CLN: applied jreback's request

* Requested in PR pandas-dev#21406

pandas/tests/frame/test_to_csv.py

@@ -882,21 +882,25 @@ def test_to_csv_line_terminators(self):
                        index=['one', 'two', 'three'])
 
         with ensure_clean() as path:
+            # case 1: CRLF as line terminator
             df.to_csv(path, line_terminator='\r\n')
 
             expected = (b',A,B\r\none,1,4\r\ntwo,2,5\r\nthree,3,6\r\n')
             with open(path, mode='rb') as f:
                 assert f.read() == expected
 
+        with ensure_clean() as path:
+            # case 2: LF as line terminator
             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
 
+        with ensure_clean() as path:
+            # case 3: The default line terminator(=os.linesep)(PR 21406)
+            df.to_csv(path)
             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:

pandas/tests/io/formats/test_to_csv.py

@@ -350,9 +350,9 @@ 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:
+            # case 1: The default line terminator(=os.linesep)(PR 21406)
+            os_linesep = os.linesep.encode('utf-8')
             expected_noarg = (
                 b'int,str_lf' + os_linesep +
                 b'1,abc' + os_linesep +
@@ -364,6 +364,8 @@ def test_to_csv_string_with_lf(self):
             with open(path, 'rb') as f:
                 assert f.read() == expected_noarg
 
+        with tm.ensure_clean('lf_test.csv') as path:
+            # case 2: LF as line terminator
             expected_lf = (
                 b'int,str_lf\n'
                 b'1,abc\n'
@@ -375,6 +377,8 @@ def test_to_csv_string_with_lf(self):
             with open(path, 'rb') as f:
                 assert f.read() == expected_lf
 
+        with tm.ensure_clean('lf_test.csv') as path:
+            # case 3: CRLF as line terminator
             # 'line_terminator' should not change inner element
             expected_crlf = (
                 b'int,str_lf\r\n'
@@ -395,9 +399,9 @@ 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:
+            # case 1: The default line terminator(=os.linesep)(PR 21406)
+            os_linesep = os.linesep.encode('utf-8')
             expected_noarg = (
                 b'int,str_crlf' + os_linesep +
                 b'1,abc' + os_linesep +
@@ -409,6 +413,8 @@ def test_to_csv_string_with_crlf(self):
             with open(path, 'rb') as f:
                 assert f.read() == expected_noarg
 
+        with tm.ensure_clean('crlf_test.csv') as path:
+            # case 2: LF as line terminator
             expected_lf = (
                 b'int,str_crlf\n'
                 b'1,abc\n'
@@ -420,6 +426,9 @@ def test_to_csv_string_with_crlf(self):
             with open(path, 'rb') as f:
                 assert f.read() == expected_lf
 
+        with tm.ensure_clean('crlf_test.csv') as path:
+            # case 3: CRLF as line terminator
+            # 'line_terminator' should not change inner element
             expected_crlf = (
                 b'int,str_crlf\r\n'
                 b'1,abc\r\n'
@@ -446,7 +455,7 @@ def test_to_csv_stdout_file(self):
         assert not sys.stdout.closed
 
     @pytest.mark.xfail(
-        os.name == 'nt',
+        compat.is_platform_windows(),
         reason=("Especially in Windows, file stream should not be passed"
                 "to csv writer without newline='' option."
                 "(https://docs.python.org/3.6/library/csv.html#csv.writer)"))

pandas/util/testing.py

@@ -2804,7 +2804,21 @@ def skipna_wrapper(x):
 
 
 def convert_rows_list_to_csv_str(rows_list):
-    csv_str = ''
-    for row in rows_list:
-        csv_str += row + os.linesep
-    return csv_str
+    """
+    Convert list of rows of csv to a single
+    csv-formatted string that fits the current OS.
+    This method is used for creating expected value of to_csv() method.
+
+    Parameters
+    ----------
+    rows_list : list
+        The list of string. Each element represents the row of csv.
+
+    Returns
+    -------
+    expected : string
+        Expected output of to_csv() in current OS
+    """
+    sep = os.linesep
+    expected = sep.join(rows_list) + sep
+    return expected