API: read_csv, to_csv line_terminator keyword inconsistency

pandas/core/generic.py

@@ -3004,6 +3004,7 @@ def to_csv(
         quoting: Optional[int] = None,
         quotechar: str = '"',
         line_terminator: Optional[str] = None,
+        lineterminator: Optional[str] = None,
         chunksize: Optional[int] = None,
         date_format: Optional[str] = None,
         doublequote: bool_t = True,
@@ -3144,7 +3145,7 @@ def to_csv(
         formatter = CSVFormatter(
             df,
             path_or_buf,
-            line_terminator=line_terminator,
+            line_terminator=line_terminator or lineterminator,
             sep=sep,
             encoding=encoding,
             errors=errors,

pandas/io/parsers.py

@@ -579,6 +579,7 @@ def read_csv(
     compression="infer",
     thousands=None,
     decimal: str = ".",
+    line_terminator=None,
     lineterminator=None,
     quotechar='"',
     quoting=csv.QUOTE_MINIMAL,
@@ -643,7 +644,7 @@ def read_csv(
         quotechar=quotechar,
         quoting=quoting,
         skipinitialspace=skipinitialspace,
-        lineterminator=lineterminator,
+        lineterminator=lineterminator or line_terminator,  # GH 9568
         header=header,
         index_col=index_col,
         names=names,

pandas/tests/frame/test_to_csv.py

@@ -1,6 +1,7 @@
 import csv
 from io import StringIO
 import os
+import re
 
 import numpy as np
 import pytest
@@ -998,6 +999,45 @@ def test_to_csv_line_terminators(self):
             with open(path, mode="rb") as f:
                 assert f.read() == expected
 
+    def test_to_csv_lineterminator_alternative_args(self):
+        # GH 9568
+        # examples from test_to_csv_line_terminators
+        # test equivalence of line_terminator vs. lineterminator keyword args
+
+        df = DataFrame({"A": [1, 2, 3], "B": [4, 5, 6]}, index=["one", "two", "three"])
+
+        # case 1: CRLF as line terminator
+
+        with tm.ensure_clean() as path:
+            df.to_csv(path, line_terminator="\r\n")
+
+            with open(path, mode="rb") as f:
+                res_line_terminator = f.read()
+
+        with tm.ensure_clean() as path:
+            df.to_csv(path, lineterminator="\r\n")
+
+            with open(path, mode="rb") as f:
+                res_lineterminator = f.read()
+
+        assert re.match(res_line_terminator, res_lineterminator)
+
+        # case 2: LF as line terminator
+
+        with tm.ensure_clean() as path:
+            df.to_csv(path, line_terminator="\n")
+
+            with open(path, mode="rb") as f:
+                res_line_terminator = f.read()
+
+        with tm.ensure_clean() as path:
+            df.to_csv(path, lineterminator="\n")
+
+            with open(path, mode="rb") as f:
+                res_lineterminator = f.read()
+
+        assert re.match(res_line_terminator, res_lineterminator)
+
     def test_to_csv_from_csv_categorical(self):
 
         # CSV with categoricals should result in the same output

pandas/tests/io/formats/test_to_csv.py

@@ -1,5 +1,6 @@
 import io
 import os
+import re
 import sys
 
 import numpy as np
@@ -330,10 +331,15 @@ def test_to_csv_multi_index(self):
     @pytest.mark.parametrize("klass", [pd.DataFrame, pd.Series])
     def test_to_csv_single_level_multi_index(self, ind, expected, klass):
         # see gh-19589
-        result = klass(pd.Series([1], ind, name="data")).to_csv(
+        # GH9568 test for equivalence between line_terminator and lineterminator
+        result_line_terminator = klass(pd.Series([1], ind, name="data")).to_csv(
             line_terminator="\n", header=True
         )
-        assert result == expected
+        result_lineterminator = klass(pd.Series([1], ind, name="data")).to_csv(
+            lineterminator="\n", header=True
+        )
+        assert re.match(result_lineterminator, result_line_terminator)
+        assert re.match(result_line_terminator, expected)
 
     def test_to_csv_string_array_ascii(self):
         # GH 10813
@@ -436,6 +442,25 @@ def test_to_csv_string_with_crlf(self):
             with open(path, "rb") as f:
                 assert f.read() == expected_crlf
 
+    def test_to_csv_string_line_terminator_alternative_args(self):
+        # GH 9568
+        # test equivalence of line_terminator vs. lineterminator keyword args
+
+        data = {"int": [1, 2, 3], "str_lf": ["abc", "d\nef", "g\nh\n\ni"]}
+        df = pd.DataFrame(data)
+
+        with tm.ensure_clean("crlf_test.csv") as path:
+            df.to_csv(path, line_terminator="\n", index=False)
+            with open(path, "rb") as f:
+                res_line_terminator = f.read()
+
+        with tm.ensure_clean("crlf_test.csv") as path:
+            df.to_csv(path, lineterminator="\n", index=False)
+            with open(path, "rb") as f:
+                res_lineterminator = f.read()
+
+        assert re.match(res_line_terminator, res_lineterminator)
+
     def test_to_csv_stdout_file(self, capsys):
         # GH 21561
         df = pd.DataFrame(