why () is () and [] is [] work in other way?

[Rotwang]

On 20/04/2012 20:10, dmitrey wrote:
> I have spent some time searching for a bug in my code, it was due to
> different work of "is" with () and []:
>>>> () is ()
> True
>>>> [] is []
> False
>
> (Python 2.7.2+ (default, Oct  4 2011, 20:03:08)
> [GCC 4.6.1] )
>
> Is this what it should be or maybe yielding unified result is better?
> D.

The reason that "[] is []" evaluates to false is because lists are 
mutable and each expression "[]" evaluates to a different object. 
Therefore you can change one without changing the other:

 >>> a, b = [], []
 >>> a.append(None)
 >>> a
[None]
 >>> b
[]

On the other hand tuples are immutable, so there's no danger in having 
each literal () evaluate to the same object - since you can't change a 
tuple but only replace a reference to a tuple with a reference to 
another tuple, there's no need to have the two "()"s in something like

 >>> a, b = (), ()

evaluate to different objects, since there is no danger that one can 
affect the value of b by doing stuff to a.

I believe it says somewhere in the Python docs that it's undefined and 
implementation-dependent whether two identical expressions have the same 
identity when the result of each is immutable, though I can't find the 
quotation right now. If my recollection is correct then you probably 
shouldn't rely on something like "() is ()" giving the same answer 
across platforms.

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