使用一个参数数组的长度作为同一个函数的另一个参数的默认值

问题描述：

这是我第一次在 SO 中提问，所以如果我不知何故做得不正确，请不要犹豫，编辑它或要求我修改它.

This is my first time asking a question in SO, so if I'm somehow not doing it properly don't hesitate to edit it or ask me to modify it.

我认为我的问题有点笼统，所以我很惊讶之前没有找到任何与此主题相关的问题.如果我错过了它并且这个问题是重复的，如果你能提供一个链接到已经回答的地方，我将不胜感激.

I think my question is kind of general, so I'm quite surprised for not having found any previous one related to this topic. If I missed it and this question is duplicated, I'll be very grateful if you could provide a link to where it was already answered.

想象一下，我需要用(至少)三个参数来实现一个函数:一个数组 a、一个 start 索引和一个 end 索引.如果未提供，start 参数应指向数组的第一个位置(start = 0)，而end 参数应设置到最后一个位置 (end = len(a) - 1).显然，定义:

Imagine I need to implement a function with (at least) three parameters: an array a, a start index and an end index. If not provided, the start parameter should refer to the first position of the array (start = 0), while the end parameter should be set to the last position (end = len(a) - 1). Obviously, the definition:

def function(a, start = 0, end = (len(a) - 1)):
    #do_something
    pass

不起作用，导致异常(NameError: name 'a' is not defined).有一些解决方法，例如使用end = -1 或end = None，并有条件地将其分配给len(a) - 1 如果在函数体内需要:

does not work, leading to an exception (NameError: name 'a' is not defined). There are some workarounds, such as using end = -1 or end = None, and conditionally assign it to len(a) - 1 if needed inside the body of the function:

def function(a, start = 0, end = -1):
    end = end if end != -1 else (len(a) -1)
    #do_something

但我觉得应该有一种更pythonic"的方式来处理这种情况，不仅是数组的长度，还有默认值是另一个(非可选)参数的函数的任何参数.遇到这样的情况你会怎么处理?条件分配是最好的选择吗?

but I have the feeling that there should be a more "pythonic" way of dealing with such situations, not only with the length of an array but with any parameter whose default value is a function of another (non optional) parameter. How would you deal with a situation like that? Is the conditional assignment the best option?

谢谢！

答

基于@NPE 在具有相关预设参数的函数，使用 -1 或(更好)None 作为标记值的替代方法是使用对象(命名对象?) 即使 None 是函数的有效值，也可以使用.例如:

Based on the answer provided by @NPE in Function with dependent preset arguments, an alternative to using -1 or (better) None as sentinel values is using an object (a named object?) which can be used even if None is a valid value of the function. For example:

default = object()

def function(a, start = 0, end = default):
    if end is default: end = (len(a) - 1)
    return start, end

允许这样的调用: function([1,2,3,4]) 返回 (0, 3)

allows a call like: function([1,2,3,4]) which returns (0, 3)

我个人觉得这个解决方案非常方便，至少对于我自己的目的

I personally find this solution quite convenient, at least for my own purpose

如果我们使用 last 而不是 default，也许代码更易读:

Maybe the code is even more readable if we use last instead of default:

last = object()

def function(a, start = 0, end = last):
    if end is last: end = (len(a) - 1)
    return start, end

使用一个参数数组的长度作为同一个函数的另一个参数的默认值

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