| msg308660 - (view) |
Author: Eric Cousineau (Eric Cousineau) * |
Date: 2017-12-19 16:29 |
| Due to how `PyObject_CallFinalizer` is written in python3, `__del__` will only *ever* be called once. In my use case, I am experimenting with a feature in `pybind11` to prevent slicing with Python class instances that inherit from pybind11-C++ base classes, which involves detecting when an instance loses all reference in Python (`Py_REFCNT(...) == 0`) but still has reference in C++ (`shared_ptr::count() > 0`), and reviving the Python portion when this situation happens. In python2, I could do this without a hitch, as a resurrected object could have its `__del__` method called multiple times (through `tp_dealloc` I believe?). But in python3, the object is marked with `_PyGC_SET_FINALIZED(...)`, thus preventing `__del__` from being called again. It'd be nice to either (a) somehow allow `__del__` to be called naturally without too much fuss or, at the least, (b) have this reflected in the documentation: https://docs.python.org/3/reference/datamodel.html#object.__del__ See attached `revive_test`. Example execution: ``` $ python2 ./revive_test.py Revive Destroy [ Done ] $ python3 ./revive_test.py Revive [ Done ] ``` |
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| msg308661 - (view) |
Author: Antoine Pitrou (pitrou) *  |
Date: 2017-12-19 16:33 |
| Thanks for the report. Apparently I forgot to update that piece of documentation when PEP 442 was implemented. |
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| msg308662 - (view) |
Author: Eric Cousineau (Eric Cousineau) * |
Date: 2017-12-19 16:46 |
| You're welcome, and thank you for the prompt response! I will say that it feels a tad odd to only have `tp_finalize` be called once for the entire lifetime of the object, while still having the option of it being resurrected. Is there any way to somehow "un-mark" the object to enable this workflow that I would like to have? My current hack is to call `_PyGC_SET_FINALIZED(self, 0)` - may I ask if there is a simpler way to do this? |
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| msg308664 - (view) |
Author: Antoine Pitrou (pitrou) * |
Date: 2017-12-19 17:13 |
| Le 19/12/2017 à 17:46, Eric Cousineau a écrit : > > My current hack is to call `_PyGC_SET_FINALIZED(self, 0)` - may I ask if there is a simpler way to do this? Well... perhaps you could create another PyObject (it's just a wrapper, right?) since the old one doesn't have any outside references to it remaining. Note that calling __del__ only once is also how PyPy works: http://doc.pypy.org/en/latest/cpython_differences.html#differences-related-to-garbage-collection-strategies If there is some demand we could expose a higher-level spelling of `_PyGC_SET_FINALIZED(self, 0)`. |
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| msg308665 - (view) |
Author: Antoine Pitrou (pitrou) * |
Date: 2017-12-19 17:47 |
| I've proposed a documentation improvement in https://github.com/python/cpython/pull/4927 . Please chime in if you see have issues with it. |
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| msg308667 - (view) |
Author: Antoine Pitrou (pitrou) * |
Date: 2017-12-19 18:48 |
| New changeset 4b965930e8625f77cb0e821daf5cc40e85b45f84 by Antoine Pitrou in branch 'master': bpo-32377: improve __del__ docs and fix mention about resurrection (#4927) https://github.com/python/cpython/commit/4b965930e8625f77cb0e821daf5cc40e85b45f84 |
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| msg308671 - (view) |
Author: Antoine Pitrou (pitrou) * |
Date: 2017-12-19 19:00 |
| New changeset dc5770b161a5e28eeff73a406cd4eddb0676c5b5 by Antoine Pitrou (Miss Islington (bot)) in branch '3.6': bpo-32377: improve __del__ docs and fix mention about resurrection (GH-4927) (#4929) https://github.com/python/cpython/commit/dc5770b161a5e28eeff73a406cd4eddb0676c5b5 |
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| msg366057 - (view) |
Author: Eric Cousineau (Eric Cousineau) * |
Date: 2020-04-09 14:55 |
| Super late response, but for this part: > Well... perhaps you could create another PyObject (it's just a wrapper, > right?) since the old one doesn't have any outside references to it > remaining. In certain cases, yes, that would be the case. I do have unittests that (hackily) check `id(o)` before and after resurrection by using a `weakref`, so I could relax that contract. However, in other cases, the "wrapper" object in Python is actually a C++ base class that has been derived from in Python. If ownership of the C++ instance (which should more-or-less own the Python portion) goes solely to C++, then the Python portion could be garbage collected, and you get a sort-of slicing effect: https://github.com/pybind/pybind11/issues/1145 Granted, within this web of issues, one alternative is to always extend the lifetime of the Python object by the C++ portion. However, that creates a (opaque?) reference cycle between C++ and CPython, so I am a bit afraid of what that would do. I would prefer to make memory management conservative if possible. Or rather: It's something cool to try out, but I have not yet had time to dig in :( |
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| msg366068 - (view) |
Author: Eric Cousineau (Eric Cousineau) * |
Date: 2020-04-09 15:25 |
| See also bpo-40240: "Expose public spelling of _PyGC_FINALIZED and _PyGC_SET_FINALIZED?" |
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