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type(None)()

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  type(None)() Hans Mulder <hansmu@xs4all.nl> - 2012-08-16 14:47 +0200
    Re: type(None)() Laszlo Nagy <gandalf@shopzeus.com> - 2012-08-16 14:54 +0200
    Re: type(None)() Chris Angelico <rosuav@gmail.com> - 2012-08-16 23:37 +1000
    Re: type(None)() Ian Kelly <ian.g.kelly@gmail.com> - 2012-08-16 07:56 -0600
    Re: type(None)() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-08-16 13:58 +0000
      Re: type(None)() Stefan Behnel <stefan_ml@behnel.de> - 2012-08-16 17:31 +0200
      Re: type(None)() Stefan Behnel <stefan_ml@behnel.de> - 2012-08-16 17:44 +0200
      Re: type(None)() Ethan Furman <ethan@stoneleaf.us> - 2012-08-16 09:06 -0700
    Re: type(None)() Robert Kern <robert.kern@gmail.com> - 2012-08-16 15:56 +0100
    Re: type(None)() MRAB <python@mrabarnett.plus.com> - 2012-08-16 16:13 +0100
    Re: type(None)() Ethan Furman <ethan@stoneleaf.us> - 2012-08-16 09:07 -0700

#27157 — type(None)()

On 8/08/12 04:14:01, Steven D'Aprano wrote:
> NoneType raises an error if you try to create a second instance. bool
> just returns one of the two singletons (doubletons?) again.
>
> py> type(None)()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: cannot create 'NoneType' instances

Why is that?

Why doesn't it just return an existing instance of the type,
like bool, int, str and other built-in non-mutable types do?

> py> type(False)() is False
> True


Just wondering,

-- HansM

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#27160

On 2012-08-16 14:47, Hans Mulder wrote:
> On 8/08/12 04:14:01, Steven D'Aprano wrote:
>> NoneType raises an error if you try to create a second instance. bool
>> just returns one of the two singletons (doubletons?) again.
>>
>> py> type(None)()
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in <module>
>> TypeError: cannot create 'NoneType' instances
> Why is that?
Because None is a singleton. It is the only instance of its class. This 
is very useful because it allows you to write conditions like this:

if obj is None:
     do_something()

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#27161

On Thu, Aug 16, 2012 at 10:47 PM, Hans Mulder <hansmu@xs4all.nl> wrote:
> Why doesn't it just return an existing instance of the type,
> like bool, int, str and other built-in non-mutable types do?
>
>> py> type(False)() is False
>> True

With int and str, it's only an optimization, and not guaranteed to happen.

>>> a=int("1234")
>>> a is int("1234")
False

>>> a=str(1234)
>>> a is str(1234)
False

But with bool, it's required, as a means of "casting to boolean". With
True/False/None, it's normal to compare them with is:

>>> a=bool("1")
>>> a is bool("2")
True

So bool() has to return one of those two actual objects, and not an equivalent.

(Note: All examples done in CPython 3.2's IDLE on Windows. Other
environments, Pythons, versions, etc, may affect exactly what these
show.)

ChrisA

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#27163

On Thu, Aug 16, 2012 at 6:47 AM, Hans Mulder <hansmu@xs4all.nl> wrote:
> On 8/08/12 04:14:01, Steven D'Aprano wrote:
>> NoneType raises an error if you try to create a second instance. bool
>> just returns one of the two singletons (doubletons?) again.
>>
>> py> type(None)()
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>> TypeError: cannot create 'NoneType' instances
>
> Why is that?
>
> Why doesn't it just return an existing instance of the type,
> like bool, int, str and other built-in non-mutable types do?

Because unlike those other types there is no use case for that.  It's
simpler to raise an error.

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#27164

On Thu, 16 Aug 2012 14:47:47 +0200, Hans Mulder wrote:

> On 8/08/12 04:14:01, Steven D'Aprano wrote:
>> NoneType raises an error if you try to create a second instance. bool
>> just returns one of the two singletons (doubletons?) again.
>>
>> py> type(None)()
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>> TypeError: cannot create 'NoneType' instances
> 
> Why is that?
> 
> Why doesn't it just return an existing instance of the type, like bool,
> int, str and other built-in non-mutable types do?

bool must return an instance, because it is designed to cast objects to a 
boolean. Since (by design) True and False are singletons (doubletons?), 
bool(x) will always return a pre-existing instance.

Other built-in immutable types do not promise to do that. For example:

py> a = float(42)
py> b = float(42)
py> a is b
False

Sometimes int and str will cache their instances, but this is an 
implementation detail subject to change without notice from version to 
version.

None, NotImplemented and Ellipsis are singletons, but unlikely bool, 
there is no common use-case for having their types return the singleton 
instance. The standard design pattern for singletons is to raise an 
exception if you try to create an instance, so they do. However, this 
behaviour really only makes sense for singletons that hold state. (If 
they hold state, you might be tempted to change that state, not realising 
that you are changing a singleton and not a second instance.)

In my opinion, this is a PITA for None and better behaviour would be to 
return the pre-existing NoneType instance, but I didn't design the 
language.


-- 
Steven

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#27171

Steven D'Aprano, 16.08.2012 15:58:
>>> NoneType raises an error if you try to create a second instance.
> In my opinion, this is a PITA for None and better behaviour would be to 
> return the pre-existing NoneType instance, but I didn't design the 
> language.

The time machine strikes again.

Python 3.3.0b1 (default:f7b59e890e30, Aug 11 2012, 05:30:10)
[GCC 4.6.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> type(None)()
>>> print(type(None)())
None

Stefan

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#27172

Ramchandra Apte, 16.08.2012 17:39:
> On 16 August 2012 21:01, Stefan Behnel wrote:
>> Steven D'Aprano, 16.08.2012 15:58:
>>>>> NoneType raises an error if you try to create a second instance.
>>> In my opinion, this is a PITA for None and better behaviour would be to
>>> return the pre-existing NoneType instance, but I didn't design the
>>> language.
>>
>> The time machine strikes again.
>>
>> Python 3.3.0b1 (default:f7b59e890e30, Aug 11 2012, 05:30:10)
>> [GCC 4.6.3] on linux
>> Type "help", "copyright", "credits" or "license" for more information.
>>>>> type(None)()
>>>>> print(type(None)())
>> None
>
> Are they the same object

Obviously. None is a singleton (as was already mentioned a couple of times
in this thread).

Stefan

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#27174

Ramchandra Apte wrote:
> Are they the same object

Yes.

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#27167

On 8/16/12 2:56 PM, Ian Kelly wrote:
> On Thu, Aug 16, 2012 at 6:47 AM, Hans Mulder <hansmu@xs4all.nl> wrote:
>> On 8/08/12 04:14:01, Steven D'Aprano wrote:
>>> NoneType raises an error if you try to create a second instance. bool
>>> just returns one of the two singletons (doubletons?) again.
>>>
>>> py> type(None)()
>>> Traceback (most recent call last):
>>>    File "<stdin>", line 1, in <module>
>>> TypeError: cannot create 'NoneType' instances
>>
>> Why is that?
>>
>> Why doesn't it just return an existing instance of the type,
>> like bool, int, str and other built-in non-mutable types do?
>
> Because unlike those other types there is no use case for that.  It's
> simpler to raise an error.

What are the use cases for the empty-argument versions of bool(), int(), 
float(), and str()?

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

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#27169

On 16/08/2012 15:56, Robert Kern wrote:
> On 8/16/12 2:56 PM, Ian Kelly wrote:
>> On Thu, Aug 16, 2012 at 6:47 AM, Hans Mulder <hansmu@xs4all.nl> wrote:
>>> On 8/08/12 04:14:01, Steven D'Aprano wrote:
>>>> NoneType raises an error if you try to create a second instance. bool
>>>> just returns one of the two singletons (doubletons?) again.
>>>>
>>>> py> type(None)()
>>>> Traceback (most recent call last):
>>>>    File "<stdin>", line 1, in <module>
>>>> TypeError: cannot create 'NoneType' instances
>>>
>>> Why is that?
>>>
>>> Why doesn't it just return an existing instance of the type,
>>> like bool, int, str and other built-in non-mutable types do?
>>
>> Because unlike those other types there is no use case for that.  It's
>> simpler to raise an error.
>
> What are the use cases for the empty-argument versions of bool(), int(),
> float(), and str()?
>
They can be used with defaultdict. For example:

counts = defaultdict(int)
for i in items:
     counts[i] += 1

Of course, an alternative would be:

counts = defaultdict(lambda: 0)

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#27175

Hans Mulder wrote:
> On 8/08/12 04:14:01, Steven D'Aprano wrote:
>> NoneType raises an error if you try to create a second instance. bool
>> just returns one of the two singletons (doubletons?) again.
>>
>> py> type(None)()
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>> TypeError: cannot create 'NoneType' instances
> 
> Why is that?

An oversight, and until a few months ago nobody had complained loud 
enough.  ;)


> Why doesn't it just return an existing instance of the type,
> like bool, int, str and other built-in non-mutable types do?

In 3.3 it now does.

~Ethan~

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