identity comparison - comparing same object returns false

Question

Could someone answer below (Output is from IDLE or check on python shell - python 2.7). For 1),2) and 3),4) I am doing exactly same operation but getting different results.

1) >>> a=0

2) >>> a is 0

True

3) >>> a=0.0

4) >>> a is 0.0

False

5) >>> 0.0 is 0.0

True

why 4) is False?

Possible duplicate of Why does comparing strings in Python using either '==' or 'is' sometimes produce a different result? — Nickil Maveli
– Nickil Maveli, Commented Feb 11, 2017 at 10:09
Since the answers don't properly account for it, let me note that (IIRC), you should expect 5) to be false too -- it only turns out to be true because of an optimization in the parser. I don't have a reference, though. — user1084944
– user1084944, Commented Feb 11, 2017 at 10:18

Dean Fenster · Accepted Answer · 2017-02-11 10:33:14Z

1

This is because an optimization in CPython that for any integer between -5 and 256 returns a reference to an already existing object, meaning a is a reference to 0, so they have the same id(). There is no such optimization for floating point numbers like 0.0, so a new object is created on assignment (meaning that id(a) != id(0.0)).

Reference: https://docs.python.org/2/c-api/int.html

edited Feb 11, 2017 at 10:33

answered Feb 11, 2017 at 10:01

Dean Fenster

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Rajesh Yogeshwar · Accepted Answer · 2017-02-11 09:57:42Z

1

The python is operator is used to test if two variables point to same object.

From the documentation:

The operators is and is not test for object identity: x is y is true if and only if x and y are the same object.

For eg.

a = 0.0

if you do b = a and then follow it up with b is a. It will return True.

Now if you do a = 0.0 and b = 0.0 and then try b is a it will return False, because now a and b are two variables pointing to two different objects.

answered Feb 11, 2017 at 9:57

Rajesh Yogeshwar

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5 Comments

Stefan Pochmann Over a year ago

I just did a = 0.0; b = 0.0 and then b is a said True.

Stefan Pochmann Over a year ago

Well you're not doing what I said. I didn't say I did a = 0.0 and then did b = 0.0. I said I did a = 0.0; b = 0.0. Try that.

Rajesh Yogeshwar Over a year ago

@StefanPochmann did you mean this imgur.com/a/rYKCL? If not, can you provide a sample?

Stefan Pochmann Over a year ago

Yes, that's what I mean. I guess you're using IPython and it does things differently. Try it in a Python shell, like the OP apparently did (as indicated by the >>>). I do get True there.

Rajesh Yogeshwar Over a year ago

@StefanPochmann Noted! and thanks. I am pretty used to IPython and do not tend to used python shell. Also, it does return True there. Something for me to ponder why :)

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