Bad dataclass post-init example

[MicaelJarniac]

BPO	44365
Nosy	@ericvsmith, @akulakov, @MicaelJarniac
Files	dataclass_inheritance_test.py: Example of Data Class Inheritance dataclass_inheritance_v2_test.py: Example of Data Class Inheritance (Updated) dataclass_inheritance_v3_test.py: Example of Data Class Inheritance (Updated, Version 3)

^{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/ericvsmith'
closed_at = None
created_at = <Date 2021-06-09.15:33:08.429>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.11', 'docs']
title = 'Bad dataclass post-init example'
updated_at = <Date 2021-10-31.17:30:04.410>
user = 'https://github.com/MicaelJarniac'

bugs.python.org fields:

activity = <Date 2021-10-31.17:30:04.410>
actor = 'da2ce7'
assignee = 'eric.smith'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2021-06-09.15:33:08.429>
creator = 'MicaelJarniac'
dependencies = []
files = ['50414', '50415', '50416']
hgrepos = []
issue_num = 44365
keywords = []
message_count = 12.0
messages = ['395427', '395449', '395537', '395541', '395573', '395596', '395861', '395999', '396029', '405406', '405408', '405410']
nosy_count = 5.0
nosy_names = ['eric.smith', 'docs@python', 'andrei.avk', 'MicaelJarniac', 'da2ce7']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44365'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']

Linked PRs

• gh-88531 Fix dataclass __post_init__/__init__ interplay documentation #107404
• [3.12] gh-88531 Fix dataclass __post_init__/__init__ interplay documentation (gh-107404) #114162
• [3.11] gh-88531 Fix dataclass __post_init__/__init__ interplay documentation (gh-107404) #114163

[MicaelJarniac]

https://docs.python.org/3/library/dataclasses.html#post-init-processing

https://github.com/python/cpython/blob/3.9/Doc/library/dataclasses.rst#post-init-processing

In the example, a base class "Rectangle" is defined, and then a "Square" class inherits from it.

On reading the example, it seems like the Square class is meant to be used like:

>> square = Square(5)

Since the Square class seems to be supposed to be a "shortcut" to creating a Rectangle with equal sides.

However, the Rectangle class has two required init arguments, and when Square inherits from it, those arguments are still required, so using Square like in the above example, with a single argument, results in an error:

>>> square = Square(5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() missing 2 required positional arguments: 'width' and 'side'

To "properly" use the Square class, it'd need to be instantiated like so:

>>> square = Square(0, 0, 5)
>>> square
Square(height=5, width=5, side=5)

Which, in my opinion, is completely counter-intuitive, and basically invalidates this example.

[ericvsmith]

Agreed that that's not a good (or even workable) example. Thanks for pointing it out.

I'll come up with something better.