deepcopy memoryview raises TypeError

[stuaxo]

Bug report

Calling deepcopy on a memoryview raises a TypeError, this looks similar to bug 82474 https://github.com/python/cpython/issues/82474

This also breaks BinaryField when using Django Polymorphic

https://github.com/django-polymorphic/django-polymorphic/issues/524 Your environment

• CPython versions tested on: 3.8, 3.10
• Operating system and architecture: Ubuntu 21.10

>>> a = memoryview(b'123')
>>> from copy import deepcopy
>>> deepcopy(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.10/copy.py", line 161, in deepcopy
    rv = reductor(4)
TypeError: cannot pickle 'memoryview' object

[corona10]

This issue looks like related to https://bugs.python.org/issue22995 and it was intended not to support deepcopy/copy. But I may need to understand the difference between https://github.com/python/cpython/issues/82474 case.

[corona10]

@serhiy-storchaka Is there any reason to prohibit copy.deepcopy/copy operations for memoryview object while allowing property object?

>>> import pickle
>>> import copy
>>> a = property()
>>> pickle.dumps(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot pickle 'property' object
>>> copy.copy(a)
<property object at 0x101017230>
>>> copy.deepcopy(a)
<property object at 0x101017230>

[serhiy-storchaka]

There are no reasons to prohibit shallow and deep copying of memoryview objects (or I do not know them). And I have not found any mention of memoryview objects in https://bugs.python.org/issue22995. Could you explain what did you mean? Did you mean some other issue?

By default, shallow and deep copying use the same mechanism as pickling. If pickling works, copying works automatically, if pickling is impossible, copying does not work by default. But if it is possible to make a copy, it can be implemented explicitly using other mechanisms.

[corona10]

There are no reasons to prohibit shallow and deep copying of memoryview objects (or I do not know them). And I have not found any mention of memoryview objects in https://bugs.python.org/issue22995. Could you explain what did you mean? Did you mean some other issue?

I found the related patch from that issue, Can you please take a look at this comment? https://github.com/python/cpython/pull/95082#discussion_r926287551

[corona10]

By default, shallow and deep copying use the same mechanism as pickling. If pickling works, copying works automatically, if pickling is impossible, copying does not work by default. But if it is possible to make a copy, it can be implemented explicitly using other mechanisms.

Okay Thank you for explain :)