Csv sniffer doesn't attempt to determine and set escape character.

[evanw2]

BPO	39092
Nosy	@evanw2

^{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 = None
closed_at = None
created_at = <Date 2019-12-18.20:50:42.912>
labels = ['type-bug', 'library', '3.9']
title = "Csv sniffer doesn't attempt to determine and set escape character."
updated_at = <Date 2019-12-18.20:50:42.912>
user = 'https://github.com/evanw2'

bugs.python.org fields:

activity = <Date 2019-12-18.20:50:42.912>
actor = 'evan.whitfield'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-12-18.20:50:42.912>
creator = 'evan.whitfield'
dependencies = []
files = []
hgrepos = []
issue_num = 39092
keywords = []
message_count = 1.0
messages = ['358645']
nosy_count = 1.0
nosy_names = ['evan.whitfield']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39092'
versions = ['Python 3.9']

Linked PRs

• gh-83273: Rewrite csv.Sniffer dialect detection using trial parsing #153694

[evanw2]

I observed a false positive for the csv sniffer has_header method. (It thought there was a header when there was not.) This is due to the fact that in has_header, it determines the csv dialect by sniffing it, and failed to determine that the file I was using had an escape character of '\'. Since it doesn't set the escape character, it then incorrectly broke the first line of the file into columns, since it encountered an escaped quote within a quoted column, and treated that as the end of that column. (It correctly determined that the dialect wasn't doublequote, but apparently still needs to have the escape character set to handle an escaped quotechar.)

I think one (or both) of these things should be done here to avoid this false positive:
1.) Allow a dialect to be passed to has_header, so that someone could specify the escape character of the dialect if it were known.
2.) Allow the sniff method of the Sniffer class to detect and set the escapechar.