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Foo.__new__ is what species of method?

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  Foo.__new__ is what species of method? Ben Finney <ben+python@benfinney.id.au> - 2015-07-14 14:45 +1000
    Re: Foo.__new__ is what species of method? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-07-14 14:54 +1000
      Re: Foo.__new__ is what species of method? Ben Finney <ben+python@benfinney.id.au> - 2015-07-14 15:17 +1000
      Re: Foo.__new__ is what species of method? Stefan Behnel <stefan_ml@behnel.de> - 2015-07-14 08:26 +0200

#93776 — Foo.__new__ is what species of method?

Howdy all,

The Python reference says of a class ‘__new__’ method::

    object.__new__(cls[, ...])

    Called to create a new instance of class cls. __new__() is a static
    method (special-cased so you need not declare it as such) that takes
    the class of which an instance was requested as its first argument.

    <URL:https://docs.python.org/3/reference/datamodel.html#object.__new__>

But a “static method” is described, elsewhere in the documentation
<URL:https://docs.python.org/3/library/functions.html#staticmethod>, as
“A static method does not receive an implicit first argument”.

What the ‘__new__’ documentation describes would match a “class method”
<URL:https://docs.python.org/3/library/functions.html#classmethod>
“A class method receives the class as implicit first argument”.

I suspect this a bug in the reference documentation for ‘__new__’, and
it should instead say “__new__ is a class method …”. Am I wrong?

-- 
 \        “Cooles & Heates: If you want just condition of warm in your |
  `\    room, please control yourself.” —air conditioner instructions, |
_o__)                                                            Japan |
Ben Finney

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#93777

On Tuesday 14 July 2015 14:45, Ben Finney wrote:

> Howdy all,
> 
> The Python reference says of a class ‘__new__’ method::
> 
>     object.__new__(cls[, ...])
> 
>     Called to create a new instance of class cls. __new__() is a static
>     method (special-cased so you need not declare it as such) that takes
>     the class of which an instance was requested as its first argument.

This is correct. __new__ is a static method and you need to explicitly 
provide the cls argument:


py> class Spam(object):
...     def __new__(cls):
...             print cls
... 
py> Spam.__new__()  # implicit first arg?
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __new__() takes exactly 1 argument (0 given)


py> Spam.__new__(Spam)
<class '__main__.Spam'>


Furthermore:

py> type(Spam.__dict__['__new__'])
<type 'staticmethod'>



> I suspect this a bug in the reference documentation for ‘__new__’, and
> it should instead say “__new__ is a class method …”. Am I wrong?

I've made that mistake in the past too :-)



-- 
Steve

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#93781

Steven D'Aprano <steve+comp.lang.python@pearwood.info> writes:

> py> class Spam(object):
> ...     def __new__(cls):
> ...             print cls
> ... 
> py> Spam.__new__()  # implicit first arg?
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: __new__() takes exactly 1 argument (0 given)

Thanks, I'm glad I checked before filing a bug report.

Hopefully I'll remember that clear test, when I need to remember which
species it is :-)

-- 
 \       “Instead of having ‘answers’ on a math test, they should just |
  `\               call them ‘impressions’, and if you got a different |
_o__)   ‘impression’, so what, can't we all be brothers?” —Jack Handey |
Ben Finney

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#93788

Steven D'Aprano schrieb am 14.07.2015 um 06:54:
> On Tuesday 14 July 2015 14:45, Ben Finney wrote:
>> The Python reference says of a class ‘__new__’ method::
>>
>>     object.__new__(cls[, ...])
>>
>>     Called to create a new instance of class cls. __new__() is a static
>>     method (special-cased so you need not declare it as such) that takes
>>     the class of which an instance was requested as its first argument.
> 
> This is correct. __new__ is a static method and you need to explicitly 
> provide the cls argument:

And it needs to be that way in order to allow superclass calls in a
subclass's __new__ method:

  class Super(object):
      def __new__(cls):
          return object.__new__(cls)

  class Sub(Super):
      def __new__(cls):
          return Super.__new__(cls)

If it was a classmethod, it would receive the class you call it on as first
argument (i.e. "Super" and "object" above), not the class you want to
instantiate (i.e. "Sub" or "Super").

Stefan

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