Fixed a bug that caused newlines on Windows to be \r\r\n in ASCII tables

CHANGES.rst

@@ -360,6 +360,9 @@ astropy.io.ascii
   method.  This can make writing ECSV significantly faster if the data do not
   actually have any masked values. [#8447]
 
+- Fixed a bug that caused newlines to be incorrect when writing out ASCII tables
+  on Windows (they were ``\r\r\n`` instead of ``\r\n``). [#8659]
+
 astropy.io.misc
 ^^^^^^^^^^^^^^^
 

astropy/io/ascii/cparser.pyx

@@ -1020,7 +1020,10 @@ cdef class FastWriter:
         opened_file = False
 
         if not hasattr(output, 'write'): # output is a filename
-            output = open(output, 'w')
+            # NOTE: we need to specify newline='', otherwise the default
+            # behavior is for Python to translate \r\n (which we write because
+            # of os.linesep) into \r\r\n. Specifying newline='' disables any
+            output = open(output, 'w', newline='')
             opened_file = True # remember to close file afterwards
         writer = core.CsvWriter(output,
                                 delimiter=self.delimiter,

astropy/io/ascii/tests/test_write.py

@@ -750,3 +750,21 @@ def test_roundtrip_masked(fmt_name_class):
     for col, col2 in zip(t.itercols(), t2.itercols()):
         assert col.dtype.kind == col2.dtype.kind
         assert np.all(col == col2)
+
+
+@pytest.mark.parametrize("fast_writer", [True, False])
+def test_write_newlines(fast_writer, tmpdir):
+
+    # Regression test for https://github.com/astropy/astropy/issues/5126
+    # On windows, when writing to a filename (not e.g. StringIO), newlines were
+    # \r\r\n instead of \r\n.
+
+    filename = tmpdir.join('test').strpath
+
+    t = table.Table([['a', 'b', 'c']], names=['col'])
+    ascii.write(t, filename, fast_writer=fast_writer)
+
+    with open(filename, 'r', newline='') as f:
+        content = f.read()
+
+    assert content == os.linesep.join(['col', 'a', 'b', 'c']) + os.linesep

astropy/io/ascii/ui.py

@@ -798,7 +798,11 @@ def write(table, output=None, format=None, Writer=None, fast_writer=True, *,
     # Write the lines to output
     outstr = os.linesep.join(lines)
     if not hasattr(output, 'write'):
-        output = open(output, 'w')
+        # NOTE: we need to specify newline='', otherwise the default
+        # behavior is for Python to translate \r\n (which we write because
+        # of os.linesep) into \r\r\n. Specifying newline='' disables any
+        # auto-translation.
+        output = open(output, 'w', newline='')
         output.write(outstr)
         output.write(os.linesep)
         output.close()