[issue15542] Documentation incorrectly suggests __init__ called after direct __new__ call

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Thanks everyone.

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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue15542] Documentation incorrectly suggests __init__ called after direct __new__ call

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset c841fb9e065c393bba5d5505238f7e286f1dcfc6 by Raymond Hettinger 
(Miss Islington (bot)) in branch '3.8':
bpo-15542: Documentation incorrectly suggests __init__ called after direct  
__new__ call (GH-15478) (GH-15506)
https://github.com/python/cpython/commit/c841fb9e065c393bba5d5505238f7e286f1dcfc6


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[issue15542] Documentation incorrectly suggests __init__ called after direct __new__ call

[miss-islington]

Change by miss-islington :


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pull_requests: +15192
pull_request: https://github.com/python/cpython/pull/15506

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[issue15542] Documentation incorrectly suggests __init__ called after direct __new__ call

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset 6b16d938d6d1ccb443815e20e8812deed274dc09 by Raymond Hettinger 
(Joannah Nanjekye) in branch 'master':
bpo-15542: Documentation incorrectly suggests __init__ called after direct  
__new__ call (GH-15478)
https://github.com/python/cpython/commit/6b16d938d6d1ccb443815e20e8812deed274dc09


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[issue15542] Documentation incorrectly suggests __init__ called after direct __new__ call

[Raymond Hettinger]

Raymond Hettinger  added the comment:

FWIW, the underlying code for this in Objects/typeobject.c::type_new().

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assignee: docs@python -> rhettinger
nosy: +rhettinger

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[issue15542] Documentation incorrectly suggests __init__ called after direct __new__ call

[Joannah Nanjekye]

Joannah Nanjekye  added the comment:

Since this has taken long on the tracker, I just opened a PR with proposed 
changes from Aaron.

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nosy: +nanjekyejoannah

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[issue15542] Documentation incorrectly suggests __init__ called after direct __new__ call

[Joannah Nanjekye]

Change by Joannah Nanjekye :


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keywords: +patch
pull_requests: +15164
stage: needs patch -> patch review
pull_request: https://github.com/python/cpython/pull/15478

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[issue15542] Documentation incorrectly suggests __init__ called after direct __new__ call

[Ankur Ankan]

Changes by Ankur Ankan ankuran...@gmail.com:


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nosy: +Ankur.Ankan

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[issue15542] Documentation incorrectly suggests __init__ called after direct __new__ call

[Aaron Staley]

New submission from Aaron Staley:

The documentation for __new__ at 
http://docs.python.org/reference/datamodel.html#object.__new__ is:

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. The remaining arguments 
are those passed to the object constructor expression (the call to the class). 
The return value of __new__() should be the new object instance (usually an 
instance of cls).

Typical implementations create a new instance of the class by invoking the 
superclass’s __new__() method using super(currentclass, cls).__new__(cls[, 
...]) with appropriate arguments and then modifying the newly-created instance 
as necessary before returning it.

If __new__() returns an instance of cls, then the new instance’s __init__() 
method will be invoked like __init__(self[, ...]), where self is the new 
instance and the remaining arguments are the same as were passed to __new__().

If __new__() does not return an instance of cls, then the new instance’s 
__init__() method will not be invoked.

__new__() is intended mainly to allow subclasses of immutable types (like int, 
str, or tuple) to customize instance creation. It is also commonly overridden 
in custom metaclasses in order to customize class creation.



The problem is in this line: If __new__() returns an instance of cls, then the 
new instance’s __init__() method will be invoked like __init__(self[, ...]), 
where self is the new instance and the remaining arguments are the same as were 
passed to __new__().

This is only true in the context of a constructor. In particular, directly 
calling cls.__new__(cls) will NOT call __init__.

If I define a class:
  
class C(object):
  def __new__(*args, **kwargs):
print 'new', args, kwargs
return object.__new__(*args,**kwargs)
  def __init__(self):
print 'init'

C() will result in __new__ and __init__ being both executed, but C.__new__(C) 
will only create the instance of C; it will not call __init__!

The original documentation described in http://bugs.python.org/issue1123716 was 
more correct:

__new__ must return an object... If you return an
existing object, the constructor call will still call
its __init__ method unless the object is an instance of
a different class...

That is __init__ is only being called in the context of an external constructor 
call.  

Proposed phrasing:
If __new__() is invoked during object construction (cls()) and it returns an 
instance of cls, then the new instance’s __init__() method will be invoked like 
__init__(self[, ...]), where self is the new instance and the remaining 
arguments are the same as were passed to the object constructor.

--
assignee: docs@python
components: Documentation
messages: 167267
nosy: Aaron.Staley, docs@python
priority: normal
severity: normal
status: open
title: Documentation incorrectly suggests __init__ called after direct  __new__ 
call
versions: Python 2.7

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[issue15542] Documentation incorrectly suggests __init__ called after direct __new__ call

[Chris Jerdonek]

Chris Jerdonek added the comment:

Whether or not the current language is technically correct, I would support 
improving its clarity.

Would you like to create a formal patch?  Also, note that the newest 
documentation is published here:

http://docs.python.org/dev/reference/datamodel.html#object.__new__

(Note that in the future, you do not need to post such full excerpts, just the 
key parts.)

--
nosy: +cjerdonek
stage:  - needs patch
type:  - enhancement
versions: +Python 3.3

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