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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