array.array should remain immutable: add Py_TPFLAGS_IMMUTABLETYPE flag

[gvanrossum]

BPO	43908
Nosy	@gvanrossum, @vstinner, @tiran, @markshannon, @serhiy-storchaka, @gvanrossum, @corona10, @pablogsal, @miss-islington, @shihai1991, @erlend-aasland, @shreyanavigyan
PRs	bpo-43908: Add Py_TPFLAGS_IMMUTABLETYPE flag #25520 bpo-43908: Make array.array type immutable #25696 bpo-43908: Make re types immutable #25697 bpo-43908: Document Static Types in the C API #25710 bpo-43973: Use Py_TPFLAGS_IMMUTABLETYPE to check for class assignments #25714 bpo-43908: check_set_special_type_attr() and type_set_annotations() now check for immutable flag #25743 bpo-43908: Mark ssl, hash, and hmac types as immutable (GH-25792) #25792 bpo-43908: Add What's New entry for Py_TPFLAGS_IMMUTABLETYPE flag #25816 [3.10] bpo-43908: Add What's New entry for Py_TPFLAGS_IMMUTABLETYPE flag (GH-25816) #26115 bpo-43908: Make heap types converted during 3.10 alpha immutable #26351 [3.10] bpo-43908: Make heap types converted during 3.10 alpha immutable (GH-26351) #26766 bpo-43908: Types with Py_TPFLAGS_IMMUTABLETYPE can now inherit the vectorcall protocol #27001
Files	patch.diff apply-to-all.diff list.txt apply-to-all.diff: modified tests_log.txt

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

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GitHub fields:

assignee = 'https://github.com/vstinner'
closed_at = None
created_at = <Date 2021-04-21.18:52:19.855>
labels = ['type-bug', 'library', '3.10', '3.11']
title = 'array.array should remain immutable: add Py_TPFLAGS_IMMUTABLETYPE flag'
updated_at = <Date 2022-01-26.17:39:44.786>
user = 'https://github.com/gvanrossum'

bugs.python.org fields:

activity = <Date 2022-01-26.17:39:44.786>
actor = 'vstinner'
assignee = 'vstinner'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-04-21.18:52:19.855>
creator = 'gvanrossum'
dependencies = []
files = ['49970', '49980', '49987', '49990', '49994']
hgrepos = []
issue_num = 43908
keywords = ['patch']
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priority = None
resolution = None
stage = 'patch review'
status = 'open'
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type = 'behavior'
url = 'https://bugs.python.org/issue43908'
versions = ['Python 3.10', 'Python 3.11']

[gvanrossum]

Hi Victor,

Sorry for making this a deferred blocker. I recall that we had a brief discussion somewhere about an accidental change to the array.array type -- this is now a heap type (Py_TPFLAGS_HEAPTYPE is set), and as a consequence it is no longer immutable.

In 3.9 this is an error:

>>> import array 
>>> array.array.foo = 1
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: can't set attributes of built-in/extension type 'array.array'
>>>

But in 3.10a7 it passes:

>>> array.array.foo = 1
>>> array.array.foo 
1
>>>

I would like this type (and other types that have been or will be converted to heap types) to remain immutable. How can we do that? I think we may need a new flag bit meaning "built-in type, immutable". This bit should not be inherited of course.

What do you think? (Feel free to close if this is a duplicate -- I couldn't find where we discussed this previously, sorry.)

[shihai1991]

I would like this type (and other types that have been or will be converted to heap types) to remain immutable.

Hi, Guido. I am interested in what have been broken in fact? or it's a defined behavior?

I think we may need a new flag bit meaning "built-in type, immutable". This bit should not be inherited of course.

Aggreed. Adding a new tp_flag is a lightweight solution.

[erlend-aasland]

FWIW, I was the one who converted arraymodule to use heap types in 75bf107 (GH-23124) in January. Sorry 'bout the accidental change of behaviour.

I think we may need a new flag bit meaning "built-in type, immutable". This bit should not be inherited of course.

+1.

See attached PoC patch. As far as I understand, type flags must be explicitly inherited in Objects/typeobject.c, no?

Should all previous static types be immutable, or are there exceptions?