csv.Dialect: Defaults are documented but not correctly implemented

Start with the csv.py entry points for csv.Dialect and csv.excel, then run the reproducer from the issue to observe the current defaults and error. Done means the documented Dialect defaults behave as described without breaking existing csv behavior; gh-100336 is already linked as work on this 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