csv.Dialect: Defaults are documented but not correctly implemented

Empieza por los puntos de entrada de csv.py para csv.Dialect y csv.excel; después, ejecuta el reproducer del issue para observar los valores predeterminados actuales y el error. Se considerará terminado cuando los valores predeterminados documentados de Dialect se comporten como se describe sin romper el comportamiento existente de csv; gh-100336 ya está vinculado como trabajo relacionado con este issue.

Bug report

In gh-68335, it was reported that the default values for csv.Dialect were not documented properly. They are, as pointed out by @smontanaro there, at https://docs.python.org/2/library/csv.html#dialects-and-formatting-parameters -- and still are for Python 3.11.

However, while I was attempting to fix gh-73138, I discovered that all arguments of csv.Dialect are None and even trigger unexpected behavior (if following the documentation):

import csv
class mydialect(csv.Dialect):
    quotechar = ":"
mydialect()

raises a _csv.Error caused by a TypeError, instead of using the documented default delimiter ,.

Traceback (most recent call last):
  File "/opt/homebrew/Cellar/python@3.11/3.11.0/Frameworks/Python.framework/Versions/3.11/lib/python3.11/csv.py", line 49, in _validate
    _Dialect(self)
TypeError: "delimiter" must be string, not NoneType

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/homebrew/Cellar/python@3.11/3.11.0/Frameworks/Python.framework/Versions/3.11/lib/python3.11/csv.py", line 45, in __init__
    self._validate()
  File "/opt/homebrew/Cellar/python@3.11/3.11.0/Frameworks/Python.framework/Versions/3.11/lib/python3.11/csv.py", line 52, in _validate
    raise Error(str(e))
_csv.Error: "delimiter" must be string, not NoneType

The discrepancy is, that the documented default values are not for the csv.Dialect class, but for its child csv.excel.
In gh-96842, a fix was prepared, improving the documentation to point this out.
I think a better fix would be to actually add the proper default values as they are written in the documentation.
The updated documentation in gh-96842 might be easily missed, and developers implementing a csv.Dialect would expect the defaults to be as documented, not as the csv.excel has them (if they want to inherit from csv.excel, they can still do so explicitly).

Your environment

• CPython versions tested on: Python 3.10, 3.11, and from the main branch `Python 3.12.0a3+
• Operating system and architecture: MacOS arm

Linked PRs

• gh-100336