which methods to use?

[Duncan Booth]

Steven D'Aprano <steve at REMOVE.THIS.cybersource.com.au> wrote:

>> py> id(object()) == id(object())
>> True
>> py> object() is object()
>> False
> 
> That's weird. How on earth does that happen?

The lifetimes of the two objects createted in the first comparison do 
not overlap: once the call to id() returns it immediately drops the 
reference to its parameter. Python's memory allocation has a tendency to 
reuse memory immediately so the second call to object() creates another 
object at the same location.

In the second comparison neither object can be released until the 'is' 
operator returns a result, so there is no memory reuse.

>> So using the `is` operator is the only safe way to test for identity.
> 
> But if an object can be garbage collected before the is operator does 
the
> comparison, how can that be safe?

The objects compared using the is operator have at least one reference, 
so they cannot be subject to garbage collection.

You might get some risky situations if you bring weakrefs into the mix. 
For example de-referencing two weak references to different objects both 
of which get garbage collected compares True because both references now 
return None.

>>> c1, c2 = C(), C()
>>> r1, r2 = weakref.ref(c1), weakref.ref(c2)
>>> r1() is r2()
False
>>> del c1
>>> del c2
>>> r1() is r2()
True