Python和Ruby中each循环引用变量问题（一个隐秘BUG?）
虽然这个问题我是在 Python 里遇到的，但是用 Ruby 解释起来比较容易一些。在 Ruby 里，遍历一个数组可以有很多种方法，最常用的两种无非是 for 和 each：
arr = ['a', 'b', 'c']
arr.each { |e|
puts e
}
for e in arr
puts e
end
通常我比较喜欢后者，似乎因为写起来比较好看，不过从效率上来说前者应该会稍微快一点，因为后者实际上是在遍历的过程中对每个元素都调用一个 lambda 函数来做的，虽然一般情况下并不明显，不过设置上下文并调用函数确实是有开销的，特别是在动态语言里面（不考虑 JIT 内联优化的话）。不过这次的问题并不是性能。然而确实跟“ each 对每个元素都会新建一个 scope 而 for 则不是”有关。
看下面一段代码：
arr = ['a', 'b', 'c']
h1 = Hash.new
h2 = Hash.new
arr.each { |e|
h1[e] = lambda { e+'!'}
}
for e in arr
h2[e] = lambda { e+'!' }
end
h1['a'].call # => ?
h2['a'].call # => ?
两个 call 分别会得到什么？应该已经猜到了吧？分别是 'a!' 和 'c!' ，后者之所以是 'c!' 是因为 for 并没有在循环的每一步都重新创建一个 scope ，因此三个 lambda 的 closure 引用到了同一个变量，而这个变量在最后一次被赋值为 'c' ，所以导致了这样的后果。
问题其实出自我在用 Python 写的一个小程序中的一段，代码类似于这样：
for prop in public_props:
setattr(proxy, 'get_%s'%prop, lambda: self.get_prop(prop))
其中 proxy 是我提供的一个代理对象，将 self 的一些公开的属性给暴露出去，因为要限制对非 public 的属性的访问，我并不想在这个 proxy 中存放任何到 self 的引用，否则在没有访问权限限制的 Python 里通过类似 proxy._orig_self.some_private_prop 的方式来访问是轻而易举的。所以最后选择了上面那样的做法。
不幸的是，由于像刚才所说的那样，for 并没有每次都单独创建 scope ，因此 closure 全部引用到了同一个变量上，导致所有的属性值取出来都是最后一个属性了。看到这样诡异的 bug ，如果是在 C/C++ 里面，我肯定要怀疑是内存或者指针的问题了。不过想了半天才终于恍然大悟！不过 Python 里面没有 Ruby 那么方便的 each 可以用，lambda 用起来也很鸡肋，所以最后通过定义一个局部的函数来解决了：
def proxy_prop(name):
setattr(proxy, 'get_%s'%prop, lambda: self.get_prop(name)
for prop in public_props:
proxy_prop(prop)
最后，还要多嘴一句，对于之前 Ruby 那个例子，如果把 each 和 for 的执行顺序颠倒过来，会得到不同的结果：
h1 = Hash.new
h2 = Hash.new
for e in arr
h2[e] = lambda { e+'!' }
end
arr.each { |e|
h1[e] = lambda { e+'!'}
}
h1['a'].call # => 'c!'
h2['a'].call # => 'c!'
现在两个都是 'c!' 了！这是因为 Ruby 1.8 的实现里面 block 的参数可以对局部变量或者全局变量之类的任何东西进行赋值，而不是通常意义上的一个 lambda 函数的参数那么简单。由于前面的 for 语句在当前作用域创建了一个 e 作为局部变量，因此 each 就直接对这个局部变量进行赋值了，这样，每次引用到的又变成了同一个东西，导致了一个隐秘的 Bug ！
值得庆幸的是，block 的这个“特性”在 Ruby 1.9 中已经被去除了，block 的参数只能是正常参数，所以就不再存在这样的问题了。希望 1.9 尽快普及吧！
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