Hash of class from instance

[Joakim Storck]

Hello,

Is there any way that I can find the hash value of a class from an
instance?
[color=blue][color=green][color=darkred]
>>> class A:[/color][/color][/color]
.... pass
....[color=blue][color=green][color=darkred]
>>> a = A()
>>> hash(A)[/color][/color][/color]
10782976[color=blue][color=green][color=darkred]
>>> hash(a)[/color][/color][/color]
12251904[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

What I want is a function that returns the value of 'hash(A)':[color=blue]
> a.getHashOfClas s()[/color]
10782976

Is this possible?

/Joakim

[Duncan Booth]

Re: Hash of class from instance

Joakim Storck wrote:
[color=blue]
> Is there any way that I can find the hash value of a class from an
> instance?
>[/color]

You only had to post the question once. It seems a strange thing to want,
but just do:

hash(a.__class_ _)

[Joakim Storck]

Re: Hash of class from instance

Thanks!

Not so strange I think, the hash values of classes will be used as keys
in a dictionary that serve as an object pool.

Sorry about the double posting, I got a 'server error' message the
first time, so I figured it hadn't gone trhough.

/Joakim

[Mick Krippendorf]

Re: Hash of class from instance

Joakim Storck wrote:[color=blue]
> [...] the hash values of classes will be used as
> keys in a dictionary that serve as an object pool. [...][/color]

That does seem unwise (as Teal'c would have uttered). The spec says:

----
hash( object)

Return the hash value of the object (if it has one). Hash values are
integers. They are used to quickly compare dictionary keys during a
dictionary lookup. Numeric values that compare equal have the same hash
value (even if they are of different types, as is the case for 1 and
1.0).
----

Normally the following should hold (not mentioned in the above spec but
true for Java, e.g.): if a and b are objects such that a equals b then
hash(a) equals hash(b). This does not imply that if hash(a) equals
hash(b) also a equals b. More formally: (a == b) -> (hash(a) ==
hash(b)).

In Python there seems to be no guarantee that different objects also
have different hash values. So let's assume we have class objects Foo
and Bar, which by some unlikely incident happen to have the same hash
values, then storing them in a dictonary under their respective hash
values (which are identical) would most probably lead into a problem,
secifically the problem that you'd end up accessing Foo when you indeed
think you are accessing Bar, or vice versa. Just try this:
[color=blue][color=green][color=darkred]
>>> my_pseudo_hash_ code = 1
>>> my_dict = {my_pseudo_hash _code:"gnarf",[/color][/color][/color]
my_pseudo_hash_ code:"snarf"}[color=blue][color=green][color=darkred]
>>> print my_dict[/color][/color][/color]

Mick.

[Steven Bethard]

Re: Hash of class from instance

Mick Krippendorf wrote:[color=blue]
> In Python there seems to be no guarantee that different objects also
> have different hash values.[/color]

Well, it's true that you can override the __hash__ method to do whatever
you want, but I believe the default for class __hash__ methods is to
return the class id, which should be different for each class:

py> class C(object):
.... pass
....
py> hash(C)
12699152
py> id(C)
12699152

Steve

[Joakim Storck]

Re: Hash of class from instance

So I guess it might be a little bit less unwise to use id() instead
then...

/Joakim

[Just]

Re: Hash of class from instance

In article <1107373593.789 905.168850@o13g 2000cwo.googleg roups.com>,
"Joakim Storck" <joakim.storck@ home.se> wrote:
[color=blue]
> So I guess it might be a little bit less unwise to use id() instead
> then...[/color]

Why don't you use the class objects themselves as dict keys?

Just

[Joakim Storck]

Re: Hash of class from instance

Simply because it didn't occur to me. So now I have
[color=blue][color=green][color=darkred]
>>> class A: pass[/color][/color][/color]
....[color=blue][color=green][color=darkred]
>>> d = {A:[A(),A(),A()]}
>>> a = d[A].pop()
>>> a[/color][/color][/color]
<__main__.A instance at 0x00F09A58>[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Thanks all!

/Joakim