id( ) function question

Laszlo Nagy gandalf at
Thu Oct 15 18:04:34 CEST 2009

Christian Heimes írta:
> Chris Rebert wrote:
>> The built-ins aren't mutable, and the singletons are each immutable
>> and/or unique; so in no case do objects that are both different and
>> mutable have the same ID.
> Correct, the fact allows you to write code like "type(egg) is str" to
> check if an object *is* an instance of str. The isistance() methods also
> returns True if the argument is a subclass of str.
>> Although I have no idea how it is that `id({}) == id({})` as a prior
>> posted showed; FWIW, I can't manage to reproduce that outcome.
> The behavior is caused by a free list in the dict implementation. When
> the reference count of a object drops to zero, it's usually removed from
> memory entirely. However some types keep a list of unused objects around
> to increase efficiency and spare some calls to malloc() and free().
> For the code "{} is {}" Python has to create two distinct dict objects.
> "id({}) == id({})" returns True under most circumstances because the
> reference count of the first dict drops to zero as soon as the id()
> function returns its memory address. The second call to id() retrieves
> the very same template from the dict type's free list thus returning the
> same memory address.
All right, I see your point now. So can we say, that the id function can 
be used to tell if two mutable objects are different as long as they are 
both alive during the comparison?

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