python - lambda 中的 Python lambda

Question

g = lambda x:[lambda x:x*1, lambda x:x*x, lambda x:x*x*x, lambda x:42][x%4](x)

[g(x) for x in xrange(12)]

这个序列的下一个值是什么？

Answer 1

你试过了吗？

>>> g = lambda x:[lambda x:x*1, lambda x:x*x, lambda x:x*x*x, lambda x:42][x%4](x)
>>> [g(x) for x in xrange(12)]
[0, 1, 8, 42, 4, 25, 216, 42, 8, 81, 1000, 42]

以下是每个值的计算方式：

[
 0,     # x is 0, x%4 is 0, so g(x) becomes (lambda x:x*1)(0) or 0*1
 1,     # x is 1, x%4 is 1, so g(x) becomes (lambda x:x*x)(1) or 1*1
 8,     # x is 2, x%4 is 2, so g(x) becomes (lambda x:x*x*x)(2) or 2*2*2
 42,    # x is 3, x%4 is 3, so g(x) becomes (lambda x:42)(3) or 42
 4,     # x is 4, x%4 is 0, so g(x) becomes (lambda x:x*1)(4) or 4*1
 25,    # x is 5, x%4 is 1, so g(x) becomes (lambda x:x*x)(5) or 5*5
 216,   # x is 6, x%4 is 2, so g(x) becomes (lambda x:x*x*x)(6) or 6*6*6
 42,    # x is 7, x%4 is 3, so g(x) becomes (lambda x:42)(7) or 42
 8,     # x is 8, x%4 is 0, so g(x) becomes (lambda x:x*1)(8) or 8*1
 81,    # x is 9, x%4 is 1, so g(x) becomes (lambda x:x*x)(9) or 9*9
 1000,  # x is 10, x%4 is 2, so g(x) becomes (lambda x:x*x*x)(10) or 10*10*10
 42     # x is 11, x%4 is 3, so g(x) becomes (lambda x:42)(11) or 42
]

基本上g(x)以x作为参数调用列表中的一个函数，当在列表推导中调用xrange它时，它将循环遍历函数，每四个调用都是同一个函数。

我知道这只是一个有助于理解 Python 的练习，但您应该注意这是非常低效的代码，因为每次调用g(). 如果您确实需要这种行为，最好只创建一个def包含多个if语句的函数（这也会使代码更具可读性）。

Answer 2

我会这样做以更好地了解正在发生的事情（如果您需要进一步解释，请发表评论）

In [44]: g = lambda x:[lambda x:x*1, lambda x:x*x, lambda x:x*x*x, lambda x:42][x%4](x)

In [45]: {x:g(x) for x in xrange(12)}
Out[45]: 
{0: 0,
 1: 1,
 2: 8,
 3: 42,
 4: 4,
 5: 25,
 6: 216,
 7: 42,
 8: 8,
 9: 81,
 10: 1000,
 11: 42}