Python列表切片，无参数 [英] Python List Slicing with None as argument

问题描述

通过反复试验，我发现了

Via trial and error I found out that

my_list = range(10)
my_list[:None] == my_list[:]

我将其用于Django查询集，因此我可以定义大小或全部使用:

I use this for django query sets so I can define a size or take all:

some_queryset[:length if length else None]

# @IanAuld
some_queryset[:length or None]


# @Bakuriu
# length works for all numbers and None if you want all elements
# does not work with False of any other False values
some_queryset[:length]

• 在切片时使用None是一种好习惯吗?
• 在任何情况下，这种方法都会出现问题吗?
• Is this good practice to use None while slicing?
• Can problems occur with this method in any case?

推荐答案

是的，可以使用None，因为它的行为由

Yes, it is fine to use None, as its behavior is specified by the documentation:

从i到j的s切片被定义为索引为k的项目序列，使得i≤k≤k. j.如果i或j大于len，请使用len.如果省略i或无，请使用0.如果省略j或无，请使用len .如果i大于或等于j，则切片为空.

The slice of s from i to j is defined as the sequence of items with index k such that i <= k < j. If i or j is greater than len(s), use len(s). If i is omitted or None, use 0. If j is omitted or None, use len(s). If i is greater than or equal to j, the slice is empty.

为切片参数之一使用None与省略切片参数相同.

Using None for one of the slice parameters is the same as omitting it.

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