What is the difference between "is" and "=="?

Question

What is the difference between these two pieces of code in the boolean world:

1 is 1

and

1 == 1

I found two web pages that describe it, but I can't see the difference since I don't know how to get different results:

http://www.informit.com/articles/article.aspx?p=459269&seqNum=10

https://docs.python.org/2/library/stdtypes.html

On the 2nd page, I found the operators. On the 1st page, which I looked at second, it described a difference that doesn't tell me when I'd do this and get different results. That's my big question, is when does doing one differ results from the other?

Obviously, there are cases where one will be true and the other false, right?

I notice I get different results when comparing integers to floats, but when else would it matter about the object type? — Rebok, Mar 16 '15 at 04:51

score 1 · Answer 1 · answered Mar 16 '15 at 04:52

1

When using variables, it can cause incorrect results:

>>> foo = [1, 2, 3]
>>> foo is [1, 2, 3]
False
>>> foo == [1, 2, 3]
True

Look at the documentation, here

== means "equal", while is is an object identity.

answered Mar 16 '15 at 04:52

Zach Gates

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score 1 · Answer 2 · answered Mar 16 '15 at 04:56

1

Equality is well-defined, type by type, depending only on the values being compared. Identity depends on whether a certain value is duplicated, or stored only once and referred to repeatedly.

For immutable values, identity's therefore an implementation detail!

Consider, for example:

2>>> x = 23
2>>> y = 23
2>>> x is y
True
2>>> x = 232323
2>>> y = 232323
2>>> x is y
False

Small values like 23 happen to be "cached" (an implementation detail!) in CPython in the hope of saving memory if they're used often; large values like 232323 are not so cached -- again, 100% an implementation detail. You'd never want to depend on such behavior!

answered Mar 16 '15 at 04:56

Alex Martelli

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Wow. I did not know small integers being interned was a CPython feature as well. – Anirudh Ramanathan Mar 16 '15 at 04:59
@AnirudhRamanathan, it currently is, it might not be in CPython 3.X for some X -- that's the very definition of an **implementation detail** one should never depend on!-) – Alex Martelli Mar 16 '15 at 05:02

score 0 · Answer 3 · answered Mar 16 '15 at 04:56

In addition to these, you can change the behavior of == for classes that you define.

class A:
    def __init__(self, a, b):
        self.a = a
        self.b = b

    def __eq__(self, other):
        #let us ignore b in our comparison!
        return self.a == other.a

a1 = A(1,2)
a2 = A(1,3)
print(a1 == a2) #prints true
print(a1 is a2) #prints false

The behavior of is cannot be changed and will always compare the memory addresses where the objects themselves are stored.

score 0 · Answer 4 · answered Mar 16 '15 at 04:57

0

a is b checks if a points to the same memory location as b a == b checks if the values of a and b are the same. e.g.

a=10 (computer stores '10' at 0xf34852)
b=10 (computer stores '10' at 0xfeac54)
a==b (true)
a is b (false)

answered Mar 16 '15 at 04:57

acenturyandabit

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What is the difference between "is" and "=="?

4 Answers4