Title: Interrupting class creation in __init_subclass__ may lead to incorrect isinstance() and issubclass() results
Type: behavior Stage:
Components: Interpreter Core Versions: Python 3.9, Python 3.8, Python 3.7, Python 3.6
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: hongweipeng, lukasz.langa, rhettinger, xitop
Priority: normal Keywords:

Created on 2019-09-10 09:09 by xitop, last changed 2021-09-17 02:24 by rhettinger.

Messages (7)
msg351599 - (view) Author: (xitop) Date: 2019-09-10 09:09
An exception in __init__subclass__ leads under certain circumstances to wrong isinstance() and issubclass() results. The exception probably leaves Python internal data in inconsistent state.

Here is a demonstration program from Stack Overflow:

--- begin --
from abc import ABCMeta

class Animal(metaclass=ABCMeta):

class Plant(metaclass=ABCMeta):
    def __init_subclass__(cls):
        assert not issubclass(cls, Animal), "Plants cannot be Animals"

class Dog(Animal):

    class Triffid(Animal, Plant):
except Exception:

print("Dog is Animal?", issubclass(Dog, Animal))
print("Dog is Plant?", issubclass(Dog, Plant))
--- end --

Result is:

Dog is Animal? True
Dog is Plant? True

Changing the order of the print statements will result in:

Dog is Plant? False
Dog is Animal? False

Another ill-behaving program and a partial analysis can be found at SO:
msg355037 - (view) Author: (xitop) Date: 2019-10-21 10:05
Same problem also in Python 3.6.8 and the new 3.8.0.
msg358966 - (view) Author: (xitop) Date: 2019-12-28 20:42
Please, could some experienced Python developer take a look at this issue reported more than 3 month ago?
msg381210 - (view) Author: hongweipeng (hongweipeng) * Date: 2020-11-17 06:40
Class `Triffid` failed to create in `__init_subclass__`, so `Triffid._abc_cache` uses its base class `_abc_cache`.
msg383796 - (view) Author: (xitop) Date: 2020-12-26 07:40
Python 3.9.1 is affected too.
msg401784 - (view) Author: (xitop) Date: 2021-09-14 16:03
2nd anniversary. Any reaction from developers would be appreciated. Thank you.
msg401998 - (view) Author: Raymond Hettinger (rhettinger) * (Python committer) Date: 2021-09-17 02:24
Confirmed that this weird behavior is still present.  Am not sure when someone will have the time and inclination to drill into the cause.
Date User Action Args
2021-09-17 02:24:12rhettingersetnosy: + rhettinger
messages: + msg401998
2021-09-17 02:22:48rhettingersetnosy: + lukasz.langa
2021-09-14 16:03:49xitopsetmessages: + msg401784
2020-12-26 07:40:18xitopsetmessages: + msg383796
versions: + Python 3.9
2020-11-17 06:40:22hongweipengsetnosy: + hongweipeng
messages: + msg381210
2019-12-28 20:42:03xitopsetmessages: + msg358966
2019-10-21 10:05:54xitopsetmessages: + msg355037
versions: + Python 3.6, Python 3.8
2019-09-10 09:09:41xitopcreate