CHANGE: changed default value of line_terminator to os.linesep

* re-defined testcases that suits conversations in PR pandas-dev#21406
* changed default value of line_terminator to os.linesep
* changed API document of DataFrame.to_csv
* changed "newline" value of "open()" from '\n' to ''
* Updated whatsnew document

related pages:
* Issue pandas-dev#20353
* PR pandas-dev#21406

Author: deflatSOCO

doc/source/whatsnew/v0.24.0.txt

@@ -23,7 +23,8 @@ Other Enhancements
 
 Backwards incompatible API changes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
+- When file path is given by string format in :func:`DataFrame.to_csv`, it disables universal newline support.
+  - Instead, default value of `line_terminator` in :func:`DataFrame.to_csv` has changed from `'\n'` to `os.linesep`(:issue:`20353`)
 .. _whatsnew_0240.api.datetimelike:
 
 Datetimelike API Changes

pandas/core/frame.py

@@ -1653,7 +1653,7 @@ def to_panel(self):
     def to_csv(self, path_or_buf=None, sep=",", na_rep='', float_format=None,
                columns=None, header=True, index=True, index_label=None,
                mode='w', encoding=None, compression=None, quoting=None,
-               quotechar='"', line_terminator='\n', chunksize=None,
+               quotechar='"', line_terminator=None, chunksize=None,
                tupleize_cols=None, date_format=None, doublequote=True,
                escapechar=None, decimal='.'):
         r"""Write DataFrame to a comma-separated values (csv) file
@@ -1690,9 +1690,10 @@ def to_csv(self, path_or_buf=None, sep=",", na_rep='', float_format=None,
         compression : string, optional
             A string representing the compression to use in the output file.
             Allowed values are 'gzip', 'bz2', 'zip', 'xz'.
-        line_terminator : string, default ``'\n'``
+        line_terminator : string, optional
             The newline character or character sequence to use in the output
-            file
+            file. Defaults to `os.linesep`, which depends on the OS in which
+            this method is called ('\n' for linux, '\r\n' for Windows, i.e.).
         quoting : optional constant from csv module
             defaults to csv.QUOTE_MINIMAL. If you have set a `float_format`
             then floats are converted to strings and thus csv.QUOTE_NONNUMERIC

pandas/io/common.py

@@ -399,10 +399,10 @@ def _get_handle(path_or_buf, mode, encoding=None, compression=None,
             f = open(path_or_buf, mode)
         elif encoding:
             # Python 3 and encoding
-            f = open(path_or_buf, mode, encoding=encoding, newline='\n')
+            f = open(path_or_buf, mode, encoding=encoding, newline='')
         elif is_text:
             # Python 3 and no explicit encoding
-            f = open(path_or_buf, mode, errors='replace', newline='\n')
+            f = open(path_or_buf, mode, errors='replace', newline='')
         else:
             # Python 3 and binary mode
             f = open(path_or_buf, mode)

pandas/io/formats/csvs.py

@@ -7,6 +7,7 @@
 
 import csv as csvlib
 import numpy as np
+import os
 
 from pandas.core.dtypes.missing import notna
 from pandas.core.dtypes.inference import is_file_like
@@ -61,6 +62,8 @@ def __init__(self, obj, path_or_buf=None, sep=",", na_rep='',
         self.doublequote = doublequote
         self.escapechar = escapechar
 
+        if line_terminator is None:
+            line_terminator = os.linesep
         self.line_terminator = line_terminator
 
         self.date_format = date_format

pandas/tests/frame/test_to_csv.py

@@ -4,6 +4,7 @@
 
 import csv
 import pytest
+import os
 
 from numpy import nan
 import numpy as np
@@ -882,12 +883,20 @@ def test_to_csv_line_terminators(self):
             with open(path, mode='rb') as f:
                 assert f.read() == expected
 
-            df.to_csv(path)  # The default line terminator remains \n
+            df.to_csv(path, line_terminator='\n')
 
             expected = (b',A,B\none,1,4\ntwo,2,5\nthree,3,6\n')
             with open(path, mode='rb') as f:
                 assert f.read() == expected
 
+            os_linesep = os.linesep.encode('utf-8')
+            df.to_csv(path)  # The default line terminator=os.linesep(PR 21406)
+
+            expected = (b',A,B' + os_linesep + b'one,1,4' + os_linesep +
+                        b'two,2,5' + os_linesep + b'three,3,6' + os_linesep)
+            with open(path, mode='rb') as f:
+                assert f.read() == expected
+
     def test_to_csv_from_csv_categorical(self):
 
         # CSV with categoricals should result in the same output as when one

pandas/tests/io/formats/test_to_csv.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 
 import sys
+import os
 import numpy as np
 import pandas as pd
 import pytest
@@ -294,20 +295,33 @@ def test_to_csv_string_with_lf(self):
         }
         df = pd.DataFrame(data)
 
+        os_linesep = os.linesep.encode('utf-8')
+
         with tm.ensure_clean('lf_test.csv') as path:
-            expected_bin = (
+            expected_noarg = (
+                b'int,str_lf' + os_linesep +
+                b'1,abc' + os_linesep +
+                b'2,"d\nef"' + os_linesep +
+                b'3,"g\nh\n\ni"' + os_linesep
+            )
+
+            df.to_csv(path, index=False)
+            with open(path, 'rb') as f:
+                assert f.read() == expected_noarg
+
+            expected_lf = (
                 b'int,str_lf\n'
                 b'1,abc\n'
                 b'2,"d\nef"\n'
                 b'3,"g\nh\n\ni"\n'
             )
 
-            df.to_csv(path, index=False)
+            df.to_csv(path, line_terminator='\n', index=False)
             with open(path, 'rb') as f:
-                assert f.read() == expected_bin
+                assert f.read() == expected_lf
 
             # 'line_terminator' should not change inner element
-            expected_bin = (
+            expected_crlf = (
                 b'int,str_lf\r\n'
                 b'1,abc\r\n'
                 b'2,"d\nef"\r\n'
@@ -316,7 +330,7 @@ def test_to_csv_string_with_lf(self):
 
             df.to_csv(path, line_terminator='\r\n', index=False)
             with open(path, 'rb') as f:
-                assert f.read() == expected_bin
+                assert f.read() == expected_crlf
 
     def test_to_csv_string_with_crlf(self):
         # GH 20353
@@ -326,19 +340,32 @@ def test_to_csv_string_with_crlf(self):
         }
         df = pd.DataFrame(data)
 
+        os_linesep = os.linesep.encode('utf-8')
+
         with tm.ensure_clean('crlf_test.csv') as path:
-            expected_bin = (
+            expected_noarg = (
+                b'int,str_crlf' + os_linesep +
+                b'1,abc' + os_linesep +
+                b'2,"d\r\nef"' + os_linesep +
+                b'3,"g\r\nh\r\n\r\ni"' + os_linesep
+            )
+
+            df.to_csv(path, index=False)
+            with open(path, 'rb') as f:
+                assert f.read() == expected_noarg
+
+            expected_lf = (
                 b'int,str_crlf\n'
                 b'1,abc\n'
                 b'2,"d\r\nef"\n'
                 b'3,"g\r\nh\r\n\r\ni"\n'
             )
 
-            df.to_csv(path, index=False)
+            df.to_csv(path, line_terminator='\n', index=False)
             with open(path, 'rb') as f:
-                assert f.read() == expected_bin
+                assert f.read() == expected_lf
 
-            expected_bin = (
+            expected_crlf = (
                 b'int,str_crlf\r\n'
                 b'1,abc\r\n'
                 b'2,"d\r\nef"\r\n'
@@ -347,4 +374,4 @@ def test_to_csv_string_with_crlf(self):
 
             df.to_csv(path, line_terminator='\r\n', index=False)
             with open(path, 'rb') as f:
-                assert f.read() == expected_bin
+                assert f.read() == expected_crlf