将 Python 切片语法传递给函数 [英] Passing Python slice syntax around to functions

问题描述

在 Python 中，是否可以准确地封装常见的切片语法并将其传递?我知道我可以使用 slice 或 __slice__ 来模拟切片.但我想传递与 __getitem__ 一起使用的方括号中的完全相同的语法.

In Python, is it possible to encapsulate exactly the common slice syntax and pass it around? I know that I can use slice or __slice__ to emulate slicing. But I want to pass the exact same syntax that I would put in the square brackets that would get used with __getitem__.

例如，假设我编写了一个函数来返回列表的某个部分.

For example, suppose I wrote a function to return some slice of a list.

def get_important_values(some_list, some_condition, slice):
    elems = filter(some_condition, some_list)
    return elems[slice]

如果我手动传入切片对象，这可以正常工作:

This works fine if I manually pass in a slice object:

In [233]: get_important_values([1,2,3,4], lambda x: (x%2) == 0, slice(0, None))
Out[233]: [2, 4]

但我想让用户通过的是完全与他们在 __getitem__ 中使用的切片相同:

But what I want to let the user pass is exactly the same slicing they would have used with __getitem__:

get_important_values([1,2,3,4], lambda x: (x%2) == 0, (0:-1) )

# or

get_important_values([1,2,3,4], lambda x: (x%2) == 0, (0:) )

显然这会产生语法错误.但是有没有什么方法可以让这个工作，而不用为 x:y:t 类型切片编写我自己的迷你解析器，并强制用户将它们作为字符串传递?

Obviously this generates a syntax error. But is there any way to make this work, without writing my own mini parser for the x:y:t type slices, and forcing the user to pass them as strings?

动机

我可以让这个示例函数返回一些直接可切片的内容，例如 filter(some_condition, some_list)，它将作为列表的整个结果.但是，在我的实际例子中，内部函数要复杂得多，如果我提前知道用户想要的切片，我可以大大简化计算.但我希望用户不必做太多额外的事情来提前告诉我切片.

I could just make this example function return something directly sliceable, such as filter(some_condition, some_list), which will be the whole result as a list. In my actual example, however, the internal function is much more complicated, and if I know the slice that the user wants ahead of time, I can greatly simplify the calculation. But I want the user to not have to do much extra to tell me the slice ahead of time.

推荐答案

也许以下内容对您有用:

Perhaps something along the following lines would work for you:

class SliceMaker(object):
  def __getitem__(self, item):
    return item

make_slice = SliceMaker()

print make_slice[3]
print make_slice[0:]
print make_slice[:-1]
print make_slice[1:10:2,...]

这个想法是您使用 make_slice[] 而不是手动创建 slice 的实例.通过这样做，您将能够充分利用熟悉的方括号语法.

The idea is that you use make_slice[] instead of manually creating instances of slice. By doing this you'll be able to use the familiar square brackets syntax in all its glory.

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