Python：为什么运算符“is”和“==”有时可以互换为字符串？

Python: Why operator “is” and “==” are sometimes interchangeable for strings?

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Possible Duplicate:
String comparison in Python: is vs. ==
Python string interning
Why does comparing strings in Python using either '==' or 'is' sometimes produce a different result?

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我不小心用了is和==来交换字符串，但我发现它们并不总是相同的。

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>>> Folder ="locales/"
>>> Folder2 ="locales/"
>>> Folder is Folder2
False
>>> Folder == Folder2
True
>>> File ="file"
>>> File2 ="file"
>>> File is File2
True
>>> File == File2
True
>>>

为什么在一种情况下，运算符是可互换的，而在另一种情况下则不可互换？

相关讨论

这个问题更清楚地说明了这一点：python中的字符串比较：is与。==

简短的回答是：==测试等价，其中is测试等价(通过对象引用)。

两个具有相同值的字符串具有相同的标识，这表明python解释器正在优化，正如Daniel Roseman确认的那样：)

==运算符调用第一个对象与第二个对象比较的内部__cmp__()方法。这适用于所有的python对象，包括字符串。is运算符比较对象标识：

Every object has an identity, a type and a value. An object’s identity never changes once it has been created; you may think of it as the object’s address in memory. The ‘is‘ operator compares the identity of two objects; the id() function returns an integer representing its identity (currently implemented as its address).

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例如。：

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s1 = 'abc'
id(s1)
>>> 140058541968080
s2 = 'abc'
id(s2)
>>> 140058541968080

# The values the same due to SPECIFIC CPython behvaiour which detects
# that the value exists in interpreter's memory thus there is no need
# to store it twice.

s1 is s2
>>> True

s2 = 'cde'
id(s2)
>>> 140058541968040
s1 is s2
>>> False