difference between 'is' and '==' [duplicate]

Question

Possible Duplicate:
Is there any difference between “foo is None” and “foo == None”?

Quite a simple question really.

Whats the difference between:

if a.b is 'something':

and

if a.b == 'something':

excuse my ignorance

Answer 1

The first checks for identity the second for equality.

Examples:

The first operation using is may or may not result in True based on where these items, i.e., strings, are stored in memory.

a='this is a very long string'
b='this is a very long string'

a is b
False

Checking, id() shows them stored at different locations.

id(a)
62751232

id(b)
62664432

The second operation (==) will give True since the strings are equal.

a == b
True

Another example showing that is can be True or False (compare with the first example), but == works the way we'd expect:

'3' is '3'
True

this implies that both of these short literals were stored in the same memory location unlike the two longer strings in the example above.

'3' == '3'
True

No surprise here, what we would have expected.

I believe is uses id() to determine if the same objects in memory are referred to (see @SvenMarnach comment below for more details)

Answer 2

a is b is true if a and b are the same object. They can compare equal, but be different objects, eg:

>>> a = [1, 2]
>>> b = [1, 2]
>>> c = b
>>> a is b
False
>>> a is c
False
>>> b is c
True
>>> a == b == c
True