: #000000"><font color="#cccccc">'''</font></span>
__consts__ = {}
<font color="#ffff00">def</font> <font color="#40ffff">__setattr__</font>(self, name, value):
<font color="#ffff00">if</font> name <font color="#ffff00">in</font> self.__consts__:
<font color="#ffff00">if</font> self.__consts__[name] == None:
self.__consts__[name] = value
<font color="#ffff00">else</font>:
<font color="#ffff00">raise</font> ConstError, <span style="background-color: #000000"><font color="#cccccc">'</font></span><font color="#ffa0a0">can</font><font color="#ffa500">\'</font><font color="#ffa0a0">t rebind const name</font><span style="background-color: #000000"><font color="#cccccc">'</font></span>
<font color="#ffff00">else</font>:
super(ConstBase, self).__setattr__(name, value)
<font color="#ffff00">def</font> <font color="#40ffff">__getattr__</font>(self, name):
<font color="#ffff00">if</font> name <font color="#ffff00">in</font> self.__consts__:
<font color="#ffff00">return</font> self.__consts__[name]
<font color="#ffff00">else</font>:
<font color="#ffff00">return</font> super(ConstBase, self).__getattr__(name, value)
<font color="#ffff00">if</font> __name__ == <span style="background-color: #000000"><font color="#cccccc">'</font></span><font color="#ffa0a0">__main__</font><span style="background-color: #000000"><font color="#cccccc">'</font></span>:
<font color="#ff80ff">import</font> doctest
doctest.testmod()
</pre>
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