> Can you flesh this out for what stumbled into being my running

example? That is, how could a regexp pattern object be part of a
cycle?

Let me try to remember when we saw this problem in the past, but

on first sigh, it seems that indeed that cannot happen in the regular case.

> And, in general, I can't add attributes to p, or change their bindings:

That's after the changes to add a new Py_TPFLAGS_IMMUTABLETYPE flag, which predates the

creation of the issue we are discussing. But in general, new heap types are not immutable by

default (check https://bugs.python.org/issue43908).

For instance, in python3.8:

>>> array.array.x = 4
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 python 3.10.0a7 (before Py_TPFLAGS_IMMUTABLETYPE):

Python 3.10.0a7 (tags/v3.10.0a7:53e55290cf, May 27 2021, 20:20:26) [Clang 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import array
>>> instance = array.array("f")
>>> type = array.array
>>> type.instance = instance

On Thu, 27 May 2021 at 20:15, Tim Peters <tim.peters@gmail.com> wrote:

[Tim]
>> And if a type pointer is the only thing being visited, then there's no point
>> unless the object can itself be reachable from the type object.

{Pablo]
> But that could happen easily for heap types as they are mutable by
> default. For instance, you set the instance in a global:
>
> type -> module -> globals -> instance -> type

Sorry, but this is apparently a rat's nest and that's too sketchy for me ;-)

Can you flesh this out for what stumbled into being my running
example? That is, how could a regexp pattern object be part of a
cycle?

>>> import re
>>> p = re.compile("ab*c")
>>> type(p)
<class 're.Pattern'>
>>> type(p).__module__
're'
>>> type(type(p).__module__)
<class 'str'>

That is, its __module__ attribute is a string, not a module object.

And, in general, I can't add attributes to p, or change their bindings:

>>> p.flags
32
>>> p.flags = re
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: readonly attribute
>>> p.newattr = re
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 're.Pattern' object has no attribute 'newattr'

I just don't see a way to trick it into being part of a cycle.

Not denying that safe is better than sorry, though.