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Python Enhancement Proposals

PEP 597 – Add optional EncodingWarning

Inada Naoki <songofacandy at>
Standards Track

Table of Contents


Add a new warning category EncodingWarning. It is emitted when the encoding argument to open() is omitted and the default locale-specific encoding is used.

The warning is disabled by default. A new -X warn_default_encoding command-line option and a new PYTHONWARNDEFAULTENCODING environment variable can be used to enable it.

A "locale" argument value for encoding is added too. It explicitly specifies that the locale encoding should be used, silencing the warning.


Using the default encoding is a common mistake

Developers using macOS or Linux may forget that the default encoding is not always UTF-8.

For example, using long_description = open("").read() in is a common mistake. Many Windows users cannot install such packages if there is at least one non-ASCII character (e.g. emoji, author names, copyright symbols, and the like) in their UTF-8-encoded file.

Of the 4000 most downloaded packages from PyPI, 489 use non-ASCII characters in their README, and 82 fail to install from source on non-UTF-8 locales due to not specifying an encoding for a non-ASCII file. [1]

Another example is logging.basicConfig(filename="log.txt"). Some users might expect it to use UTF-8 by default, but the locale encoding is actually what is used. [2]

Even Python experts may assume that the default encoding is UTF-8. This creates bugs that only happen on Windows; see [3], [4], [5], and [6] for example.

Emitting a warning when the encoding argument is omitted will help find such mistakes.

Explicit way to use locale-specific encoding

open(filename) isn’t explicit about which encoding is expected:

  • If ASCII is assumed, this isn’t a bug, but may result in decreased performance on Windows, particularly with non-Latin-1 locale encodings
  • If UTF-8 is assumed, this may be a bug or a platform-specific script
  • If the locale encoding is assumed, the behavior is as expected (but could change if future versions of Python modify the default)

From this point of view, open(filename) is not readable code.

encoding=locale.getpreferredencoding(False) can be used to specify the locale encoding explicitly, but it is too long and easy to misuse (e.g. one can forget to pass False as its argument).

This PEP provides an explicit way to specify the locale encoding.

Prepare to change the default encoding to UTF-8

Since UTF-8 has become the de-facto standard text encoding, we might default to it for opening files in the future.

However, such a change will affect many applications and libraries. If we start emitting DeprecationWarning everywhere the encoding argument is omitted, it will be too noisy and painful.

Although this PEP doesn’t propose changing the default encoding, it will help enable that change by:

  • Reducing the number of omitted encoding arguments in libraries before we start emitting a DeprecationWarning by default.
  • Allowing users to pass encoding="locale" to suppress the current warning and any DeprecationWarning added in the future, as well as retaining consistent behavior if later Python versions change the default, ensuring support for any Python version >=3.10.



Add a new EncodingWarning warning class as a subclass of Warning. It is emitted when the encoding argument is omitted and the default locale-specific encoding is used.

Options to enable the warning

The -X warn_default_encoding option and the PYTHONWARNDEFAULTENCODING environment variable are added. They are used to enable EncodingWarning.

sys.flags.warn_default_encoding is also added. The flag is true when EncodingWarning is enabled.

When the flag is set, io.TextIOWrapper(), open() and other modules using them will emit EncodingWarning when the encoding argument is omitted.

Since EncodingWarning is a subclass of Warning, they are shown by default (if the warn_default_encoding flag is set), unlike DeprecationWarning.


io.TextIOWrapper will accept "locale" as a valid argument to encoding. It has the same meaning as the current encoding=None, except that io.TextIOWrapper doesn’t emit EncodingWarning when encoding="locale" is specified.


io.text_encoding() is a helper for functions with an encoding=None parameter that pass it to io.TextIOWrapper() or open().

A pure Python implementation will look like this:

def text_encoding(encoding, stacklevel=1):
    """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 function 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:
        if sys.flags.warn_default_encoding:
            import warnings
                "'encoding' argument not specified.",
                EncodingWarning, stacklevel + 2)
        encoding = "locale"
    return encoding

For example, pathlib.Path.read_text() can use it like this:

def read_text(self, encoding=None, errors=None):
    encoding = io.text_encoding(encoding)
    with'r', encoding=encoding, errors=errors) as f:

By using io.text_encoding(), EncodingWarning is emitted for the caller of read_text() instead of read_text() itself.

Affected standard library modules

Many standard library modules will be affected by this change.

Most APIs accepting encoding=None will use io.text_encoding() as written in the previous section.

Where using the locale encoding as the default encoding is reasonable, encoding="locale" will be used instead. For example, the subprocess module will use the locale encoding as the default for pipes.

Many tests use open() without encoding specified to read ASCII text files. They should be rewritten with encoding="ascii".


Opt-in warning

Although DeprecationWarning is suppressed by default, always emitting DeprecationWarning when the encoding argument is omitted would be too noisy.

Noisy warnings may lead developers to dismiss the DeprecationWarning.

“locale” is not a codec alias

We don’t add “locale” as a codec alias because the locale can be changed at runtime.

Additionally, TextIOWrapper checks os.device_encoding() when encoding=None. This behavior cannot be implemented in a codec.

Backward Compatibility

The new warning is not emitted by default, so this PEP is 100% backwards-compatible.

Forward Compatibility

Passing "locale" as the argument to encoding is not forward-compatible. Code using it will not work on Python older than 3.10, and will instead raise LookupError: unknown encoding: locale.

Until developers can drop Python 3.9 support, EncodingWarning can only be used for finding missing encoding="utf-8" arguments.

How to Teach This

For new users

Since EncodingWarning is used to write cross-platform code, there is no need to teach it to new users.

We can just recommend using UTF-8 for text files and using encoding="utf-8" when opening them.

For experienced users

Using open(filename) to read text files encoded in UTF-8 is a common mistake. It may not work on Windows because UTF-8 is not the default encoding.

You can use -X warn_default_encoding or PYTHONWARNDEFAULTENCODING=1 to find this type of mistake.

Omitting the encoding argument is not a bug when opening text files encoded in the locale encoding, but encoding="locale" is recommended in Python 3.10 and later because it is more explicit.

Reference Implementation


The latest discussion thread is:

  • Why not implement this in linters?
    • encoding="locale" and io.text_encoding() must be implemented in Python.
    • It is difficult to find all callers of functions wrapping open() or TextIOWrapper() (see the io.text_encoding() section).
  • Many developers will not use the option.
    • Some will, and report the warnings to libraries they use, so the option is worth it even if many developers don’t enable it.
    • For example, I found [7] and [8] by running pip install -U pip, and [9] by running tox with the reference implementation. This demonstrates how this option can be used to find potential issues.



Last modified: 2023-09-09 17:39:29 GMT