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PEP 3120 -- Using UTF-8 as the default source encoding

Title:Using UTF-8 as the default source encoding
Author:Martin von Löwis <martin at>
Type:Standards Track


This PEP proposes to change the default source encoding from ASCII to UTF-8. Support for alternative source encodings [1] continues to exist; an explicit encoding declaration takes precedence over the default.

A Bit of History

In Python 1, the source encoding was unspecified, except that the source encoding had to be a superset of the system's basic execution character set (i.e. an ASCII superset, on most systems). The source encoding was only relevant for the lexis itself (bytes representing letters for keywords, identifiers, punctuation, line breaks, etc). The contents of a string literal was copied literally from the file on source.

In Python 2.0, the source encoding changed to Latin-1 as a side effect of introducing Unicode. For Unicode string literals, the characters were still copied literally from the source file, but widened on a character-by-character basis. As Unicode gives a fixed interpretation to code points, this algorithm effectively fixed a source encoding, at least for files containing non-ASCII characters in Unicode literals.

PEP 263 identified the problem that you can use only those Unicode characters in a Unicode literal which are also in Latin-1, and introduced a syntax for declaring the source encoding. If no source encoding was given, the default should be ASCII. For compatibility with Python 2.0 and 2.1, files were interpreted as Latin-1 for a transitional period. This transition ended with Python 2.5, which gives an error if non-ASCII characters are encountered and no source encoding is declared.


With PEP 263, using arbitrary non-ASCII characters in a Python file is possible, but tedious. One has to explicitly add an encoding declaration. Even though some editors (like IDLE and Emacs) support the declarations of PEP 263, many editors still do not (and never will); users have to explicitly adjust the encoding which the editor assumes on a file-by-file basis.

When the default encoding is changed to UTF-8, adding non-ASCII text to Python files becomes easier and more portable: On some systems, editors will automatically choose UTF-8 when saving text (e.g. on Unix systems where the locale uses UTF-8). On other systems, editors will guess the encoding when reading the file, and UTF-8 is easy to guess. Yet other editors support associating a default encoding with a file extension, allowing users to associate .py with UTF-8.

For Python 2, an important reason for using non-UTF-8 encodings was that byte string literals would be in the source encoding at run-time, allowing then to output them to a file or render them to the user as-is. With Python 3, all strings will be Unicode strings, so the original encoding of the source will have no impact at run-time.


The parser needs to be changed to accept bytes > 127 if no source encoding is specified; instead of giving an error, it needs to check that the bytes are well-formed UTF-8 (decoding is not necessary, as the parser converts all source code to UTF-8, anyway).

IDLE needs to be changed to use UTF-8 as the default encoding.