memoryview does not support subclassing

[mdehoon]

BPO	41285
Nosy	@methane, @skrah
Superseder	bpo-13797: Allow objects implemented in pure Python to export PEP 3118 buffers

^{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 = None
closed_at = <Date 2020-07-15.17:38:42.139>
created_at = <Date 2020-07-12.14:23:24.421>
labels = ['expert-C-API', 'type-feature', '3.10']
title = 'memoryview does not support subclassing'
updated_at = <Date 2020-07-15.17:38:42.138>
user = 'https://bugs.python.org/mdehoon'

bugs.python.org fields:

activity = <Date 2020-07-15.17:38:42.138>
actor = 'skrah'
assignee = 'none'
closed = True
closed_date = <Date 2020-07-15.17:38:42.139>
closer = 'skrah'
components = ['C API']
creation = <Date 2020-07-12.14:23:24.421>
creator = 'mdehoon'
dependencies = []
files = []
hgrepos = []
issue_num = 41285
keywords = []
message_count = 6.0
messages = ['373554', '373601', '373668', '373693', '373697', '373705']
nosy_count = 3.0
nosy_names = ['mdehoon', 'methane', 'skrah']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '13797'
type = 'enhancement'
url = 'https://bugs.python.org/issue41285'
versions = ['Python 3.10']

[mdehoon]

Currently memoryview does not support subclassing:

>>> class B(memoryview): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: type 'memoryview' is not an acceptable base type

Subclassing memoryview can be useful when

• class A supports the buffer protocol;
• class B wraps class A and should support the buffer protocol provided by class A;
• class A does not support subclassing.

In this situation,

class B(memoryview):
    def __new__(cls, a):
        return super(B, cls).__new__(cls, a)

where a is an instance of class A, would let instances of B support the buffer protocol provided by a.

Is there any particular reason why memoryview does not support subclassing?

[methane]

Would you be more specific about why you need that feature on the Python-ideas mailing list or "Ideas" category on discuss.python.org?

[mdehoon]

Thank you, I have posted an explanation in the "Ideas" category of discuss.python.org.

[skrah]

If I understand this correctly, I think this may be a duplicate of bpo-13797.

Something like a __getbuffer__ protocol would be needed. I'm not
saying we *should* go that route, just stating the requirements
from my perspective.

[mdehoon]

You are correct, this is indeed a duplicate of bpo-13797. My apologies for not finding this issue before opening a new one. If there are no objections, I will close this issue as a duplicate.

[skrah]

Yes, let's make bpo-13797 a superseder.