Python 中的 OR 行为: [英] OR behaviour in python:

问题描述

我编写了以下代码，如果传递的数字是 1、0 或 2 的字符串表示，我想要做的就是打印 yes，对于其他所有内容都为 false:

I have written the following piece of code, all i want to do is print a yes for if the number passed is a string representation of 1, 0 or 2 and for everything else a false:

number=raw_input()
if number is "1" or "0" or "2":
    print "Yes"
else:
     print "no"

我知道如何使预期的逻辑工作，但我只是想让 need 知道为什么它会为我传递给 raw_input 的任何数字打印 yes.我希望答案尽可能详细，因为我不明白为什么会失败，这对我来说似乎足够 pythonic

I know how to make the intended logic work, but i just want need to know why it prints yes for any number i pass to raw_input. I'd like the answer to be as detailed as possible, because i cant understand why it would fail, it seems pythonic enough to me

推荐答案

问题是你的代码对于 Python 来说是这样的:

The problem is that your code, to Python, reads like this:

if (number is "1") or "0" or "2":

并且由于任何非空字符串的计算结果为 True，因此它始终为 True.

And as any non-empty string evaluates to True, it's always True.

做你想做的事，一个很好的语法是:

To do what you want to do, a nice syntax is:

if number in {"1", "0", "2"}:

注意我在这里使用了一个集合——虽然在这种情况下(只有三个值)检查集合比列表更快，因为集合的成员资格测试是 O(1)而不是 O(n).

Note my use of a set here - while it doesn't matter too much in this case (with only three values) checking against a set is faster than a list, as a membership test for a set is O(1) instead of O(n).

这样写起来更好更容易:

This is simply a nicer and easier of writing this:

if number == "1" or number == "0" or number == "2":

这就是你想要的.

请注意，在进行值比较时，您应该始终使用 == 而不是 is - is 是身份检查(这两个值是同一个对象).通常，您应该将 is 用于诸如 is True 或 is None 之类的东西.

Note when making a comparison for value you should always use == not is - is is an identity check (the two values are the same object). Generally you should use is for stuff like is True or is None.

如果你想把它当作一个数字来处理，你可以这样做:

If you wanted to handle this as a number, you could do something like this:

try:
   value = int(number)
except ValueError:
   value = None
if value is not None and 0 <= value <= 2:
    ...

在您想与大量数字进行比较的情况下，这可能更有用.请注意我使用 Python 有用的比较链(0 <= value <= 2 而不是 0 <= value 和 value <= 2).

Which could be more useful in situations where you want to compare to a large range of numbers. Note my use of Python's useful comparison chaining (0 <= value <= 2 rather than 0 <= value and value <= 2).

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