lamba的奇怪行为:列表内的getattr(obj，x)

问题描述

在以下示例中:

class A(object):
    pass
    prop1 = 1
    prop2 = 2
    prop3 = 3
    prop4 = 4

obj = A()
tmp = ['prop1', 'prop2', 'prop3', 'prop4']
getter = [ lambda: getattr(obj, x) for x in tmp ]

调用getter时总是得到4:

[getter[i]() for i in range(4)]
#[4, 4, 4, 4]

为什么！?

推荐答案

这是lambda的一个非常普遍的问题.最终，变量x是在调用函数时(而不是在创建函数时)查找的.因此，在循环结束时，x的值是'prop4'，所有lambda都将为您提供相同的功能.

This is a very common problem with lambdas. Ultimately, the variable x is looked up when the function is called, not when it is created. As such, at the end of your loop, the value of x is 'prop4' and all your lambdas will give you the same thing.

通常建议的解决方法是在lambda中使用默认参数.创建函数时会对其进行评估.

The commonly proposed fix is to use a default argument in your lambda. It gets evaluated when the function is created.

lambda x=x: getattr(obj,x)

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