在循环中创建函数

问题描述

我试图在循环内部创建函数并将它们存储在字典中。
问题是，字典中的所有条目似乎都会映射到最后创建的函数。代码如下所示：

  
 $ b 通过强制早绑定轻松修复：将 def f（）：更改为 def f （k = k）：  - 默认值（右边 k 在 k = k 是参数名称 k 的默认值，它是<$ c $中左侧的 k  c> k = k ）在 def 时间内查找，而不是在 call 时间，所以基本上它们是专门寻找早期绑定的一种方法。
 
 
 
如果您担心 f 获得一个额外的参数（因此可能被错误地调用），还有一种更复杂的方式，它将一个闭包作为函数工厂使用： 
 
 
  def make_f（kwargs，k）：
 def f（）：
 print k，kwargs [k] 
 return f 
  
并在循环中使用 f = make_f（kwargs，k）而不是 def 语句。
 
I'm trying to create functions inside of a loop and storing them in a dictionary.The problem is that all entries in the dictionary seem end up mapping to the last created function. The code goes like this:
d = {}
def test(**kwargs):
    for k in kwargs:
        def f():
            print k, kwargs[k]
        d[k] = f
        f()

test(foo=1, bar=2)
print 'should print the same output as before'
d['foo']()
d['bar']()
This outputs:
foo 1
bar 2
should print the same output as before
bar 2
bar 2
Any idea why?
 解决方案 
You're running into a problem with late binding -- each function looks up k as late as possible (thus, when called outside test, this happens after the end of the loop).  
Easily fixed by forcing early binding: change def f(): to def f(k=k): -- default values (the right-hand k in k=k is a default value for argument name k, which is the left-hand k in k=k) are looked up at def time, not at call time, so essentially they're a way to specifically looking for early binding.
If you're worried about f getting an extra argument (and thus potentially being called erroneously), there's a more sophisticated way which involved using a closure as a "function factory":
def make_f(kwargs, k):
    def f():
        print k, kwargs[k]
    return f
and in your loop use f = make_f(kwargs, k) instead of the def statement.
                        
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