[issue46938] dataclass __post_init__ recursion

[Eric V. Smith]

Eric V. Smith  added the comment:

Yeah, I've come to the conclusion that it's not so simple, either. I'm also 
thinking that advising to call the base __init__ is a mistake.

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[issue46938] dataclass __post_init__ recursion

[Bar Harel]

Bar Harel  added the comment:

Actually I'm not sure if the fix is so simple. What happens if B does not 
inherit from dataclass, but still inherits from A? Do we want to use the new 
__post_init__? I would assume we do, meaning we don't necessarily want to 
attach the __post_init__ to the dataclass, but to subclasses.

If so, the suggestion to call __init__() from __post_init__() might be the 
problematic part, and is what conceptually breaks the inheritance linearization.

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[issue46938] dataclass __post_init__ recursion

[Bar Harel]

Bar Harel  added the comment:

@Eric, I can easily fix it if you wish :-)

Just wondered if it's intended or not, as it looked like a bug but the 
documentation was somewhat endorsing it and I got confused.

Either case, a simple fix.

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[issue46938] dataclass __post_init__ recursion

[Eric V. Smith]

Eric V. Smith  added the comment:

I think this is a bug in the code. I'll have a PR ready shortly.

But since it's a non-trivial change, I'm going to target it for 3.11 only.

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assignee: docs@python -> eric.smith
versions:  -Python 3.10, Python 3.9

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[issue46938] dataclass __post_init__ recursion

[Alex Waygood]

Change by Alex Waygood :


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nosy: +eric.smith

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[issue46938] dataclass __post_init__ recursion

[Bar Harel]

New submission from Bar Harel :

Not sure if a continuance of https://bugs.python.org/issue44365 or not, but the 
suggestion to call super().__init__() in __post__init__ will cause infinite 
recursion if the superclass also contains __post__init__:

>>> @d
... class A:
...  test: int
...  def __post_init__(self):
...pass

>>> @d
... class B(A):
...  test2: int
...  def __post_init__(self):
...super().__init__(test=1)

>>> B(test2=1, test=3) <-- infinite recursion.

This is caused by line 564 
(https://github.com/python/cpython/blob/4716f70c8543d12d18c64677af650d479b99edac/Lib/dataclasses.py#L564)
 checking for post init on current class, and calling it on self (child class).

Not sure if the bug is in the documentation/suggestion, or if it's a bug in the 
implementation, in which case we need to call the current class's __post_init__.

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assignee: docs@python
components: Documentation, Library (Lib)
messages: 414613
nosy: bar.harel, docs@python
priority: normal
severity: normal
status: open
title: dataclass __post_init__ recursion
type: behavior
versions: Python 3.10, Python 3.11, Python 3.9

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