RecursionError when using property() inside classes

[cheny0y0]

BPO	46776
Nosy	@stevendaprano, @cheny0y0

^{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 = None
closed_at = <Date 2022-02-17.13:13:30.966>
created_at = <Date 2022-02-17.12:48:24.227>
labels = ['interpreter-core', 'type-bug', '3.8', '3.9', '3.10', '3.11', '3.7', 'invalid']
title = 'RecursionError when using property() inside classes'
updated_at = <Date 2022-02-17.13:18:51.286>
user = 'https://github.com/cheny0y0'

bugs.python.org fields:

activity = <Date 2022-02-17.13:18:51.286>
actor = 'steven.daprano'
assignee = 'none'
closed = True
closed_date = <Date 2022-02-17.13:13:30.966>
closer = 'steven.daprano'
components = ['Interpreter Core']
creation = <Date 2022-02-17.12:48:24.227>
creator = 'prasechen'
dependencies = []
files = []
hgrepos = []
issue_num = 46776
keywords = []
message_count = 3.0
messages = ['413403', '413406', '413407']
nosy_count = 2.0
nosy_names = ['steven.daprano', 'prasechen']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46776'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']

[cheny0y0]

A simple class definition:
class Foo: bar = property(lambda self: self.bar)
And get the value of Foo.bar, it returns correctly, <property object at 0x**********>.
And get the value of Foo().bar, it raises RecursionError:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <lambda>
  File "<stdin>", line 1, in <lambda>
  File "<stdin>", line 1, in <lambda>
  [Previous line repeated 996 more times]
RecursionError: maximum recursion depth exceeded

[stevendaprano]

This is not a bug, Python is working correctly here. The interpreter is doing exactly what you told it to do, and then raising a RecursionError before you can crash the system.

You have a property instance.bar which returns instance.bar, which returns instance.bar, which returns instance.bar... forever, or until RecursionError is triggered.

Your class is the same as:

class Foo:
    @property
    def bar(self):
        return self.bar

So when you call instance.bar, it looks up bar, which is the property, which looks up bar, which is the property, which looks up bar, and so on.

What did you expect to happen?

[stevendaprano]

Maybe you expected to do this:

class C:
    def __init__(self):
        self._value = 999

    @property
    def bar(self):
        return self._value

obj = C()
obj.bar  # returns 999