Asp Forum - Classes and modules are singletons?

Steven D'Aprano

3/6/2008 1:45:00 AM

I recall that Python guarantees that module objects are singletons, and
that this must hold for any implementation, not just CPython: you can
only ever create one instance of a module via the import mechanism. But
my google-foo is obviously weak today, I cannot find where the Python
language reference guarantees that. Can somebody please point me at the
link making that guarantee?

(Note: you can create multiple modules with the same name and state using
new.module. I don't think that counts, although it may be a good way to
win bar bets with your Python buddies.)

But what about classes? Are they singletons? Obviously classes aren't
Singleton classes, that is, given an arbitrary class C you can create
multiple instances of C. But what about class objects themselves? I've
found a few odd references to "classes are singletons", but nothing in
the language reference.

I've done some experimentation, e.g.:

>>> import module
>>> from module import Class
>>> module.Class is Class
True

but I'm not sure if that's (1) meaningful or (2) implementation-specific.

--
Steven

15 Answers

Terry Reedy

3/6/2008 2:06:00 AM

"Steven D'Aprano" <steve@REMOVE-THIS-cybersource.com.au> wrote in message
news:13suj3u2d17f205@corp.supernews.com...
|I recall that Python guarantees that module objects are singletons, and
| that this must hold for any implementation, not just CPython: you can
| only ever create one instance of a module via the import mechanism. But
| my google-foo is obviously weak today, I cannot find where the Python
| language reference guarantees that. Can somebody please point me at the
| link making that guarantee?
|
| (Note: you can create multiple modules with the same name and state using
| new.module. I don't think that counts, although it may be a good way to
| win bar bets with your Python buddies.)
|
|
| But what about classes? Are they singletons? Obviously classes aren't
| Singleton classes, that is, given an arbitrary class C you can create
| multiple instances of C. But what about class objects themselves? I've
| found a few odd references to "classes are singletons", but nothing in
| the language reference.
|
| I've done some experimentation, e.g.:
|
| >>> import module
| >>> from module import Class
| >>> module.Class is Class
| True
|
|
| but I'm not sure if that's (1) meaningful or (2) implementation-specific.

If I understand your question, classes are not singletons:
>>> ll=[]
>>> for i in range(2):
import string
ll[i]=string

>>> ll[0] is ll[1]
True
>>> for i in range(2):
class C: pass
ll[i] = C

>>> ll[0] is ll[1]
False

tjr

Steven D'Aprano

3/6/2008 2:32:00 AM

On Wed, 05 Mar 2008 21:05:31 -0500, Terry Reedy wrote:

> If I understand your question, classes are not singletons:
>>>> ll=[]
>>>> for i in range(2):
> import string
> ll[i]=string

Where's the IndexError? :-)

>>>> ll[0] is ll[1]
> True

But yes, modules are singletons in that way, at least if you go through
the import mechanism.

>>>> for i in range(2):
> class C: pass
> ll[i] = C
>
>
>>>> ll[0] is ll[1]
> False

Ah, but each time around the loop you create a *new class* that just
happens to be called C. An alternative way to see similar behaviour is:

def foo(x=None):
class C(object):
X = x
return C

Naturally foo() is foo() gives False -- although both classes are called
C, they are different classes that just happen to have the same state.

I accept my question about classes being singletons is not well-formed,
not even in my own mind. I guess one way of asking is, for any two class
objects (not instances) C1 and C2, does "C1 == C2" imply "C1 is C2"?

--
Steven

Micah Cowan

3/6/2008 3:39:00 AM

Steven D'Aprano <steve@REMOVE-THIS-cybersource.com.au> writes:

> I recall that Python guarantees that module objects are singletons, and
> that this must hold for any implementation, not just CPython: you can
> only ever create one instance of a module via the import mechanism. But
> my google-foo is obviously weak today, I cannot find where the Python
> language reference guarantees that. Can somebody please point me at the
> link making that guarantee?

It's not an absolute strict guarantee; it's just implied by the fact
that "import" uses any appropriate objects already found in
sys.modules.

>>> import sys
>>> ll = []
>>> for i in range(2):
.... import string
.... ll.append(string)
.... del sys.modules['string']
....
>>> ll[0] is ll[1]
False

(I'm sure there are very good reasons never to do what I just did there.)

--
Micah J. Cowan
Programmer, musician, typesetting enthusiast, gamer...
http://micah.c...

Aaron Brady

3/6/2008 3:44:00 AM

On Mar 5, 8:31 pm, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.au> wrote:
> On Wed, 05 Mar 2008 21:05:31 -0500, Terry Reedy wrote:
> > If I understand your question, classes are not singletons:
> >>>> ll=[]
> >>>> for i in range(2):
> > import string
> > ll[i]=string
>
> Where's the IndexError? :-)
>
> >>>> ll[0] is ll[1]
> > True
>
> But yes, modules are singletons in that way, at least if you go through
> the import mechanism.
>
> >>>> for i in range(2):
> > class C: pass
> > ll[i] = C
>
> >>>> ll[0] is ll[1]
> > False
>
> Ah, but each time around the loop you create a *new class* that just
> happens to be called C. An alternative way to see similar behaviour is:
>
> def foo(x=None):
> class C(object):
> X = x
> return C
>
> Naturally foo() is foo() gives False -- although both classes are called
> C, they are different classes that just happen to have the same state.
>
> I accept my question about classes being singletons is not well-formed,
> not even in my own mind. I guess one way of asking is, for any two class
> objects (not instances) C1 and C2, does "C1 == C2" imply "C1 is C2"?

>>> class metaC( type ):
... def __eq__( self, other ):
... return random.choice([ True, False ])
...
>>> class C( metaclass= metaC ):
... pass
...
>>> import random
>>> C==C
True
>>> C==C
False
>>> C==C
False
>>> C==C
True
>>> C==C
False
>>>

You made me think of this. What relevance it has is open to
debate. ...In a separate thread.

http://en.wikipedia.org/wiki...

It's my Signal-channel signal signal.

Aaron Brady

3/6/2008 3:52:00 AM

> > > If I understand your question, classes are not singletons:
> > >>>> ll=[]
> > >>>> for i in range(2):
> > > import string
> > > ll[i]=string
>
> > Where's the IndexError? :-)
>
> > I accept my question about classes being singletons is not well-formed,
> > not even in my own mind. I guess one way of asking is, for any two class
> > objects (not instances) C1 and C2, does "C1 == C2" imply "C1 is C2"?

Similarly, no.

>>> class metaC( type ):
... def __eq__( self, other ):
... return True
...
>>> class C( metaclass= metaC ):
... pass
...
>>> class D( metaclass= metaC ):
... pass
...
>>> C==D
True
>>> C is D
False

+1 raised eyebrow

Aaron Brady

3/6/2008 4:35:00 AM

On Mar 5, 9:51 pm, castiro...@gmail.com wrote:
> > > > If I understand your question, classes are not singletons:
> > > >>>> ll=[]
> > > >>>> for i in range(2):
> > > > import string
> > > > ll[i]=string
>
> > > Where's the IndexError? :-)
>
> > > I accept my question about classes being singletons is not well-formed,
> > > not even in my own mind. I guess one way of asking is, for any two class
> > > objects (not instances) C1 and C2, does "C1 == C2" imply "C1 is C2"?
>
> Similarly, no.
>
> >>> class metaC( type ):
>
> ... def __eq__( self, other ):
> ... return True
> ...>>> class C( metaclass= metaC ):
>
> ... pass
> ...>>> class D( metaclass= metaC ):
>
> ... pass
> ...>>> C==D
> True
> >>> C is D
>
> False
>
> +1 raised eyebrow

Well, you're not going to believe me, but a <scaryword>"bar"</
scaryword> just had an idea.

Don't forget:
class C( metaclass= metaC )
class metaC( metaclass= metametaC )
class metametaC( type ): bor hor hor.

Now for the academics, is it telling a joke? If so, that's pretty
fine--- but.

Aaron Brady

3/6/2008 6:25:00 AM

> I accept my question about classes being singletons is not well-formed,
> not even in my own mind. I guess one way of asking is, for any two class
> objects (not instances) C1 and C2, does "C1 == C2" imply "C1 is C2"?

C and D are instances of metaC in that.

class metaC( type ):
def what( self ):
return self

class C( metaclass= metaC ): pass
class D( metaclass= metaC ): pass

assert C is C.what() and D is D.what()
#print( C.what(), D.what() )

C= metaC('C',(),{})
D= metaC('D',(),{})
assert C is C.what() and D is D.what()
#print( C.what(), D.what() )

furthermore.

Carl Banks

3/6/2008 2:31:00 PM

On Mar 5, 8:44 pm, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.au> wrote:
> But what about classes? Are they singletons? Obviously classes aren't
> Singleton classes, that is, given an arbitrary class C you can create
> multiple instances of C. But what about class objects themselves? I've
> found a few odd references to "classes are singletons", but nothing in
> the language reference.

Probably because "singleton" is the wrong word. A singleton means
there is one instance of a type; classes are instances of "type" which
can have many instances so classes are not singletons.

Anyway, the answer to what you are probably asking is No. Try this:

>>>import module
>>>c1 = module.Someclass
>>>reload(module)
>>>c2 = module.Someclass
>>>c1 is c2

For that matter, try this:

>>>import module
>>>c1 = module.Someclass
>>>module.Someclass = some_other_class()
>>>c2 = module.Someclass
>>>c1 is c2

Carl Banks

Aaron Brady

3/6/2008 7:07:00 PM

On Mar 6, 8:30 am, Carl Banks <pavlovevide...@gmail.com> wrote:
> On Mar 5, 8:44 pm, Steven D'Aprano <st...@REMOVE-THIS-
>
> cybersource.com.au> wrote:
> > But what about classes? Are they singletons? Obviously classes aren't
> > Singleton classes, that is, given an arbitrary class C you can create
> > multiple instances of C. But what about class objects themselves? I've
> > found a few odd references to "classes are singletons", but nothing in
> > the language reference.
>
> Probably because "singleton" is the wrong word. A singleton means
> there is one instance of a type; classes are instances of "type" which
> can have many instances so classes are not singletons.
>
> Anyway, the answer to what you are probably asking is No. Try this:
>
> >>>import module
> >>>c1 = module.Someclass
> >>>reload(module)
> >>>c2 = module.Someclass
> >>>c1 is c2

What about

>>> o= object()
>>> b1= o.someattr
>>> reload( o )
>>> b2= o.someattr
>>> b1 is b2

?

Marc 'BlackJack' Rintsch

3/6/2008 8:58:00 PM

On Thu, 06 Mar 2008 11:06:50 -0800, castironpi wrote:

> On Mar 6, 8:30Â am, Carl Banks <pavlovevide...@gmail.com> wrote:
>> Anyway, the answer to what you are probably asking is No. Â Try this:
>>
>> >>>import module
>> >>>c1 = module.Someclass
>> >>>reload(module)
>> >>>c2 = module.Someclass
>> >>>c1 is c2
>
> What about
>
>>>> o= object()
>>>> b1= o.someattr
>>>> reload( o )
>>>> b2= o.someattr
>>>> b1 is b2
>
> ?

You are really a bit thick, a troll, or a bot.

*plonk*

Ciao,
Marc 'BlackJack' Rintsch

comp.lang.python

Classes and modules are singletons?

Steven D'Aprano

Terry Reedy

Steven D'Aprano

Micah Cowan

Aaron Brady

Aaron Brady

Aaron Brady

Aaron Brady

Carl Banks

Aaron Brady

Marc 'BlackJack' Rintsch

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