Python-将长/整数值与==和进行比较 [英] Python - comparing long/integer values with == and is

问题描述

可能重复：

Python是

昨晚在某处发生了奇怪的变化

Ran into something odd last night where doing

if max_urls is 0:
    max_urls = 10

总是返回false ...即使max_urls为0...。当我做完

would always return false... even when max_urls was 0.... it was getting assigned from the database. When I did a

print type(max_urls)

会返回

<type 'long'> 0

这看似正确，但始终返回false。

which seemed right but it would always return false.

如果我将其更改为

if max_urls == 0:
    max_urls = 10

然后最终它在0时将返回true。为什么==和is之间的差异？

then finally it would return true when it was 0. Why the difference between == and is?

推荐答案

== 是值比较，是是对象标识（内存位置）比较。您会经常看到像 max_urls为0 这样的比较会给出预期的结果，因为通常 会在Python中缓存较小的值，但是您始终希望在检查相等性时使用 == 而不是是，因为不能依赖此行为。

== is a value comparison, is is an object identity (memory location) comparison. You will often see that comparisons like max_urls is 0 will give the intended result because small values are usually cached in Python, but you always want to be using == instead of is when checking equality because this behavior cannot be relied upon.

下面是一个简短的示例，说明了这一点：

Here is a brief example illustrating this:

>>> a = 0
>>> (a == 0, a is 0)
(True, True)
>>> a = 1000
>>> (a == 1000, a is 1000)
(True, False)

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