Backport PR pandas-dev#39253: REGR: codecs.open() is always opened in…

… text mode

doc/source/whatsnew/v1.2.1.rst

@@ -15,6 +15,7 @@ including other versions of pandas.
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
 - Fixed regression in :meth:`~DataFrame.to_csv` that created corrupted zip files when there were more rows than ``chunksize`` (:issue:`38714`)
+- Fixed regression in :meth:`~DataFrame.to_csv` opening ``codecs.StreamReaderWriter`` in binary mode instead of in text mode (:issue:`39247`)
 - Fixed regression in :meth:`read_csv` and other read functions were the encoding error policy (``errors``) did not default to ``"replace"`` when no encoding was specified (:issue:`38989`)
 - Fixed regression in :func:`read_excel` with non-rawbyte file handles (:issue:`38788`)
 - Fixed regression in :meth:`DataFrame.to_stata` not removing the created file when an error occured (:issue:`39202`)

pandas/io/common.py

@@ -1,6 +1,7 @@
 """Common IO api utilities"""
 
 import bz2
+import codecs
 from collections import abc
 import dataclasses
 import gzip
@@ -844,9 +845,12 @@ def file_exists(filepath_or_buffer: FilePathOrBuffer) -> bool:
 
 def _is_binary_mode(handle: FilePathOrBuffer, mode: str) -> bool:
     """Whether the handle is opened in binary mode"""
+    # classes that expect string but have 'b' in mode
+    text_classes = (codecs.StreamReaderWriter,)
+    if isinstance(handle, text_classes):
+        return False
+
     # classes that expect bytes
-    binary_classes = [BufferedIOBase, RawIOBase]
+    binary_classes = (BufferedIOBase, RawIOBase)
 
-    return isinstance(handle, tuple(binary_classes)) or "b" in getattr(
-        handle, "mode", mode
-    )
+    return isinstance(handle, binary_classes) or "b" in getattr(handle, "mode", mode)

pandas/tests/io/test_common.py

@@ -1,6 +1,7 @@
 """
 Tests for the pandas.io.common functionalities
 """
+import codecs
 from io import StringIO
 import mmap
 import os
@@ -426,3 +427,19 @@ def test_default_errors():
         file = Path(path)
         file.write_bytes(b"\xe4\na\n1")
         tm.assert_frame_equal(pd.read_csv(file, skiprows=[0]), pd.DataFrame({"a": [1]}))
+
+
+@pytest.mark.parametrize("encoding", [None, "utf-8"])
+@pytest.mark.parametrize("format", ["csv", "json"])
+def test_codecs_encoding(encoding, format):
+    # GH39247
+    expected = tm.makeDataFrame()
+    with tm.ensure_clean() as path:
+        with codecs.open(path, mode="w", encoding=encoding) as handle:
+            getattr(expected, f"to_{format}")(handle)
+        with codecs.open(path, mode="r", encoding=encoding) as handle:
+            if format == "csv":
+                df = pd.read_csv(handle, index_col=0)
+            else:
+                df = pd.read_json(handle)
+    tm.assert_frame_equal(expected, df)