Does '!=' equivelent to 'is not'

[pirata]

I'm a bit confusing about whether "is not" equivelent to "!="

if a != b:
...

if a is not b:
...
What's the difference between "is not" and "!=" or they are the same thing?

[John Salerno]

pirata wrote:

I'm a bit confusing about whether "is not" equivelent to "!="

if a != b:
...

if a is not b:
...
What's the difference between "is not" and "!=" or they are the same thing?

No, they are not the same thing. == and != test to see if the *value* of
two variables are the same. Like so:

>>a = 'hello world'
b = 'hello world'
a == b

True

a and b both have the value of 'hello world', so they are equal

is and is not, however, do not test for value equivalence, they test for
object identity. In other words, they test to see if the object the two
variables reference are the same object in memory, like so:

>>a is b

False

a and b are assigned to two different objects that happen to have the
same value, but nevertheless there are two separate 'hello world'
objects in memory, and therefore you cannot say that a *is* b

Now look at this:

>>c = d = 'hello world'
c == d

True

>>c is d

True

In this case, they are again the same value, but now the is test also
shows that they are the same *object* as well, because both c and d
refer to the same 'hello world' object in memory. There is only one this
time.

!= and is not work the same:

>>a != b

False

>>a is not b

True

>>c != d

False

>>c is not d

False

>>>

Hope that helps!

[Erik Max Francis]

pirata wrote:

I'm a bit confusing about whether "is not" equivelent to "!="

if a != b:
...

if a is not b:
...
What's the difference between "is not" and "!=" or they are the same thing?

The `==` operator tests equality. The `is` operator tests identity.

If you don't specifically intend to test for identity, use `==`. If you
don't know what identity tests are for (with the exception of testing
for None-ness), then you don't need it.

--
Erik Max Francis && ma*@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Do we ask what profit the little bird hopes for in singing?
-- Johannes Kepler, 1571-1630

[Dan Bishop]

On Jun 16, 10:29*pm, pirata <[e-mail] validwrote:

I'm a bit confusing about whether "is not" equivelent to "!="

if a != b:
* ...

if a is not b:
* ...

What's the difference between "is not" and "!=" or they are the same thing?

"is not" is the logical negation of the "is" operator, while "!=" is
the logical negation of the "==" operator. The difference is equality
versus identity.

>>a = [1, 2, 3]
b = [1, 2, 3]
a == b

True

>>a is b

False

[John Salerno]

John Salerno wrote:

== and != test to see if the *value* of
two variables are the same.

Let me just clarify this. It might seem like a picky point, but I think
it's pretty important when learning Python.

I don't really mean the value of the variables themselves, I mean the
values that the variables refer to. The variables themselves aren't
actually the objects, nor do they have values, exactly. Instead, they
*refer* to objects in memory, and it is these objects that we are
testing the values and identities of. For example, using that previous code:

a ---'hello world'

b ---'hello world'

c ---\
---'hello world'
d ---/

a and b were assigned to two separate objects (it doesn't matter that
they happen to be the same value). As you can see above, a and b refer
to different things.

c and d, however, were assigned simultaneously to the same object, and
therefore refer to a single object in memory. This is why "c is d" is
True, but "a is b" is False.

[Leo Jay]

On Tue, Jun 17, 2008 at 11:29 AM, pirata <pi****@mars.in validwrote:

I'm a bit confusing about whether "is not" equivelent to "!="

if a != b:
...

if a is not b:
...
What's the difference between "is not" and "!=" or they are the same thing?

The 'is' is used to test do they point to the exactly same object.
The '==' is used to test are their values equal.

same objects are equal, but equal don't have to be the same object.

and be very careful to the dirty corner of python:

>>a = 100000
b = 100000
a is b

False

>>a == b

True

>>a = 5
b = 5
a is b

True

>>a == b

True

>>>

which is caused by small object cache mechanism.
--
Best Regards,
Leo Jay

[Lie]

On Jun 17, 11:07*am, "Leo Jay" <python.leo...@ gmail.comwrote:

On Tue, Jun 17, 2008 at 11:29 AM, pirata <[e-mail] validwrote:

I'm a bit confusing about whether "is not" equivelent to "!="

if a != b:
*...

if a is not b:
*...

What's the difference between "is not" and "!=" or they are the same thing?

The 'is' is used to test do they point to the exactly same object.
The '==' is used to test are their values equal.

same objects are equal, but equal don't have to be the same object.

and be very careful to the dirty corner of python:

No you don't have to be careful, you should never rely on it in the
first place.

Basically 'a is b' and 'not(a is b)' is similar to 'id(a) == id(b)'
and 'not(id(a) == id(b))'

You use 'is' when you want to test whether two variable/names are
actually the same thing (whether they actually refers to the same spot
on memory). The '==' equality comparison just test whether two
objects' values can be considered equal.

[Gabriel Genellina]

En Tue, 17 Jun 2008 02:25:42 -0300, Lie <Li******@gmail .comescribió:

On Jun 17, 11:07*am, "Leo Jay" <python.leo...@ gmail.comwrote:

>On Tue, Jun 17, 2008 at 11:29 AM, pirata <[e-mail] validwrote:

What's the difference between "is not" and "!=" or they are the same thing?

The 'is' is used to test do they point to the exactly same object.
The '==' is used to test are their values equal.

and be very careful to the dirty corner of python:

[example with "5 is 5" but "100000 is not 100000"]

No you don't have to be careful, you should never rely on it in the
first place.

Basically 'a is b' and 'not(a is b)' is similar to 'id(a) == id(b)'
and 'not(id(a) == id(b))'

No.

You use 'is' when you want to test whether two variable/names are
actually the same thing (whether they actually refers to the same spot
on memory). The '==' equality comparison just test whether two
objects' values can be considered equal.

Yes, *that* is true. The above statement is not. A counterexample:

py[] is []
False
pyid([])==id([])
True

Even dissimilar objects may have the same id:

pyclass A: pass
....
pyclass B: pass
....
pyA() is B()
False
pyA() == B()
False
pyid(A())==id(B ())
True

Comparing id(a) with id(b) is only meaningful when a and b are both alive at the same time. If their lifetimes don't overlap, id(a) and id(b) are not related in any way. So I think that trying to explain object identity in terms of the id function is a mistake.

--
Gabriel Genellina

[Duncan Booth]

"Leo Jay" <py***********@ gmail.comwrote:

same objects are equal, but equal don't have to be the same object.

same objects are often equal, but not always:

>>inf = 2e200*2e200
ind = inf/inf
ind==ind

False

>>ind is ind

True

--
Duncan Booth http://kupuguy.blogspot.com

[Derek Martin]

On Tue, Jun 17, 2008 at 04:33:03AM -0300, Gabriel Genellina wrote:

Basically 'a is b' and 'not(a is b)' is similar to 'id(a) == id(b)'
and 'not(id(a) == id(b))'

No.

Sure it is... he said "similar".. . not identical. They are not the
same, but they are similar.

Saying a flat "no" alone, without qualifying your statement is
generally interpreted as rude in English... It's kind of like how you
talk to children when they're too young to understand the explanation.
Yucky.

--
Derek D. Martin
http://www.pizzashack.org/
GPG Key ID: 0x81CFE75D
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