Q about object identity

[vronskij@gmail.com]

Hello,

I am testing object identity.

If I do it from the interpreter, I get strange results.

>>print [] is []

False

>>print id([]), id([])

3083942700 3083942700



Why is that? Isn't this an error?


If I test it in a script, all is OK.

#!/usr/bin/python

a = []
b = []

print a == b
print a is b

print id(a), id(b)

print id(None), id(None)
print id(True), id(True)


On my computer, I got these results:

True
False
3083769036 3083793516
135553336 135553336
135526444 135526444

All as expected.


jan bodnar

[Christian Heimes]

Re: Q about object identity

vronskij@gmail. com schrieb:

Hello,
>
I am testing object identity.
>
If I do it from the interpreter, I get strange results.
>

>>>print [] is []

False
>

>>>print id([]), id([])

3083942700 3083942700
>
>
>
Why is that? Isn't this an error?

No, it's not an error. You are getting this result because the list
implementation keeps a bunch of unused list objects in a free list. It's
an optimization trick. Python has to create two different list objects
for "[] is []" while it can reuse the same list object for id([]) == id([]).

Christian

[vronskij@gmail.com]

Re: Q about object identity

On 3. Jún, 23:08 h., Christian Heimes <li...@cheimes. dewrote:

vrons...@gmail. com schrieb:
>

Hello,

>

I am testing object identity.

>

If I do it from the interpreter, I get strange results.

>

>>print [] is []

False

>

>>print id([]), id([])

3083942700 3083942700

>

Why is that? Isn't this an error?

>
No, it's not an error. You are getting this result because the list
implementation keeps a bunch of unused list objects in a free list. It's
an optimization trick. Python has to create two different list objects
for "[] is []" while it can reuse the same list object for id([]) == id([]).
>
Christian

Aha.

Thanks.

jan bodnar

[Marc 'BlackJack' Rintsch]

Re: Q about object identity

On Tue, 03 Jun 2008 23:08:46 +0200, Christian Heimes wrote:

vronskij@gmail. com schrieb:

>Hello,
>>
>I am testing object identity.
>>
>If I do it from the interpreter, I get strange results.
>>

>>>>print [] is []

>False
>>

>>>>print id([]), id([])

>3083942700 3083942700
>>
>>
>>
>Why is that? Isn't this an error?

>
No, it's not an error. You are getting this result because the list
implementation keeps a bunch of unused list objects in a free list. It's
an optimization trick. Python has to create two different list objects
for "[] is []" while it can reuse the same list object for id([]) == id([]).

I don't think you need optimization tricks for the explanation. In ``[]
is []`` there need to be two lists that exist at the same time to be
compared by the ``is`` operator. With ``id([]) == id([])`` just the
id numbers have to exist at the same time, so the execution may look like
this:

• create empty list
• call `id()` with it
• remember first id number
• when `id()` returns, the list is not referenced anymore and can be
garbage collected
• create empty list, and here the list object might get allocated at the
same memory location as the first empty list → same id.
• call `id()` with it
• remember second id number
• garbage collect second empty list
• compare both numbers

Ciao,
Marc 'BlackJack' Rintsch

[Ethan Furman]

Re: Q about object identity

vronskij@gmail. com wrote:

Hello,
>
I am testing object identity.
>
If I do it from the interpreter, I get strange results.
>
>

>>>>*print [] is []*

>
*False*
>
>

>>>>print id([]), id([])

>
3083942700 3083942700
>
>
>
Why is that? Isn't this an error?
>
>
If I test it in a script, all is OK.
>
#!/usr/bin/python
>
a = []
b = []
>
print a == b
*print a is b*
>
print id(a), id(b)
>
print id(None), id(None)
print id(True), id(True)
>
>
On my computer, I got these results:
>
True
*False*
3083769036 3083793516
135553336 135553336
135526444 135526444
>
All as expected.
>
>
jan bodnar

Assuming I'm not missing something obvious here, the results from the
script are the same as those from the interpreter -- the _is_ returns
False in both cases.

[Tommy Grav]

Re: Q about object identity


On Jun 6, 2008, at 4:27 PM, Ethan Furman wrote:

vronskij@gmail. com wrote:

>Hello,
>I am testing object identity.
>If I do it from the interpreter, I get strange results.

>>>>*print [] is []*

>*False*

>>>>print id([]), id([])

>3083942700 3083942700
>Why is that? Isn't this an error?

in the first statement the two []'s are in memory at the same
time and have different id() signatures. For the second statement
the first id([]) is evaluated and release from memory and then
the second id([]) is evaluated and release from memory. Since
only one of them exist at a time they can have the same id()
signature.

Cheers
Tommy