is implemented with id ?

Aahz aahz at pythoncraft.com
Sun Nov 4 06:12:10 CET 2012


In article <50959154$0$6880$e4fe514c at news2.news.xs4all.nl>,
Hans Mulder  <hansmu at xs4all.nl> wrote:
>On 3/11/12 20:41:28, Aahz wrote:
>> In article <50475822$0$6867$e4fe514c at news2.news.xs4all.nl>,
>> Hans Mulder  <hansmu at xs4all.nl> wrote:
>>> On 5/09/12 15:19:47, Franck Ditter wrote:
>>>>
>>>> - I should have said that I work with Python 3. Does that matter ?
>>>> - May I reformulate the queston : "a is b" and "id(a) == id(b)"
>>>>   both mean : "a et b share the same physical address". Is that True ?
>>>
>>> Yes.
>>>
>>> Keep in mind, though, that in some implementation (e.g.  Jython), the
>>> physical address may change during the life time of an object.
>>>
>>> It's usually phrased as "a and b are the same object".  If the object
>>> is mutable, then changing a will also change b.  If a and b aren't
>>> mutable, then it doesn't really matter whether they share a physical
>>> address.
>> 
>> That last sentence is not quite true.  intern() is used to ensure that
>> strings share a physical address to save memory.
>
>That's a matter of perspective: in my book, the primary advantage of
>working with interned strings is that I can use 'is' rather than '=='
>to test for equality if I know my strings are interned.  The space
>savings are minor; the time savings may be significant.

As others have pointed out, using ``is`` with strings is a Bad Habit
likely leading to nasty, hard-to-find bugs.

intern() costs time, but saves considerable space in any application
with lots of duplicate computed strings (hundreds of megabytes in some
cases).
-- 
Aahz (aahz at pythoncraft.com)           <*>         http://www.pythoncraft.com/

"....Normal is what cuts off your sixth finger and your tail..."  --Siobhan


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