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?](http://stackoverflow.com/questions/1504717/why-does-comparing-strings-in-python-using-either-or-is-sometimes-produce) — Nickil Maveli, Feb 11 '17 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. — , Feb 11 '17 at 10:18

score 1 · Answer 1 · answered Feb 11 '17 at 09:57

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 '17 at 09:57

Rajesh Yogeshwar

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I just did `a = 0.0; b = 0.0` and then `b is a` said `True`. – Stefan Pochmann Feb 11 '17 at 10:30
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*. – Stefan Pochmann Feb 11 '17 at 10:51
@StefanPochmann did you mean this http://imgur.com/a/rYKCL? If not, can you provide a sample? – Rajesh Yogeshwar Feb 11 '17 at 11:02
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. – Stefan Pochmann Feb 11 '17 at 11:17
@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 :) – Rajesh Yogeshwar Feb 11 '17 at 11:27

Dean Fenster · Accepted Answer · 2017-02-11T10:33:14.470

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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 '17 at 10:33

answered Feb 11 '17 at 10:01

Dean Fenster

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identity comparison - comparing same object returns false

2 Answers2