使用lambdas进行Python列表理解 [英] Python list comprehension with lambdas

问题描述

我正在运行Python 3.4.2，我对代码的行为感到困惑.我正在尝试创建一个递增次数的可调用多项式函数列表:

I'm running Python 3.4.2, and I'm confused at the behavior of my code. I'm trying to create a list of callable polynomial functions with increasing degree:

bases = [lambda x: x**i for i in range(3)]

但是出于某种原因，它会这样做:

But for some reason it does this:

print([b(5) for b in bases])
# [25, 25, 25]

为什么bases似乎是列表理解中最后一个lambda表达式的列表重复了?

Why is bases seemingly a list of the last lambda expression, in the list comprehension, repeated?

推荐答案

问题，这是是 在lambda函数中引用的i是 lambda函数称为.当时i的值是它的最后一个值 绑定到for-loop结束的时间，即2.

The problem, which is a classic "gotcha", is that the i referenced in the lambda functions is not looked up until the lambda function is called. At that time, the value of i is the last value it was bound to when the for-loop ended, i.e. 2.

如果将i绑定到lambda函数的定义中的默认值，则每个i都将成为局部变量，并且在lambda为定义而不是被调用.

If you bind i to a default value in the definition of the lambda functions, then each i becomes a local variable, and its default value is evaluated and bound to the function at the time the lambda is defined rather than called.

因此，当调用lambda时，现在在本地范围中查找i，并使用其默认值:

Thus, when the lambda is called, i is now looked up in the local scope, and its default value is used:

In [177]: bases = [lambda x, i=i: x**i for i in range(3)]

In [178]: print([b(5) for b in bases])
[1, 5, 25]

供参考:

• Python作用域和命名空间

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