subclassing types.SimpleNamespace does not work

[bitdancer]

BPO	16160
Nosy	@tiran, @bitdancer, @ericsnowcurrently
Files	issue16160.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = 'https://github.com/ericsnowcurrently'
closed_at = <Date 2012-10-17.05:57:18.585>
created_at = <Date 2012-10-08.01:10:07.134>
labels = ['type-bug']
title = 'subclassing types.SimpleNamespace does not work'
updated_at = <Date 2012-10-17.05:57:18.561>
user = 'https://github.com/bitdancer'

bugs.python.org fields:

activity = <Date 2012-10-17.05:57:18.561>
actor = 'python-dev'
assignee = 'eric.snow'
closed = True
closed_date = <Date 2012-10-17.05:57:18.585>
closer = 'python-dev'
components = []
creation = <Date 2012-10-08.01:10:07.134>
creator = 'r.david.murray'
dependencies = []
files = ['27524']
hgrepos = []
issue_num = 16160
keywords = ['patch']
message_count = 8.0
messages = ['172355', '172357', '172510', '172552', '172565', '172617', '172827', '173133']
nosy_count = 5.0
nosy_names = ['christian.heimes', 'Arfrever', 'r.david.murray', 'python-dev', 'eric.snow']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue16160'
versions = ['Python 3.3', 'Python 3.4']

[bitdancer]

Python 3.3.0+ (3.3:152d85b2da3a, Oct  6 2012, 13:17:54) 
[GCC 4.6.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from types import SimpleNamespace
>>> class Foo(SimpleNamespace):
...   pass
... 
>>> y = Foo(bar=8, foo=9)
>>> y
namespace()

It doesn't work to define an __init__ method, either, which is what I really wanted to do. (I was subclassing to get the nice repr).

[tiran]

I have verified that the __init__ function isn't executed when SimpleNamespace is subclasses. I guess that's happening:

http://docs.python.org/py3k/c-api/typeobj.html?highlight=tp_init#PyTypeObject.tp_init

"If the tp_new function returns an instance of some other type that is not a subtype of the original type, no tp_init function is called; if tp_new returns an instance of a subtype of the original type, the subtype’s tp_init is called."

namespace_new always returns a namespace object no matter what. As namespace is not a subclass of the Foo (the other way around), the type check in type_call() fails and __init__ isn't called. The tp_new method needs a fix.

That looks all wrong to me:

>>> import types
>>> class SubNS(types.SimpleNamespace):
...     pass
... 
>>> SubNS.__new__(SubNS)
namespace()

That's about right:

>>> class SubStr(str):
...     pass
... 
>>> type(SubStr.__new__(SubStr))
<class '__main__.SubStr'>

[ericsnowcurrently]

Yikes. I'll get a patch up tonight.

[ericsnowcurrently]

Here's a patch that fixes subclass support. It was supposed to work in the first place. Guess I had tunnel vision at the time.

The fix is essentially a copy of the code in dict_new() in Objects/dictobject.c. I left out the part about releasing GC if the type is not a subclass.

Note: see bpo-15022, bpo-15004, and bpo-15003 for other things SimpleNamespace is missing. I can get those done if it would be meaningful to you.

[bitdancer]

Thanks. I don't have a need for those in my current application at the current time.

[ericsnowcurrently]

Here's an updated patch that addresses Éric's review comments.

[ericsnowcurrently]

Am I good to commit this?

[python-dev]

New changeset c5124145e79e by Eric Snow in branch '3.3':
Close bpo-16160: Subclass support now works for types.SimpleNamespace. Thanks to RDM for noticing.
http://hg.python.org/cpython/rev/c5124145e79e

New changeset 47b9732696eb by Eric Snow in branch 'default':
merge for issue bpo-16160: Subclass support now works for types.SimpleNamespace.
http://hg.python.org/cpython/rev/47b9732696eb