Python gotcha - "is" operator on integers

emrebeyler (74) in python • 7 years ago (edited)

Screen Shot 2018-04-17 at 11.50.36 PM.png

I have been into this "quirk" again, today. You have probably no idea if you didn't encounter it yourself before.

The current implementation keeps an array of integer objects for all integers between -5 and 256, when you create an int in that range you actually just get back a reference to the existing object. So it should be possible to change the value of 1. I suspect the behaviour of Python in this case is undefined. :-) ^ref.

Have a look at Python3 docs and see the stackoverflow discussions to learn more.

And, please be safe with the small integers.

python

7 years ago by emrebeyler (74)

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themarkymark (78) · 7 years ago (edited)

== tests if they are equal, and is is testing if they are the same object.
They are the same value so == succeeds, but they are not the same reference so is fails.

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danielsaori (63) · 7 years ago

That can really cause some confusion. :)
Using is and testing with small integers, only to later find out it fails with larger values.

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leprechaun (56) · 7 years ago (edited)

You should only use is with None. For there is and can only ever be one object of type NoneType. If a routine may return None, we know it isthe same constant None object used everywhere in Python.

That is if a==None and b==None then a is b. However, no such identity should be assumed for values other than None.