如何在Python中保存列表理解中的重复计算？

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k+d的结果存储在列表理解中，以便在列表理解中引用它。在Python3中是可能的吗？

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)

但是我希望避免append操作的副作用。

Answer 1

在列表理解中，只有for循环可以为新名称赋值，所以如果你需要一个“局部”变量来引用一个计算，你需要想办法添加一个额外的循环。

您可以在计算k + d的单个元素元组上添加另一个循环

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]