The PEP is here: https://www.python.org/dev/peps/pep-0597/ I created a thread on discuss.python.org https://discuss.python.org/t/pep-597-raise-a-warning-when-encoding-is-omitte... but I want more feedback. Please reply on this mail or discuss.python.org. ## Summary This PEP doesn't propose to change the default text encoding yet, but introduce warning for code using default text encoding only in dev mode. This opt-in warning is useful for: * Estimate the impact of changing the default text encoding. * Find bugs they use default text encoding where it should be UTF-8. ## PEP Abstract https://www.python.org/dev/peps/pep-0597/#id6 This PEP proposes: - TextIOWrapper raises a PendingDeprecationWarning when the encoding option is not specified, and dev mode is enabled. - Add encoding="locale" option to TextIOWrapper. It behaves like encoding=None but don't raise a warning. Motivation https://www.python.org/dev/peps/pep-0597/#id7 Omitting encoding is common mistake https://www.python.org/dev/peps/pep-0597/#id8 Developers using macOS or Linux may forget that the default encoding is not always UTF-8. For example, long_description = open("README.md").read() in setup.py is a common mistake. Many Windows users can not install the package if there is at least one non-ASCII character (e.g. emoji) in the README.md file. For example, 489 packages of the 4000 most downloaded packages from PyPI used non-ASCII characters in README. And 82 packages of them can not be installed from source package when locale encoding is ASCII. [1 https://www.python.org/dev/peps/pep-0597/#id2] They used default encoding to read README or TOML file. Another example is logging.basicConfig(filename="log.txt"). Some users expect UTF-8 is used by default, but locale encoding is used actually. [2 https://www.python.org/dev/peps/pep-0597/#id3] Even Python experts assume that default encoding is UTF-8. It creates bugs that happen only on Windows. See [3 https://www.python.org/dev/peps/pep-0597/#id4] and [4 https://www.python.org/dev/peps/pep-0597/#id5]. Raising a warning when the encoding option is omitted will help to find such mistakes. Prepare to change the default encoding to UTF-8 https://www.python.org/dev/peps/pep-0597/#id9 We chose to use locale encoding for the default text encoding in Python 3.0. But UTF-8 has been adopted very widely since then. We might change the default text encoding to UTF-8 in the future. But this change will affect many applications and libraries. Many DeprecationWarning will be raised if we start raising the warning by default. It will be too noisy. While this PEP doesn't cover the change, this PEP will help to reduce the number of DeprecationWarning in the future. Specification https://www.python.org/dev/peps/pep-0597/#id10 Raising a PendingDeprecationWarning https://www.python.org/dev/peps/pep-0597/#id11 TextIOWrapper raises the PendingDeprecationWarning when the encoding option is omitted, and dev mode is enabled. encoding="locale" option https://www.python.org/dev/peps/pep-0597/#id12 When encoding="locale" is specified to the TextIOWrapper, it behaves same to encoding=None. In detail, the encoding is chosen by: 1. os.device_encoding(buffer.fileno()) 2. locale.getpreferredencoding(False) This option can be used to suppress the PendingDeprecationWarning. io.text_encoding https://www.python.org/dev/peps/pep-0597/#id13 TextIOWrapper is used indirectly in most cases. For example, open, and pathlib.Path.read_text() use it. Warning to these functions doesn't make sense. Callers of these functions should be warned instead. io.text_encoding(encoding, stacklevel=1) is a helper function for it. Pure Python implementation will be like this: def text_encoding(encoding, stacklevel=1): """ Helper function to choose the text encoding. When encoding is not None, just return it. Otherwise, return the default text encoding ("locale" for now), and raise a PendingDeprecationWarning in dev mode. This function can be used in APIs having encoding=None option. But please consider encoding="utf-8" for new APIs. """ if encoding is None: if sys.flags.dev_mode: import warnings warnings.warn( "'encoding' option is not specified. The default encoding " "will be changed to 'utf-8' in the future", PendingDeprecationWarning, stacklevel + 2) encoding = "locale" return encoding pathlib.Path.read_text() can use this function like this: 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() subprocess module doesn't warn https://www.python.org/dev/peps/pep-0597/#id14 While the subprocess module uses TextIOWrapper, it doesn't raise PendingDeprecationWarning. It uses the "locale" encoding by default. Rationale https://www.python.org/dev/peps/pep-0597/#id15 "locale" is not a codec alias https://www.python.org/dev/peps/pep-0597/#id16 We don't add the "locale" to the codec alias because locale can be changed in runtime. Additionally, TextIOWrapper checks os.device_encoding() when encoding=None. This behavior can not be implemented in the codec. Use a PendingDeprecationWarning https://www.python.org/dev/peps/pep-0597/#id17 This PEP doesn't make decision about changing default text encoding. So we use PendingDeprecationWarning instead of DeprecationWarning for now. Raise warning only in dev mode https://www.python.org/dev/peps/pep-0597/#id18 This PEP will produce a huge amount of PendingDeprecationWarning. It will be too noisy for most Python developers. We need to fix warnings in standard library, pip, and major dev tools like pytest before raise this warning by default. subprocess module doesn't warn https://www.python.org/dev/peps/pep-0597/#id19 The default encoding for PIPE is relating to the encoding of the stdio. It should be discussed later. Reference Implementation https://www.python.org/dev/peps/pep-0597/#id20 https://github.com/python/cpython/pull/19481 References https://www.python.org/dev/peps/pep-0597/#id21 [1] "Packages can't be installed when encoding is not UTF-8" ( https://github.com/methane/pep597-pypi-ascii) [2] "Logging - Inconsistent behaviour when handling unicode" ( https://bugs.python.org/issue37111) [3] Packaging tutorial in packaging.python.org didn't specify encoding to read a README.md (https://github.com/pypa/packaging.python.org/pull/682) [4] json.tool had used locale encoding to read JSON files. ( https://bugs.python.org/issue33684) Copyright https://www.python.org/dev/peps/pep-0597/#id22 This document has been placed in the public domain. Source: https://github.com/python/peps/blob/master/pep-0597.rst -- Inada Naoki