Python序列子列表按特定顺序生成-IterTools？

Question

I would like to generate sublists in a specific way and a specific order.

The idea of the algorithm is the following :

• Outer loop: remove the before last item
• Inner loop: remove the first item

Here's an example starting with the [A, B, C ,D] list :

[A, B, C, D]
[B, C, D] (remove first item)
[C, D]  (remove first item)
[A, B, D]  (remove before last item)
[B, D]  (remove first item)
[A, D]  (remove before last item)
[D] (remove before last item)

The only constraint I have is that the the sublist of length one containing the last item alone have to be last the sublist generated.

I do not have any difficulties to implent this algorithm in a naive iterative way. But I'd strongly prefer to do it in a more pythonic way, like with list comprehensions or maybe the itertools lib.

Any advice please ?

Answer 1

Can be accomplished with a double nested comprehension + some index math:

>>> data = ['A', 'B', 'C', 'D']
>>>
>>> [data[i:j-1]+[data[-1]] for j in range(len(data),0,-1) for i in range(0, j-1)] + [data[-1]]
[['A', 'B', 'C', 'D'], ['B', 'C', 'D'], ['C', 'D'], ['A', 'B', 'D'], ['B', 'D'], ['A', 'D'], ['D']]

The last element is a special case because the sub-list whose combinations are prepended to it is empty, ie the length of the list slice decreases in the following fashion:

3 2 1
2 1
1
0