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bpo-30825: guess lineterminator in csv.Sniffer by vmax · Pull Request #2529 · python/cpython · GitHub

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bpo-30825: guess lineterminator in csv.Sniffer #2529

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bpo-30825: guess lineterminator in csv.Sniffer #2529

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6f46019
bpo-30825: guess lineterminator in csv.Sniffer
vmax Jul 1, 2017
524ff18
Fix unit test to cover weird or no terminator case (os.linesep)
vmax Feb 12, 2018
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24 changes: 19 additions & 5 deletions Lib/csv.py
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Expand Up @@ -13,6 +13,7 @@

from collections import OrderedDict
from io import StringIO
import os

__all__ = ["QUOTE_MINIMAL", "QUOTE_ALL", "QUOTE_NONNUMERIC", "QUOTE_NONE",
"Error", "Dialect", "__doc__", "excel", "excel_tab",
Expand Down Expand Up @@ -165,7 +166,7 @@ def writerows(self, rowdicts):

class Sniffer:
'''
"Sniffs" the format of a CSV file (i.e. delimiter, quotechar)
"Sniffs" the format of a CSV file (i.e. delimiter, quotechar, lineterminator)
Returns a Dialect object.
'''
def __init__(self):
Expand All @@ -178,18 +179,19 @@ def sniff(self, sample, delimiters=None):
Returns a dialect (or None) corresponding to the sample
"""

lineterminator = self._guess_lineterminator(sample)
quotechar, doublequote, delimiter, skipinitialspace = \
self._guess_quote_and_delimiter(sample, delimiters)
if not delimiter:
delimiter, skipinitialspace = self._guess_delimiter(sample,
delimiters)
delimiters,
lineterminator)

if not delimiter:
raise Error("Could not determine delimiter")

class dialect(Dialect):
_name = "sniffed"
lineterminator = '\r\n'
quoting = QUOTE_MINIMAL
# escapechar = ''

Expand All @@ -198,6 +200,7 @@ class dialect(Dialect):
# _csv.reader won't accept a quotechar of ''
dialect.quotechar = quotechar or '"'
dialect.skipinitialspace = skipinitialspace
dialect.lineterminator = lineterminator

return dialect

Expand Down Expand Up @@ -278,7 +281,7 @@ def _guess_quote_and_delimiter(self, data, delimiters):
return (quotechar, doublequote, delim, skipinitialspace)


def _guess_delimiter(self, data, delimiters):
def _guess_delimiter(self, data, delimiters, lineterminator):
"""
The delimiter /should/ occur the same number of times on
each row. However, due to malformed data, it may not. We don't want
Expand All @@ -297,7 +300,7 @@ def _guess_delimiter(self, data, delimiters):
additional chunks as necessary.
"""

data = list(filter(None, data.split('\n')))
data = list(filter(None, data.split(lineterminator)))

ascii = [chr(c) for c in range(127)] # 7-bit ASCII

Expand Down Expand Up @@ -447,3 +450,14 @@ def has_header(self, sample):
hasHeader -= 1

return hasHeader > 0


def _guess_lineterminator(self, sample):
# Guess line terminator based on presence
terminators = ('\r\n', '\n', '\r')

for terminator in terminators:
if terminator in sample:
return terminator

terryjreedy Feb 7, 2018

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

This is the core of the patch. The sequence looks right.

return os.linesep

terryjreedy Feb 7, 2018

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

In case user submits a sample with an unusual or no line terminator.
15 changes: 15 additions & 0 deletions Lib/test/test_csv.py
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@
import csv
import gc
import pickle
import os
from test import support
from itertools import permutations
from textwrap import dedent
Expand Down Expand Up @@ -1018,6 +1019,20 @@ def test_doublequote(self):
dialect = sniffer.sniff(self.sample9)
self.assertTrue(dialect.doublequote)

def test_guess_lineterminator(self):
sniffer = csv.Sniffer()
dialect = sniffer.sniff(r"Date;Value\r\n2010-01-01;10")
self.assertEqual(dialect.lineterminator, '\r\n')
dialect = sniffer.sniff(r"Date;Value\n2010-01-01;10")
self.assertEqual(dialect.lineterminator, '\n')

terryjreedy Feb 7, 2018

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

This assumes that the line terminator for multiline triple-quoted string literals is '\n'. We require that '\r\n' in code files be converted to '\n' before merging, but the test should not assume '\n' for all developer and user systems where the test might be run. So sample8 should be replaced by a literal.

Since the sample strings need not be long, they can all be literals defined here. Make all 3 the same except for the terminator. This will make the test both crystal clear and independent of system and optional local settings.
dialect = sniffer.sniff(r"Date;Value\r2010-01-01;10")
self.assertEqual(dialect.lineterminator, '\r')
dialect = sniffer.sniff(r"Date;Value\v2010-01-01;10")
self.assertEqual(dialect.lineterminator, os.linesep)
dialect = sniffer.sniff(r"Date;Value")
self.assertEqual(dialect.lineterminator, os.linesep)

terryjreedy Feb 7, 2018

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Add tests to cover the last line of _guess_lineterminator: return os.linesep. Add a test with the same literal, but with an oddball terminator, such as '\v' (vertical tab), and another with a single line with no control chars.

class NUL:
def write(s, *args):
pass
Expand Down
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2018-02-12-05-10-28.bpo-30825.t0RXql.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
csv.Sniffer now detects line terminator instead of defaulting to '\r\n'
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