Possible Duplicate:
Python ‘==’ vs ‘is’ comparing strings, ‘is’ fails sometimes, why?
In Python, what is the difference between these two statements:
if x is "odp":
if x == "odp":
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codeforester
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Nick Heiner
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3dupe http://stackoverflow.com/questions/1504717/python-vs-is-comparing-strings-is-fails-sometimes-why – SilentGhost Mar 10 '10 at 18:11
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1@SilentGhost I disagree. The linked question contains the answer to this question, but to anyone stumbling around looking for a relatively simple answer to the question asked here, the linked question will not seem at all the same. – David Berger Mar 10 '10 at 18:30
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@David: and simple was already asked (quite recently too), you free to spend *your* time to find where it is, questions that have the same answers are duplicates in my book. – SilentGhost Mar 10 '10 at 18:40
4 Answers
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The == operator tests for equality
The is keyword tests for object identity; whether we are talking about the same object. Note that multiple variables may refer to the same object.
Justin Ethier
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The is operator compares the identity while the == operator compares the value. Essentially x is y is the same as id(x) == id(y)
Wolph
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For implementation reasons, "odp" is a bad example, but you should not use is unless you want the possibility of two identical strings to evaluate to false:
>>> lorem1 = "lorem ipsum dolor sit amet"
>>> lorem2 = " ".join(["lorem", "ipsum", "dolor", "sit", "amet"])
>>> lorem1 == lorem2
True
>>> lorem1 is lorem2
False
As others have said, is tests identity, not equality. In this case, I have two separate strings with the same contents. However, you should not depend on this either:
>>> odp1 = "odp"
>>> odp2 = "".join(["o", "d", "p"])
>>> odp1 == odp2
True
>>> odp1 is odp2
True
In other words, you should never use is to compare strings.
P.S.
In Python 2.7.10 >>> odp1 is odp2 returns False.
