循环中的Lambda [英] Lambda in a loop

问题描述

考虑以下代码段:

# directorys == {'login': <object at ...>, 'home': <object at ...>}
for d in directorys:
    self.command["cd " + d] = (lambda : self.root.change_directory(d))

我希望创建一个具有以下两个功能的字典:

I expect to create a dictionary of two function as following :

# Expected :
self.command == {
    "cd login": lambda: self.root.change_directory("login"),
    "cd home": lambda: self.root.change_directory("home")
}

但看起来生成的两个lambda函数完全相同:

but it looks like the two lambda function generated are exactly the same :

# Result :
self.command == {
    "cd login": lambda: self.root.change_directory("login"),
    "cd home": lambda: self.root.change_directory("login")   # <- Why login ?
}

我真的不明白为什么.您有什么建议吗?

I really don't understand why. Do you have any suggestions ?

推荐答案

您需要为创建的每个函数绑定d.一种方法是将其作为具有默认值的参数传递:

You need to bind d for each function created. One way to do that is to pass it as a parameter with a default value:

lambda d=d: self.root.change_directory(d)

现在，函数中的d即使其名称相同，也使用该参数，并且在创建函数时将评估该参数的默认值.为了帮助您看到这一点:

Now the d inside the function uses the parameter, even though it has the same name, and the default value for that is evaluated when the function is created. To help you see this:

lambda bound_d=d: self.root.change_directory(bound_d)

记住默认值是如何工作的，例如对于可变对象(如列表和字典)，因为您要绑定对象.

Remember how default values work, such as for mutable objects like lists and dicts, because you are binding an object.

这种带有默认值的参数的惯用法足够普遍，但是如果您对函数参数进行内省并根据其存在来确定要做什么，则可能会失败.您可以使用另一个闭包来避免使用该参数:

This idiom of parameters with default values is common enough, but may fail if you introspect function parameters and determine what to do based on their presence. You can avoid the parameter with another closure:

(lambda d=d: lambda: self.root.change_directory(d))()
# or
(lambda d: lambda: self.root.change_directory(d))(d)

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