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，但我对代码的行为感到困惑.我正在尝试创建一个递增程度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 在列表推导式中似乎是重复的最后一个 lambda 表达式的列表?

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

推荐答案

问题，这是一个经典陷阱"，是在 lambda 函数中引用的 i 是 直到这lambda 函数被称为.那个时候i的值就是它的最后一个值for-loop 结束时绑定到，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 函数定义中的默认值，则每个 i 都成为局部变量，并且它的默认值在 lambda 被定义而不是被调用时被评估并绑定到函数.

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 lambda 被调用时，i 现在会在 本地范围 中查找，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 范围和命名空间

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