bpo-43510: Implement PEP 597 opt-in EncodingWarning. (GH-19481)

See [PEP 597](https://www.python.org/dev/peps/pep-0597/).

* Add `-X warn_default_encoding` and `PYTHONWARNDEFAULTENCODING`.
* Add EncodingWarning
* Add io.text_encoding()
* open(), TextIOWrapper() emits EncodingWarning when encoding is omitted and warn_default_encoding is enabled.
* _pyio.TextIOWrapper() uses UTF-8 as fallback default encoding used when failed to import locale module. (used during building Python)
* bz2, configparser, gzip, lzma, pathlib, tempfile modules use io.text_encoding().
* What's new entry

Author: methane

Doc/c-api/init_config.rst

@@ -583,6 +583,15 @@ PyConfig
 
       Default: ``0``.
 
+   .. c:member:: int warn_default_encoding
+
+      If non-zero, emit a :exc:`EncodingWarning` warning when :class:`io.TextIOWrapper`
+      uses its default encoding. See :ref:`io-encoding-warning` for details.
+
+      Default: ``0``.
+
+      .. versionadded:: 3.10
+
    .. c:member:: wchar_t* check_hash_pycs_mode
 
       Control the validation behavior of hash-based ``.pyc`` files:

Doc/library/exceptions.rst

@@ -741,6 +741,15 @@ The following exceptions are used as warning categories; see the
    Base class for warnings related to Unicode.
 
 
+.. exception:: EncodingWarning
+
+   Base class for warnings related to encodings.
+
+   See :ref:`io-encoding-warning` for details.
+
+   .. versionadded:: 3.10
+
+
 .. exception:: BytesWarning
 
    Base class for warnings related to :class:`bytes` and :class:`bytearray`.

Doc/library/io.rst

@@ -106,6 +106,56 @@ stream by opening a file in binary mode with buffering disabled::
 The raw stream API is described in detail in the docs of :class:`RawIOBase`.
 
 
+.. _io-text-encoding:
+
+Text Encoding
+-------------
+
+The default encoding of :class:`TextIOWrapper` and :func:`open` is
+locale-specific (:func:`locale.getpreferredencoding(False) <locale.getpreferredencoding>`).
+
+However, many developers forget to specify the encoding when opening text files
+encoded in UTF-8 (e.g. JSON, TOML, Markdown, etc...) since most Unix
+platforms use UTF-8 locale by default. This causes bugs because the locale
+encoding is not UTF-8 for most Windows users. For example::
+
+   # May not work on Windows when non-ASCII characters in the file.
+   with open("README.md") as f:
+       long_description = f.read()
+
+Additionally, while there is no concrete plan as of yet, Python may change
+the default text file encoding to UTF-8 in the future.
+
+Accordingly, it is highly recommended that you specify the encoding
+explicitly when opening text files. If you want to use UTF-8, pass
+``encoding="utf-8"``. To use the current locale encoding,
+``encoding="locale"`` is supported in Python 3.10.
+
+When you need to run existing code on Windows that attempts to opens
+UTF-8 files using the default locale encoding, you can enable the UTF-8
+mode. See :ref:`UTF-8 mode on Windows <win-utf8-mode>`.
+
+.. _io-encoding-warning:
+
+Opt-in EncodingWarning
+^^^^^^^^^^^^^^^^^^^^^^
+
+.. versionadded:: 3.10
+   See :pep:`597` for more details.
+
+To find where the default locale encoding is used, you can enable
+the ``-X warn_default_encoding`` command line option or set the
+:envvar:`PYTHONWARNDEFAULTENCODING` environment variable, which will
+emit an :exc:`EncodingWarning` when the default encoding is used.
+
+If you are providing an API that uses :func:`open` or
+:class:`TextIOWrapper` and passes ``encoding=None`` as a parameter, you
+can use :func:`text_encoding` so that callers of the API will emit an
+:exc:`EncodingWarning` if they don't pass an ``encoding``. However,
+please consider using UTF-8 by default (i.e. ``encoding="utf-8"``) for
+new APIs.
+
+
 High-level Module Interface
 ---------------------------
 
@@ -143,6 +193,32 @@ High-level Module Interface
    .. versionadded:: 3.8
 
 
+.. function:: text_encoding(encoding, stacklevel=2)
+
+   This is a helper function for callables that use :func:`open` or
+   :class:`TextIOWrapper` and have an ``encoding=None`` parameter.
+
+   This function returns *encoding* if it is not ``None`` and ``"locale"`` if
+   *encoding* is ``None``.
+
+   This function emits an :class:`EncodingWarning` if
+   :data:`sys.flags.warn_default_encoding <sys.flags>` is true and *encoding*
+   is None. *stacklevel* specifies where the warning is emitted.
+   For example::
+
+      def read_text(path, encoding=None):
+          encoding = io.text_encoding(encoding)  # stacklevel=2
+          with open(path, encoding) as f:
+              return f.read()
+
+   In this example, an :class:`EncodingWarning` is emitted for the caller of
+   ``read_text()``.
+
+   See :ref:`io-text-encoding` for more information.
+
+   .. versionadded:: 3.10
+
+
 .. exception:: BlockingIOError
 
    This is a compatibility alias for the builtin :exc:`BlockingIOError`
@@ -869,6 +945,8 @@ Text I/O
    *encoding* gives the name of the encoding that the stream will be decoded or
    encoded with.  It defaults to
    :func:`locale.getpreferredencoding(False) <locale.getpreferredencoding>`.
+   ``encoding="locale"`` can be used to specify the current locale's encoding
+   explicitly. See :ref:`io-text-encoding` for more information.
 
    *errors* is an optional string that specifies how encoding and decoding
    errors are to be handled.  Pass ``'strict'`` to raise a :exc:`ValueError`
@@ -920,6 +998,9 @@ Text I/O
       locale encoding using :func:`locale.setlocale`, use the current locale
       encoding instead of the user preferred encoding.
 
+   .. versionchanged:: 3.10
+      The *encoding* argument now supports the ``"locale"`` dummy encoding name.
+
    :class:`TextIOWrapper` provides these data attributes and methods in
    addition to those from :class:`TextIOBase` and :class:`IOBase`:
 

Doc/using/cmdline.rst

@@ -453,6 +453,9 @@ Miscellaneous options
    * ``-X pycache_prefix=PATH`` enables writing ``.pyc`` files to a parallel
      tree rooted at the given directory instead of to the code tree. See also
      :envvar:`PYTHONPYCACHEPREFIX`.
+   * ``-X warn_default_encoding`` issues a :class:`EncodingWarning` when the
+     locale-specific default encoding is used for opening files.
+     See also :envvar:`PYTHONWARNDEFAULTENCODING`.
 
    It also allows passing arbitrary values and retrieving them through the
    :data:`sys._xoptions` dictionary.
@@ -482,6 +485,9 @@ Miscellaneous options
 
       The ``-X showalloccount`` option has been removed.
 
+   .. versionadded:: 3.10
+      The ``-X warn_default_encoding`` option.
+
    .. deprecated-removed:: 3.9 3.10
       The ``-X oldparser`` option.
 
@@ -907,6 +913,15 @@ conflict.
 
    .. versionadded:: 3.7
 
+.. envvar:: PYTHONWARNDEFAULTENCODING
+
+   If this environment variable is set to a non-empty string, issue a
+   :class:`EncodingWarning` when the locale-specific default encoding is used.
+
+   See :ref:`io-encoding-warning` for details.
+
+   .. versionadded:: 3.10
+
 
 Debug-mode variables
 ~~~~~~~~~~~~~~~~~~~~

Doc/whatsnew/3.10.rst

@@ -454,6 +454,30 @@ For the full specification see :pep:`634`.  Motivation and rationale
 are in :pep:`635`, and a longer tutorial is in :pep:`636`.
 
 
+.. _whatsnew310-pep597:
+
+Optional ``EncodingWarning`` and ``encoding="locale"`` option
+-------------------------------------------------------------
+
+The default encoding of :class:`TextIOWrapper` and :func:`open` is
+platform and locale dependent. Since UTF-8 is used on most Unix
+platforms, omitting ``encoding`` option when opening UTF-8 files
+(e.g. JSON, YAML, TOML, Markdown) is very common bug. For example::
+
+   # BUG: "rb" mode or encoding="utf-8" should be used.
+   with open("data.json") as f:
+       data = json.laod(f)
+
+To find this type of bugs, optional ``EncodingWarning`` is added.
+It is emitted when :data:`sys.flags.warn_default_encoding <sys.flags>`
+is true and locale-specific default encoding is used.
+
+``-X warn_default_encoding`` option and :envvar:`PYTHONWARNDEFAULTENCODING`
+are added to enable the warning.
+
+See :ref:`io-text-encoding` for more information.
+
+
 New Features Related to Type Annotations
 ========================================
 

Include/cpython/initconfig.h

@@ -153,6 +153,7 @@ typedef struct PyConfig {
     PyWideStringList warnoptions;
     int site_import;
     int bytes_warning;
+    int warn_default_encoding;
     int inspect;
     int interactive;
     int optimization_level;

Include/internal/pycore_initconfig.h

@@ -102,6 +102,7 @@ typedef struct {
     int isolated;             /* -I option */
     int use_environment;      /* -E option */
     int dev_mode;             /* -X dev and PYTHONDEVMODE */
+    int warn_default_encoding;     /* -X warn_default_encoding and PYTHONWARNDEFAULTENCODING */
 } _PyPreCmdline;
 
 #define _PyPreCmdline_INIT \

Include/pyerrors.h

@@ -146,6 +146,7 @@ PyAPI_DATA(PyObject *) PyExc_FutureWarning;
 PyAPI_DATA(PyObject *) PyExc_ImportWarning;
 PyAPI_DATA(PyObject *) PyExc_UnicodeWarning;
 PyAPI_DATA(PyObject *) PyExc_BytesWarning;
+PyAPI_DATA(PyObject *) PyExc_EncodingWarning;
 PyAPI_DATA(PyObject *) PyExc_ResourceWarning;
 
 

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)

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