instance comparison

[King]

I am facing a problem where I am really confused about it.

I am trying to compare to instances using:

if inst1 == inst2

These instances have a overridden method __str__ which returns same
string. The condition result is true although they are different
instances.

If I use:

if id(inst1) == id(inst2)

The results is false. What's happening here?

If I am trying to store this pair in a set where other instances are
already stored, but it's not storing it. Although I am sure that there
is no pair already stored of same but even if Set is comparing by
using string return by __str__ method then it won't.

How do I solve this?

Is this mean when you have overridden __str__ method then it comapre
with results of __str__ or else it will comapre whether they are the
same instances?

[John Machin]

Re: instance comparison

On Jul 24, 6:50 pm, King <animator...@gm ail.comwrote:

I am facing a problem where I am really confused about it.
>
I am trying to compare to instances using:
>
if inst1 == inst2
>
These instances have a overridden method __str__ which returns same
string. The condition result is true although they are different
instances.
>
If I use:
>
if id(inst1) == id(inst2)
>
The results is false. What's happening here?
>
If I am trying to store this pair in a set where other instances are
already stored, but it's not storing it. Although I am sure that there
is no pair already stored of same but even if Set is comparing by
using string return by __str__ method then it won't.
>
How do I solve this?
>
Is this mean when you have overridden __str__ method then it comapre
with results of __str__

No. Do you have a __cmp__ method, or an __eq__ method? Any other
__twounderscore s__ methods?

It is impossible to tell what is going on from your description.
Please supply the source of a *short* class that demonstrates the
problem, and demonstrate it.

[Fredrik Lundh]

Re: instance comparison

King wrote:

Is this mean when you have overridden __str__ method then it comapre
with results of __str__ or else it will comapre whether they are the
same instances?

Comparisons uses __cmp__ or the rich comparison set; see

http://docs.python.org/ref/customization.html

for details.

Sets and dictionaries, also relies on hashing (the __hash__ method, also
described on the linked page).

The __str__ method only affects printing and conversion to string.

</F>

[King]

Re: instance comparison


The only methods I do have in class is __init__ and __str__.
How ever inst1 and inst2 is coming from a dictionary where I stored
them with a unique id.

inst1 = stored[id]
inst2 = stored[id]

Is this makes a difference? I will rip down the piece of code giving
me problem and post.

[Fredrik Lundh]

Re: instance comparison

King wrote:

The only methods I do have in class is __init__ and __str__.
How ever inst1 and inst2 is coming from a dictionary where I stored
them with a unique id.
>
inst1 = stored[id]
inst2 = stored[id]
>
Is this makes a difference?

unlikely (well, if that's the literal code, both variables will point to
the same variable, but I guess that's not what you meant).

I will rip down the piece of code giving me problem and post.

please do. and don't be surprised if you find the problem while doing
so ;-)

</F>

[King]

Re: instance comparison

No,

The the class is not subclass of another one. Problem still persist.
The code is pretty huge and I am trying to post the information as
clear as possible.

[Fredrik Lundh]

Re: instance comparison

King skrev:

The the class is not subclass of another one. Problem still persist.
The code is pretty huge and I am trying to post the information as
clear as possible.

feel free to *add* stuff to the following example until it breaks, if
that's easier:

>>class Spam:

.... def __init__(self, label):
.... self.label = label
.... def __str__(self):
.... return self.label
....

>>a = Spam("an object")
>>a

<__main__.Spa m instance at 0xb7e8faac>

>>print a

an object

>>b = Spam("an object")
>>b

<__main__.Spa m instance at 0xb7e8fb6c>

>>print b

an object

>>a == b

False

>>a is b

False

>>str(a) == str(b)

True

</F>

[Petite Abeille]

Re: instance comparison


On Jul 24, 2008, at 7:53 PM, King wrote:

The the class is not subclass of another one. Problem still persist.
The code is pretty huge and I am trying to post the information as
clear as possible.

Mark V. Shaney, from Dissociated Press, I presume?

--
PA.

http://alt.textdrive.com/nanoki/