Python get default encoding

PEP 263 – Defining Python Source Code Encodings

This PEP proposes to introduce a syntax to declare the encoding of a Python source file. The encoding information is then used by the Python parser to interpret the file using the given encoding. Most notably this enhances the interpretation of Unicode literals in the source code and makes it possible to write Unicode literals using e.g. UTF-8 directly in an Unicode aware editor.

Problem

In Python 2.1, Unicode literals can only be written using the Latin-1 based encoding “unicode-escape”. This makes the programming environment rather unfriendly to Python users who live and work in non-Latin-1 locales such as many of the Asian countries. Programmers can write their 8-bit strings using the favorite encoding, but are bound to the “unicode-escape” encoding for Unicode literals.

Proposed Solution

I propose to make the Python source code encoding both visible and changeable on a per-source file basis by using a special comment at the top of the file to declare the encoding.

To make Python aware of this encoding declaration a number of concept changes are necessary with respect to the handling of Python source code data.

Defining the Encoding

Python will default to ASCII as standard encoding if no other encoding hints are given.

To define a source code encoding, a magic comment must be placed into the source files either as first or second line in the file, such as:

or (using formats recognized by popular editors):

#!/usr/bin/python # vim: set fileencoding= : 

More precisely, the first or second line must match the following regular expression:

The first group of this expression is then interpreted as encoding name. If the encoding is unknown to Python, an error is raised during compilation. There must not be any Python statement on the line that contains the encoding declaration. If the first line matches the second line is ignored.

To aid with platforms such as Windows, which add Unicode BOM marks to the beginning of Unicode files, the UTF-8 signature \xef\xbb\xbf will be interpreted as ‘utf-8’ encoding as well (even if no magic encoding comment is given).

If a source file uses both the UTF-8 BOM mark signature and a magic encoding comment, the only allowed encoding for the comment is ‘utf-8’. Any other encoding will cause an error.

Examples

These are some examples to clarify the different styles for defining the source code encoding at the top of a Python source file:

With interpreter binary and using Emacs style file encoding comment:

#!/usr/bin/python # -*- coding: latin-1 -*- import os, sys . #!/usr/bin/python # -*- coding: iso-8859-15 -*- import os, sys . #!/usr/bin/python # -*- coding: ascii -*- import os, sys . 

# This Python file uses the following encoding: utf-8 import os, sys . 

#!/usr/local/bin/python # coding: latin-1 import os, sys . 

#!/usr/local/bin/python import os, sys . 

#!/usr/local/bin/python # latin-1 import os, sys . 

#!/usr/local/bin/python # # -*- coding: latin-1 -*- import os, sys . 

#!/usr/local/bin/python # -*- coding: utf-42 -*- import os, sys . 

Concepts

The PEP is based on the following concepts which would have to be implemented to enable usage of such a magic comment:

1. The complete Python source file should use a single encoding. Embedding of differently encoded data is not allowed and will result in a decoding error during compilation of the Python source code. Any encoding which allows processing the first two lines in the way indicated above is allowed as source code encoding, this includes ASCII compatible encodings as well as certain multi-byte encodings such as Shift_JIS. It does not include encodings which use two or more bytes for all characters like e.g. UTF-16. The reason for this is to keep the encoding detection algorithm in the tokenizer simple.
2. Handling of escape sequences should continue to work as it does now, but with all possible source code encodings, that is standard string literals (both 8-bit and Unicode) are subject to escape sequence expansion while raw string literals only expand a very small subset of escape sequences.
3. Python’s tokenizer/compiler combo will need to be updated to work as follows:
   1. read the file
   2. decode it into Unicode assuming a fixed per-file encoding
   3. convert it into a UTF-8 byte string
   4. tokenize the UTF-8 content
   5. compile it, creating Unicode objects from the given Unicode data and creating string objects from the Unicode literal data by first reencoding the UTF-8 data into 8-bit string data using the given file encoding

   Note that Python identifiers are restricted to the ASCII subset of the encoding, and thus need no further conversion after step 4.

   Implementation

   For backwards-compatibility with existing code which currently uses non-ASCII in string literals without declaring an encoding, the implementation will be introduced in two phases:

   1. Allow non-ASCII in string literals and comments, by internally treating a missing encoding declaration as a declaration of “iso-8859-1”. This will cause arbitrary byte strings to correctly round-trip between step 2 and step 5 of the processing, and provide compatibility with Python 2.2 for Unicode literals that contain non-ASCII bytes. A warning will be issued if non-ASCII bytes are found in the input, once per improperly encoded input file.
   2. Remove the warning, and change the default encoding to “ascii”.

   The builtin compile() API will be enhanced to accept Unicode as input. 8-bit string input is subject to the standard procedure for encoding detection as described above.

   If a Unicode string with a coding declaration is passed to compile() , a SyntaxError will be raised.

   SUZUKI Hisao is working on a patch; see [2] for details. A patch implementing only phase 1 is available at [1].

   Phases

   Implementation of steps 1 and 2 above were completed in 2.3, except for changing the default encoding to “ascii”.

   The default encoding was set to “ascii” in version 2.5.

   Scope

   This PEP intends to provide an upgrade path from the current (more-or-less) undefined source code encoding situation to a more robust and portable definition.

   References

   History

   • 1.10 and above: see CVS history
   • 1.8: Added ‘.’ to the coding RE.
   • 1.7: Added warnings to phase 1 implementation. Replaced the Latin-1 default encoding with the interpreter’s default encoding. Added tweaks to compile() .
   • 1.4 — 1.6: Minor tweaks
   • 1.3: Worked in comments by Martin v. Loewis: UTF-8 BOM mark detection, Emacs style magic comment, two phase approach to the implementation

   Copyright

   This document has been placed in the public domain.

   Источник

   PEP 686 – Make UTF-8 mode default

   This PEP proposes enabling UTF-8 mode by default.

   With this change, Python consistently uses UTF-8 for default encoding of files, stdio, and pipes.

   Motivation

   UTF-8 becomes de facto standard text encoding.

   • The default encoding of Python source files is UTF-8.
   • JSON, TOML, YAML use UTF-8.
   • Most text editors, including Visual Studio Code and Windows Notepad use UTF-8 by default.
   • Most websites and text data on the internet use UTF-8.
   • And many other popular programming languages, including Node.js, Go, Rust, and Java uses UTF-8 by default.

   Changing the default encoding to UTF-8 makes it easier for Python to interoperate with them.

   Additionally, many Python developers using Unix forget that the default encoding is platform dependent. They omit to specify encoding=»utf-8″ when they read text files encoded in UTF-8 (e.g. JSON, TOML, Markdown, and Python source files). Inconsistent default encoding causes many bugs.

   Specification

   Enable UTF-8 mode by default

   Python will enable UTF-8 mode by default from Python 3.15.

   Users can still disable UTF-8 mode by setting PYTHONUTF8=0 or -X utf8=0 .

   locale.getencoding()

   Since UTF-8 mode affects locale.getpreferredencoding(False) , we need an API to get locale encoding regardless of UTF-8 mode.

   locale.getencoding() will be added for this purpose. It returns locale encoding too, but ignores UTF-8 mode.

   When warn_default_encoding option is specified, locale.getpreferredencoding() will emit EncodingWarning like open() (see also PEP 597).

   This API was added in Python 3.11.

   Fixing encoding=»locale» option

   PEP 597 added the encoding=»locale» option to the TextIOWrapper . This option is used to specify the locale encoding explicitly. TextIOWrapper should use locale encoding when the option is specified, regardless of default text encoding.

   But TextIOWrapper uses «UTF-8″ in UTF-8 mode even if encoding=»locale» is specified for now. This behavior is inconsistent with the PEP 597 motivation. It is because we didn’t expect making UTF-8 mode default when Python changes its default text encoding.

   This inconsistency should be fixed before making UTF-8 mode default. TextIOWrapper should use locale encoding when encoding=»locale» is passed even in UTF-8 mode.

   This issue was fixed in Python 3.11.

   Backward Compatibility

   Most Unix systems use UTF-8 locale and Python enables UTF-8 mode when its locale is C or POSIX. So this change mostly affects Windows users.

   When a Python program depends on the default encoding, this change may cause UnicodeError , mojibake, or even silent data corruption. So this change should be announced loudly.

   This is the guideline to fix this backward compatibility issue:

   1. Disable UTF-8 mode.
   2. Use EncodingWarning (PEP 597) to find every places UTF-8 mode affects.
      • If encoding option is omitted, consider using encoding=»utf-8″ or encoding=»locale» .
      • If locale.getpreferredencoding() is used, consider using «utf-8» or locale.getencoding() .
   3. Test the application with UTF-8 mode.

   Preceding examples

   • Ruby changed the default external_encoding to UTF-8 on Windows in Ruby 3.0 (2020).
   • Java changed the default text encoding to UTF-8 in JDK 18. (2022).

   Both Ruby and Java have an option for backward compatibility. They don’t provide any warning like PEP 597’s EncodingWarning in Python for use of the default encoding.

   Rejected Alternative

   Deprecate implicit encoding

   Deprecating the use of the default encoding is considered.

   But there are many cases that the default encoding is used for reading/writing only ASCII text. Additionally, such warnings are not useful for non-cross platform applications run on Unix.

   So forcing users to specify the encoding everywhere is too painful. Emitting a lot of DeprecationWarning will lead users ignore warnings.

   PEP 387 requires adding a warning for backward incompatible changes. But it doesn’t require using DeprecationWarning . So using optional EncodingWarning doesn’t violate the PEP 387.

   Java also rejected this idea in JEP 400.

   Use PYTHONIOENCODING for PIPEs

   To ease backward compatibility issue, using PYTHONIOENCODING as the default encoding of PIPEs in the subprocess module is considered.

   With this idea, users can use legacy encoding for subprocess.Popen(text=True) even in UTF-8 mode.

   But this idea makes “default encoding” complicated. And this idea is also backward incompatible.

   So this idea is rejected. Users can disable UTF-8 mode until they replace text=True with encoding=»utf-8″ or encoding=»locale» .

   How to teach this

   For new users, this change reduces things that need to teach. Users don’t need to learn about text encoding in their first year. They should learn it when they need to use non-UTF-8 text files.

   For existing users, see the Backward compatibility section.

   Copyright

   This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.

   Источник