On 10/24/2011 03:21 PM, Johan Martinez wrote:
On Mon, Oct 24, 2011 at 2:07 PM, Andreas Perstinger<
andreas.perstin...@gmx.net> wrote:
On 2011-10-24 20:04, Johan Martinez wrote:
Hi,
I am struggling to understand Python string immutability. I am able to
modify Python string object after initializing/assigning it a value.
s = "First"
print s.__class__
<type 'str'>
print s
First
s = "Second"
print s
Second
Dave, Sander and Wayne have already explained why you aren't modifying
string objects in your example.
With the id()-function you can also see what is happening:
s = "First"
id(s)
3077110080L # In CPython this is the memory address of the object
# with the name 's' (in your case "First")
s = "Second"
id(s)
3077110304L # You see that 's' refers now to another address
id("First")
3077110080L # But "First" is still on the same address as before
id("Second")
3077110304L # And this proves that "Second" is at the address
# which 's' refers to
Bye, Andreas
Great, that's really helpful Andreas.
thanks,
jM.
Unfortunately, that trick is not guaranteed to work. The only reason
that id("First") gives the same value as s="First"; id(s) is that
"First" is one of the few magic values that get cached. Small
non-negative integers and short strings without spaces tend to be in
that category, but you can't count on it.
Try
a = 400
b = 400
print id(a), id(b)
it'll probably print different numbers, if the value 400 isn't one of
the "cached" values.
Worse is that an id() can be reused once an object is garbage
collected. So I could do something like:
a = 600
print id(a)
del a
b = 400
print id(b)
and it might print the same value. There are more subtle cases,
but I wanted to keep it simple.
An id() is guaranteed to be unique across all objects that exist at the
same moment. So as long as you're comparing id's of two objects that
are both present, you're fine.
I'm afraid to really understand id(), you have to understand how the
object model works, so you can't use id() to prove it.
--
DaveA
_______________________________________________
Tutor maillist - Tutor@python.org
To unsubscribe or change subscription options:
http://mail.python.org/mailman/listinfo/tutor
