Curiously:

奇怪的是：

>>> a = 123
>>> b = 123
>>> a is b
True
>>> a = 123.
>>> b = 123.
>>> a is b
False

Seems a is b being more or less defined as id(a) == id(b). It is easy to make bugs this way:

似乎a是b或多或少被定义为id(A)==id(B)。用这种方式制造错误很容易：

basename, ext = os.path.splitext(fname)
if ext is '.mp3':
    # do something
else:
    # do something else

Some fnames unexpectedly ended up in the else block. The fix is simple, we should use ext == '.mp3' instead, but nonetheless if ext is '.mp3' on the surface seems like a nice pythonic way to write this and it's more readable than the "correct" way.

有些名字出人意料地出现在Else块中。解决方法很简单，我们应该使用ext==‘.mp3’，但是如果ext表面上是‘.mp3’，似乎是一种很好的编写方法，而且比“正确”的方法更具可读性。

Since strings are immutable, what are the technical details of why it's wrong? When is an identity check better, and when is an equality check better?

既然字符串是不变的，那么它为什么是错误的技术细节是什么呢？什么时候身份检查更好，什么时候平等检查更好？

They are fundamentally different.

它们从根本上是不同的。

1. == compares by calling the __eq__ method


2. is returns true if and only if the two references are to the same object

So in comparision with say Java:

与Java相比：

1. is is the same as == for objects


2. == is the same as equals for objects

As far as I can tell, is checks for object identity equivalence. As there's no compulsory "string interning", two strings that just happen to have the same characters in sequence are, typically, not the same string object.

据我所知，是检查对象身份的等价性。因为没有强制的“字符串内嵌”，所以恰好在序列中有相同字符的两个字符串通常不是相同的字符串对象。

When you extract a substring from a string (or, really, any subsequence from a sequence), you will end up with two different objects, containing the same value(s).

当您从一个字符串(或者，实际上是一个序列中的任何子序列)中提取一个子字符串时，您将得到两个不同的对象，它们包含相同的值(S)。

So, use is when and only when you are comparing object identities. Use == when comparing values.

因此，当且仅当您比较对象身份时才使用。在比较值时使用==。

Simple rule for determining if to use is or == in Python

在Python中确定是使用is还是==的简单规则

Here is an easy rule (unless you want to go to theory in Python interpreter or building frameworks doing funny things with Python objects):

这里有一条简单的规则(除非您想要学习有关Python解释器的理论知识，或者使用Python对象构建框架来做一些有趣的事情)：

Use is only for None comparison.

使用仅用于不进行比较。

if foo is None

Otherwise use ==.

否则，请使用==。

if x == 3

Then you are on the safe side. The rationale for this is already explained int the above comments. Don't use is if you are not 100% sure why to do it.

那么你就是安全的一方。这样做的理由已经在上述评论中解释过了。如果你不能100%确定为什么要这么做，就不要使用。

It would be also useful to define a class like this to be used as the default value for constants used in your API. In this case, it would be more correct to use is than the == operator.

定义一个这样的类作为API中使用的常量的缺省值也很有用。在这种情况下，使用is比使用==运算符更正确。

class Sentinel(object):
    """A constant object that does not change even when copied."""
    def __deepcopy__(self, memo):
        # Always return the same object because this is essentially a constant.
        return self

    def __copy__(self):
        # called via copy.copy(x)
        return self

You should be warned by PyCharm when you use is with a literal with a warning such as SyntaxWarning: "is" with a literal. Did you mean "=="?. So, when comparing with a literal, always use ==. Otherwise, you may prefer using is in order to compare objects through their references.

当您将is与带有警告的文本一起使用时，您应该得到PyCharm的警告，例如，将语法警告：“is”与文本一起使用。你是说“==”吗？因此，在与文字进行比较时，请始终使用==。否则，您可能更喜欢使用is，以便通过引用比较对象。