diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 4804ed2..0f182d4 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -40,6 +40,29 @@
 _CHECK_ERRORS = _IOBASE_EMITS_UNRAISABLE
 
 
+def text_encoding(encoding, stacklevel=2):
+    """
+    A helper function to choose the text encoding.
+
+    When encoding is not None, just return it.
+    Otherwise, return the default text encoding (i.e. "locale").
+
+    This function emits an EncodingWarning if *encoding* is None and
+    sys.flags.warn_default_encoding is true.
+
+    This can be used in APIs with an encoding=None parameter
+    that pass it to TextIOWrapper or open.
+    However, please consider using encoding="utf-8" for new APIs.
+    """
+    if encoding is None:
+        encoding = "locale"
+        if sys.flags.warn_default_encoding:
+            import warnings
+            warnings.warn("'encoding' argument not specified.",
+                          EncodingWarning, stacklevel + 1)
+    return encoding
+
+
 def open(file, mode="r", buffering=-1, encoding=None, errors=None,
          newline=None, closefd=True, opener=None):
 
@@ -248,6 +271,7 @@ def open(file, mode="r", buffering=-1, encoding=None, errors=None,
         result = buffer
         if binary:
             return result
+        encoding = text_encoding(encoding)
         text = TextIOWrapper(buffer, encoding, errors, newline, line_buffering)
         result = text
         text.mode = mode
@@ -2004,19 +2028,22 @@ class TextIOWrapper(TextIOBase):
     def __init__(self, buffer, encoding=None, errors=None, newline=None,
                  line_buffering=False, write_through=False):
         self._check_newline(newline)
-        if encoding is None:
+        encoding = text_encoding(encoding)
+
+        if encoding == "locale":
             try:
-                encoding = os.device_encoding(buffer.fileno())
+                encoding = os.device_encoding(buffer.fileno()) or "locale"
             except (AttributeError, UnsupportedOperation):
                 pass
-            if encoding is None:
-                try:
-                    import locale
-                except ImportError:
-                    # Importing locale may fail if Python is being built
-                    encoding = "ascii"
-                else:
-                    encoding = locale.getpreferredencoding(False)
+
+        if encoding == "locale":
+            try:
+                import locale
+            except ImportError:
+                # Importing locale may fail if Python is being built
+                encoding = "utf-8"
+            else:
+                encoding = locale.getpreferredencoding(False)
 
         if not isinstance(encoding, str):
             raise ValueError("invalid encoding: %r" % encoding)
diff --git a/Lib/bz2.py b/Lib/bz2.py
index ce07ebe..1da3ce6 100644
--- a/Lib/bz2.py
+++ b/Lib/bz2.py
@@ -311,6 +311,7 @@ def open(filename, mode="rb", compresslevel=9,
     binary_file = BZ2File(filename, bz_mode, compresslevel=compresslevel)
 
     if "t" in mode:
+        encoding = io.text_encoding(encoding)
         return io.TextIOWrapper(binary_file, encoding, errors, newline)
     else:
         return binary_file
diff --git a/Lib/configparser.py b/Lib/configparser.py
index 924cc56..3b4cb5e 100644
--- a/Lib/configparser.py
+++ b/Lib/configparser.py
@@ -690,6 +690,7 @@ def read(self, filenames, encoding=None):
         """
         if isinstance(filenames, (str, bytes, os.PathLike)):
             filenames = [filenames]
+        encoding = io.text_encoding(encoding)
         read_ok = []
         for filename in filenames:
             try:
diff --git a/Lib/gzip.py b/Lib/gzip.py
index 1369157..0a8993b 100644
--- a/Lib/gzip.py
+++ b/Lib/gzip.py
@@ -62,6 +62,7 @@ def open(filename, mode="rb", compresslevel=_COMPRESS_LEVEL_BEST,
         raise TypeError("filename must be a str or bytes object, or a file")
 
     if "t" in mode:
+        encoding = io.text_encoding(encoding)
         return io.TextIOWrapper(binary_file, encoding, errors, newline)
     else:
         return binary_file
diff --git a/Lib/io.py b/Lib/io.py
index fbce6ef..01f1df8 100644
--- a/Lib/io.py
+++ b/Lib/io.py
@@ -54,7 +54,7 @@
 from _io import (DEFAULT_BUFFER_SIZE, BlockingIOError, UnsupportedOperation,
                  open, open_code, FileIO, BytesIO, StringIO, BufferedReader,
                  BufferedWriter, BufferedRWPair, BufferedRandom,
-                 IncrementalNewlineDecoder, TextIOWrapper)
+                 IncrementalNewlineDecoder, text_encoding, TextIOWrapper)
 
 OpenWrapper = _io.open # for compatibility with _pyio
 
diff --git a/Lib/lzma.py b/Lib/lzma.py
index 0817b87..c8b1970 100644
--- a/Lib/lzma.py
+++ b/Lib/lzma.py
@@ -302,6 +302,7 @@ def open(filename, mode="rb", *,
                            preset=preset, filters=filters)
 
     if "t" in mode:
+        encoding = io.text_encoding(encoding)
         return io.TextIOWrapper(binary_file, encoding, errors, newline)
     else:
         return binary_file
diff --git a/Lib/pathlib.py b/Lib/pathlib.py
index 531a699..5c9284b 100644
--- a/Lib/pathlib.py
+++ b/Lib/pathlib.py
@@ -1241,6 +1241,8 @@ def open(self, mode='r', buffering=-1, encoding=None,
         Open the file pointed by this path and return a file object, as
         the built-in open() function does.
         """
+        if "b" not in mode:
+            encoding = io.text_encoding(encoding)
         return io.open(self, mode, buffering, encoding, errors, newline,
                        opener=self._opener)
 
@@ -1255,6 +1257,7 @@ def read_text(self, encoding=None, errors=None):
         """
         Open the file in text mode, read it, and close the file.
         """
+        encoding = io.text_encoding(encoding)
         with self.open(mode='r', encoding=encoding, errors=errors) as f:
             return f.read()
 
@@ -1274,6 +1277,7 @@ def write_text(self, data, encoding=None, errors=None, newline=None):
         if not isinstance(data, str):
             raise TypeError('data must be str, not %s' %
                             data.__class__.__name__)
+        encoding = io.text_encoding(encoding)
         with self.open(mode='w', encoding=encoding, errors=errors, newline=newline) as f:
             return f.write(data)
 
diff --git a/Lib/site.py b/Lib/site.py
index 5f1b31e..939893e 100644
--- a/Lib/site.py
+++ b/Lib/site.py
@@ -170,7 +170,9 @@ def addpackage(sitedir, name, known_paths):
     fullname = os.path.join(sitedir, name)
     _trace(f"Processing .pth file: {fullname!r}")
     try:
-        f = io.TextIOWrapper(io.open_code(fullname))
+        # locale encoding is not ideal especially on Windows. But we have used
+        # it for a long time. setuptools uses the locale encoding too.
+        f = io.TextIOWrapper(io.open_code(fullname), encoding="locale")
     except OSError:
         return
     with f:
diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 4b011e4..2b78549 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -693,7 +693,7 @@ def _use_posix_spawn():
 _USE_POSIX_SPAWN = _use_posix_spawn()
 
 
-class Popen(object):
+class Popen:
     """ Execute a child program in a new process.
 
     For a complete description of the arguments see the Python documentation.
@@ -844,6 +844,13 @@ def __init__(self, args, bufsize=-1, executable=None,
 
         self.text_mode = encoding or errors or text or universal_newlines
 
+        # PEP 597: We suppress the EncodingWarning in subprocess module
+        # for now (at Python 3.10), because we focus on files for now.
+        # This will be changed to encoding = io.text_encoding(encoding)
+        # in the future.
+        if self.text_mode and encoding is None:
+            self.encoding = encoding = "locale"
+
         # How long to resume waiting on a child after the first ^C.
         # There is no right value for this.  The purpose is to be polite
         # yet remain good for interactive users trying to exit a tool.
diff --git a/Lib/tempfile.py b/Lib/tempfile.py
index 4b2547c..efcf7a7 100644
--- a/Lib/tempfile.py
+++ b/Lib/tempfile.py
@@ -543,6 +543,9 @@ def NamedTemporaryFile(mode='w+b', buffering=-1, encoding=None,
     if _os.name == 'nt' and delete:
         flags |= _os.O_TEMPORARY
 
+    if "b" not in mode:
+        encoding = _io.text_encoding(encoding)
+
     (fd, name) = _mkstemp_inner(dir, prefix, suffix, flags, output_type)
     try:
         file = _io.open(fd, mode, buffering=buffering,
@@ -583,6 +586,9 @@ def TemporaryFile(mode='w+b', buffering=-1, encoding=None,
         """
         global _O_TMPFILE_WORKS
 
+        if "b" not in mode:
+            encoding = _io.text_encoding(encoding)
+
         prefix, suffix, dir, output_type = _sanitize_params(prefix, suffix, dir)
 
         flags = _bin_openflags
@@ -638,6 +644,7 @@ def __init__(self, max_size=0, mode='w+b', buffering=-1,
         if 'b' in mode:
             self._file = _io.BytesIO()
         else:
+            encoding = _io.text_encoding(encoding)
             self._file = _io.TextIOWrapper(_io.BytesIO(),
                             encoding=encoding, errors=errors,
                             newline=newline)
diff --git a/Lib/test/exception_hierarchy.txt b/Lib/test/exception_hierarchy.txt
index 763a6c8..6c5e821 100644
--- a/Lib/test/exception_hierarchy.txt
+++ b/Lib/test/exception_hierarchy.txt
@@ -61,4 +61,5 @@
            +-- ImportWarning
            +-- UnicodeWarning
            +-- BytesWarning
+           +-- EncodingWarning
            +-- ResourceWarning
diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
index 6833b25..646cd06 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -389,6 +389,7 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
 
         'site_import': 1,
         'bytes_warning': 0,
+        'warn_default_encoding': 0,
         'inspect': 0,
         'interactive': 0,
         'optimization_level': 0,
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 3768b62..c731302 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -4249,6 +4249,29 @@ def test_check_encoding_errors(self):
         proc = assert_python_failure('-X', 'dev', '-c', code)
         self.assertEqual(proc.rc, 10, proc)
 
+    def test_check_encoding_warning(self):
+        # PEP 597: Raise warning when encoding is not specified
+        # and sys.flags.warn_default_encoding is set.
+        mod = self.io.__name__
+        filename = __file__
+        code = textwrap.dedent(f'''\
+            import sys
+            from {mod} import open, TextIOWrapper
+            import pathlib
+
+            with open({filename!r}) as f:           # line 5
+                pass
+
+            pathlib.Path({filename!r}).read_text()  # line 8
+        ''')
+        proc = assert_python_ok('-X', 'warn_default_encoding', '-c', code)
+        warnings = proc.err.splitlines()
+        self.assertEqual(len(warnings), 2)
+        self.assertTrue(
+            warnings[0].startswith(b"<string>:5: EncodingWarning: "))
+        self.assertTrue(
+            warnings[1].startswith(b"<string>:8: EncodingWarning: "))
+
 
 class CMiscIOTest(MiscIOTest):
     io = io
diff --git a/Lib/test/test_pickle.py b/Lib/test/test_pickle.py
index 1f5cb10..23c7bd2 100644
--- a/Lib/test/test_pickle.py
+++ b/Lib/test/test_pickle.py
@@ -483,7 +483,8 @@ def test_exceptions(self):
                 if exc in (BlockingIOError,
                            ResourceWarning,
                            StopAsyncIteration,
-                           RecursionError):
+                           RecursionError,
+                           EncodingWarning):
                     continue
                 if exc is not OSError and issubclass(exc, OSError):
                     self.assertEqual(reverse_mapping('builtins', name),
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index fca05e6..5b004c2 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -591,7 +591,8 @@ def test_sys_flags(self):
                  "inspect", "interactive", "optimize",
                  "dont_write_bytecode", "no_user_site", "no_site",
                  "ignore_environment", "verbose", "bytes_warning", "quiet",
-                 "hash_randomization", "isolated", "dev_mode", "utf8_mode")
+                 "hash_randomization", "isolated", "dev_mode", "utf8_mode",
+                 "warn_default_encoding")
         for attr in attrs:
             self.assertTrue(hasattr(sys.flags, attr), attr)
             attr_type = bool if attr == "dev_mode" else int