what is the keyword "is" for?

[Kirk McDonald]

daniel wrote:
> I'm so confused by the keyword "is" and "==" equal sign, it seems they
> could be exchanged in some contexts, but not in others, what's the
> difference between them in terms of comparation?
> 
> thanks...
> 
> daniel
> 

  'is' compares object identity. == compares values.

 >>> a = [1, 2, 3]
 >>> b = [1, 2, 3]
 >>> a is b
False
 >>> a == b
True

In this example, a and b refer to two separate lists that happen to hold 
the same values. Thus, 'is' returns False, and == returns True. On the 
other hand:

 >>> c = d = [4, 5, 6]
 >>> c is d
True
 >>> c == d
True

Here, c and d refer to the same list. Therefore, 'is' returns true (they 
both refer to the same object), as does == (an object is always equal to 
itself, unless you overload the equality check in a weird way).

The distinction can easily be seen if we try to mutate these lists:

 >>> a.append(4)
 >>> a is b
False
 >>> a == b
False
 >>> c.append(7)
 >>> c is d
True
 >>> c == d
True

When we mutate a, b is not affected. They are two different lists, and 
changing 'a' makes it so they are no longer equal.

When we mutate c, d IS affected; they refer to the same list.

You can easily confuse yourself if you ever talk about applying 'is' to 
(for example) integers. Python may re-use certain small integers when 
you might not expect it to; this is done in the interests of efficiency. 
If you only compare the /values/ of numbers (with ==), then you will 
never notice this.

 >>> a = 1
 >>> b = 1
 >>> c = 1000000
 >>> d = 1000000
 >>> a is b
True
 >>> c is d
False

-Kirk McDonald