gh-80678: Add comment on preferred csv delimiters

[slateny]

https://docs.python.org/dev/library/csv.html#csv.Sniffer.sniff

See

cpython/Lib/csv.py

Line 177 in 1499d73

self.preferred= [',', '\t', ';', ' ', ':']

for the list of preferred delimiters, and see

cpython/Lib/csv.py

Lines 369 to 375 in 1499d73

	# if there's more than one, fall back to a 'preferred' list
	iflen(delims) >1:
	fordinself.preferred:
	ifdindelims.keys():
	skipinitialspace= (data[0].count(d) ==
	data[0].count("%c "%d))
	return (d, skipinitialspace)

for choosing the preferred delimiters

• Issue: Undocumented behavior in csv.Sniffer (preferred delimiters) #80678

[eendebakpt]

Why is the documentation here not added to the def sniff docstring? That would be a natural place for documentation that is available from the command line or IDE.

[slateny]

I'm not sure what information should go into the docstring vs in the docs. I imagine there's some duplicate, but surely not everything in the docs goes into the docstring and vice-versa. The docstring of sniff is looking a bit empty though, so it could use some work for sure.

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[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[StanFromIreland]

This is a little inaccurate, it is in that specific order. Maybe something like ",, ... : in that order"

[serhiy-storchaka]

Thank you for the patch, and sorry that it was left without attention for so long.

I documented this in GH-154336, with the order made explicit and with a note that the preference wins no matter how many times each delimiter occurs, which is the case reported in the issue.