Calling `help` executes @classmethod @property decorated methods

[randolf-scholz]

BPO	45356
Nosy	@rhettinger, @terryjreedy, @ambv, @berkerpeksag, @serhiy-storchaka, @wimglenn, @corona10, @pablogsal, @akulakov, @sirosen, @randolf-scholz, @AlexWaygood
PRs	bpo-45356: fix incorrect access of class property in pydoc and inspect #29239
Files	classmethod_property.py classmethod_property.py ClassPropertyIdea.py

^{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 = 'https://github.com/rhettinger'
closed_at = None
created_at = <Date 2021-10-03.19:45:55.963>
labels = ['type-bug', '3.11']
title = 'Calling `help` executes @classmethod @property decorated methods'
updated_at = <Date 2022-02-17.20:03:40.349>
user = 'https://github.com/randolf-scholz'

bugs.python.org fields:

activity = <Date 2022-02-17.20:03:40.349>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-10-03.19:45:55.963>
creator = 'randolf.scholz'
dependencies = []
files = ['50325', '50326', '50344']
hgrepos = []
issue_num = 45356
keywords = ['patch']
message_count = 23.0
messages = ['403109', '403110', '403111', '403504', '403505', '403578', '403582', '403611', '403613', '403653', '403699', '403713', '405100', '405115', '405121', '405142', '405143', '406600', '406605', '406606', '407493', '407499', '413451']
nosy_count = 13.0
nosy_names = ['rhettinger', 'terry.reedy', 'grahamd', 'lukasz.langa', 'berker.peksag', 'serhiy.storchaka', 'wim.glenn', 'corona10', 'pablogsal', 'andrei.avk', 'sirosen0', 'randolf.scholz', 'AlexWaygood']
pr_nums = ['29239']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45356'
versions = ['Python 3.11']

Linked PRs

• gh-89519: Remove classmethod descriptor chaining, deprecated since 3.11 #110163

[randolf-scholz]

I noticed some strange behaviour when calling help on a class inheriting from a class or having itself @classmethod @Property decorated methods.

from time import sleep
from abc import ABC, ABCMeta, abstractmethod

class MyMetaClass(ABCMeta):
 @classmethod
 @property
 def expensive_metaclass_property(cls):
 """This may take a while to compute!"""
 print("computing metaclass property"); sleep(3)
 return "Phew, that was a lot of work!"

 
class MyBaseClass(ABC, metaclass=MyMetaClass):
 @classmethod
 @property
 def expensive_class_property(cls):
 """This may take a while to compute!"""
 print("computing class property .."); sleep(3)
 return "Phew, that was a lot of work!"
 
 @property
 def expensive_instance_property(self):
 """This may take a while to compute!"""
 print("computing instance property ..."); sleep(3)
 return "Phew, that was a lot of work!"

class MyClass(MyBaseClass):
 """Some subclass of MyBaseClass"""
 
help(MyClass)

Calling help(MyClass) will cause expensive_class_property to be executed 4 times (!)

The other two properties, expensive_instance_property and expensive_metaclass_property are not executed.

Secondly, only expensive_instance_property is listed as a read-only property; expensive_class_property is listed as a classmethod and expensive_metaclass_property is unlisted.

The problem is also present in '3.10.0rc2 (default, Sep 28 2021, 17:57:14) [GCC 10.2.1 20210110]'

Stack Overflow thread: https://stackoverflow.com/questions/69426309

[randolf-scholz]

I updated the script with dome more info. The class-property gets actually executed 5 times when calling help(MyClass)

Computing class property of <class '__main__.MyClass'> ...DONE!
Computing class property of <class '__main__.MyClass'> ...DONE!
Computing class property of <class '__main__.MyClass'> ...DONE!
Computing class property of <class '__main__.MyBaseClass'> ...DONE!
Computing class property of <class '__main__.MyClass'> ...DONE!

[AlexWaygood]

See also: https://bugs.python.org/issue44904

[terryjreedy]

Randolf, what specific behaviors do you consider to be bugs that should be fixed. What would a test of the the changed behavior look like?

This should perhaps be closed as a duplicate of bpo-44904. Randolf, please check and say what you thing.

[terryjreedy]

On current 3.9, 3.10, 3.11, on Windows running in IDLE, I see
computing class property ..
computing class property ..
computing class property ..
computing class property ..
computing class property ..
Help ...

[randolf-scholz]

@terry I think the problem boils down to the fact that @classmethod @property decorated methods end up not being real properties.

Calling MyBaseClass.__dict__ reveals:

mappingproxy({'__module__': '__main__',
 'expensive_class_property': <classmethod at 0x7f893e95dd60>,
 'expensive_instance_property': <property at 0x7f893e8a5860>,
 '__dict__': <attribute '__dict__' of 'MyBaseClass' objects>,
 '__weakref__': <attribute '__weakref__' of 'MyBaseClass' objects>,
 '__doc__': None,
 '__abstractmethods__': frozenset(),
 '_abc_impl': <_abc._abc_data at 0x7f893fb98740>})

Two main issues:

1. Any analytics or documentation tool that has special treatment for properties may not identify 'expensive_class_property' as a property if they simply check via isinstance(func, property). Of course, this could be fixed by the tool-makers by doing a conditional check:

isinstance(func, property) or (`isinstance(func, classmethod)` and `isinstance(func.__func__, property)`

2. expensive_class_property does not implement getter, setter, deleter. This seems to be the real dealbreaker, for example, if we do

MyBaseClass.expensive_class_property = 2
MyBaseClass().expensive_instance_property = 2

Then the first line erroneously executes, such that MyBaseClass.dict['expensive_class_property'] is now int instead of classmethod, while the second line correctly raises AttributeError: can't set attribute since the setter method is not implemented.

[randolf-scholz]

If fact, in the current state it seem that it is impossible to implement real class-properties, for a simple reason:

descriptor.__set__ is only called when setting the attribute of an instance, but not of a class!!

import math

class TrigConst: 
 const = math.pi
 def __get__(self, obj, objtype=None):
 print("__get__ called")
 return self.const
 
 def __set__(self, obj, value):
 print("__set__ called")
 self.const = value
 

class Trig:
 const = TrigConst() # Descriptor instance

Trig().const # calls TrigConst.__get__
Trig().const = math.tau # calls TrigConst.__set__
Trig.const # calls TrigConst.__get__
Trig.const = math.pi # overwrites TrigConst attribute with float.

[rhettinger]

I'm merging bpo-44904 into this one because they are related.