如何在 Python 的列表理解中保存重复计算？

Question

在以下 Python 代码中：

keyboards = [3, 1]
drivers = [5, 2, 8]
upper_limit = 10
sums = [k + d for k in keyboards for d in drivers if (k + d) <= upper_limit]

我想将 的结果存储k+d在列表理解中，以便它可以在列表理解中引用。在 Python3 中可能吗？

我知道我们可以做到以下几点：

sums = []
for k in keyboards:
    for d in drivers:
        s = k + d
        if s <= upper_limit:
            sums.append(s)

但我希望避免 append 的副作用操作。

Answer 1

只有for循环才能在列表推导式中分配新名称，因此如果您需要一个“本地”变量来引用计算，则需要找到添加额外循环的方法。

您可以在计算的单个元素元组上添加另一个循环k + d：

sums = [kd for k in keyboards for d in drivers for kd in (k + d,) if d <= upper_limit]

或者您也可以使用生成器表达式：

sums = [kd for kd in (k + d for k in keyboards for d in drivers) if kd <= upper_limit]

在后一种情况下，您可以先将该表达式存储为单独的变量：

kd_calc = (k + d for k in keyboards for d in drivers)
sums = [kd for kd in kd_calc if kd <= upper_limit]

演示：

>>> keyboards = [3, 1]
>>> drivers = [5, 2, 8]
>>> upper_limit = 10
>>> [kd for k in keyboards for d in drivers for kd in (k + d,) if d <= upper_limit]
[8, 5, 11, 6, 3, 9]
>>> [kd for kd in (k + d for k in keyboards for d in drivers) if kd <= upper_limit]
[8, 5, 6, 3, 9]
>>> kd_calc = (k + d for k in keyboards for d in drivers)
>>> [kd for kd in kd_calc if kd <= upper_limit]
[8, 5, 6, 3, 9]