[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Eryk Sun]

Change by Eryk Sun :


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versions: +Python 3.10, Python 3.8, Python 3.9 -Python 2.7, Python 3.5, Python 
3.6, Python 3.7

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Carl Dunham]

Changes by Carl Dunham :


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nosy: +Carl Dunham

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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resolution: fixed -> 
stage: resolved -> 
versions: +Python 3.7 -Python 3.4

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Bohuslav "Slavek" Kabrda]

Bohuslav "Slavek" Kabrda added the comment:

Hi all, it seems to me that this change has been reverted not only in 2.7, but 
also in 3.5 (changeset: 101549:c8df1877d1bc). Benjamin, was this intentional? 
If so, perhaps this issue should be reopened and not marked as resolved.

Thanks a lot!

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Roundup Robot]

Roundup Robot added the comment:

New changeset 3ff84a3eeb6b by Benjamin Peterson in branch '2.7':
Backed out changeset e7062dd9085e (#25731)
https://hg.python.org/cpython/rev/3ff84a3eeb6b

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Benjamin Peterson]

Benjamin Peterson added the comment:

I'll have to think about how to fix this while maintaining compatiblity with 
obscure cases like above.

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[jcristau]

jcristau added the comment:

Well yes, but as far as I can tell that's why python used B.__new__ for C 
before your change, as that has a compatible layout.

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Benjamin Peterson]

Benjamin Peterson added the comment:

I believe the correct behavior is actually

Traceback (most recent call last):
  File "", line 1, in 
  File "bar.py", line 7, in 
C()
TypeError: foo.A.__new__(C) is not safe, use foo.B.__new__()

This is because A comes before B in the mro, and, indeed, constructing C with 
A.__new__ is unsafe. In fact, reordering A and B in the definition of C fixes 
everything.

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Barry A. Warsaw]

Changes by Barry A. Warsaw :


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status: closed -> open

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[jcristau]

jcristau added the comment:

This change in 2.7 seems to break things:

$ cat foo.pxd 
cdef class B:
cdef object b
$ cat foo.pyx 
cdef class A:
pass

cdef class B:
def __init__(self, b):
self.b = b
$ cat bar.py
from foo import A, B

class C(A, B):
def __init__(self):
B.__init__(self, 1)

C()
$ cython foo.pyx && gcc -I/usr/include/python2.7 -Wall -shared -fPIC -o foo.so 
foo.c
$ python -c 'import bar'
Segmentation fault

C's tp_new is set to A's tp_new function, thus the b slot is never initialized 
to Py_None, and C's __init__ calls DECREF on a NULL pointer.

Reverting changeset e7062dd9085e makes things work again, with C's tp_new being 
B's tp_new.

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Roundup Robot]

Roundup Robot added the comment:

New changeset 3c9512d8ac0d by Benjamin Peterson in branch '3.5':
set tp_new from the class in the hierarchy that actually owns the descriptor 
(closes #25731)
https://hg.python.org/cpython/rev/3c9512d8ac0d

New changeset e7062dd9085e by Benjamin Peterson in branch '2.7':
set tp_new from the class in the hierarchy that actually owns the descriptor 
(closes #25731)
https://hg.python.org/cpython/rev/e7062dd9085e

New changeset a7953ee29f1c by Benjamin Peterson in branch 'default':
merge 3.5 (#25731)
https://hg.python.org/cpython/rev/a7953ee29f1c

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nosy: +python-dev
resolution:  -> fixed
stage:  -> resolved
status: open -> closed

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Bohuslav "Slavek" Kabrda]

Bohuslav "Slavek" Kabrda added the comment:

Hi, I'm maintainer of flexmock [1] and some users of my library have started 
hitting this bug [2] - when one tries to mock __new__ and then revert this mock 
at the end of testcase to the original one (either by deleting the mock __new__ 
or replacing mock __new__ by the old one), then one gets "TypeError: object() 
takes no parameters".

I think that solution proposed by Eryk is correct, but I don't have that much 
insight into Python internals to confirm that 100 %. I would appreciate this 
getting fixed. Thanks!

[1] https://github.com/bkabrda/flexmock
[2] https://github.com/bkabrda/flexmock/issues/13

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Eryk Sun]

Changes by Eryk Sun :


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nosy: +benjamin.peterson, twouters

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Eryk Sun]

Eryk Sun added the comment:

For "del X.__new__", type_setattro in Objects/typeobject.c indirectly calls 
update_one_slot. This finds object.__new__ fom the base object class when it 
looks up __new__ on the type. Since __new__ for built-in types is special-cased 
to be a built-in method instead of a slot wrapper, update_one_slot takes the 
branch for "Py_TYPE(descr) == &PyCFunction_Type", which assigns the existing 
tp_new to "specific". In this case that's slot_tp_new instead of object_new due 
to the previous assignment of the "new" function to X.__new__.  

slot_tp_new looks up and calls __new__, which in this case, as noted above, is 
object.__new__. This built-in method calls tp_new_wrapper, which calls the 
wrapped tp_new function. In this case that's object_new. If the type's tp_init 
is object_init (i.e. not overridden) or tp_new is not object_new (i.e. 
overridden), then object_new raises a TypeError when called with arguments. The 
problem in this case is that __new__ isn't overridden anymore. 

It seems to me that update_one_slot needs to install the tp_new that 
tp_new_wrapper would call, e.g. 

specific = (void *)(
 (PyTypeObject *)PyCFunction_GET_SELF(descr))->tp_new;

In this case that's object_new. Thus after "del X.__new__", X would be restored 
as if __new__ was never overridden.

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nosy: +eryksun
versions: +Python 2.7, Python 3.4, Python 3.6

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[issue25731] Assigning and deleting __new__ attr on the class does not allow to create instances of this class

[Uosiu]

New submission from Stanisław Skonieczny (Uosiu):

When moving from python 2.7 to 3.5 I have found a problem with patching __new__ 
method on the class. It was done this way:
'''
patch('foo.bar.MyClass.__new__', return_value=mocked_instance)
'''
In works with python 2.7, but in 3.5 it fails with:
'''
TypeError: object() takes no parameters
'''

I have created minimal scenario that ilustrates this bug cause:
'''
class X:
def __init__(self, a):
pass


def new(cls, a):
pass


X(1)
X.__new__ = new
X(1)
del X.__new__
X(1)
'''
Setting __new__ attribute and then deleting it has some side effect.

--
components: Interpreter Core
files: new_patch_fails.py
messages: 255337
nosy: Stanisław Skonieczny (Uosiu)
priority: normal
severity: normal
status: open
title: Assigning and deleting __new__ attr on the class does not allow to 
create instances of this class
type: behavior
versions: Python 3.5
Added file: http://bugs.python.org/file41161/new_patch_fails.py

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