PEP: 3131

Title: Supporting Non-ASCII Identifiers

Author: Martin von Löwis <martin@v.loewis.de>

Status: Final

Type: Standards Track

Created: 01-May-2007

Python-Version: 3.0

Post-History:

Abstract

This PEP suggests to support non-ASCII letters (such as accented characters,

Cyrillic, Greek, Kanji, etc.) in Python identifiers.

Rationale

Python code is written by many people in the world who are not

familiar with the English language, or even well-acquainted with the

Latin writing system. Such developers often desire to define classes

and functions with names in their native languages, rather than having

to come up with an (often incorrect) English translation of the

concept they want to name. By using identifiers in their native

language, code clarity and maintainability of the code among

speakers of that language improves.

For some languages, common transliteration systems exist (in particular, for the

Latin-based writing systems). For other languages, users have larger

difficulties to use Latin to write their native words.

Common Objections

Some objections are often raised against proposals similar to this one.

People claim that they will not be able to use a library if to do so they have

to use characters they cannot type on their keyboards. However, it is the

choice of the designer of the library to decide on various constraints for using

the library: people may not be able to use the library because they cannot get

physical access to the source code (because it is not published), or because

licensing prohibits usage, or because the documentation is in a language they

cannot understand. A developer wishing to make a library widely available needs

to make a number of explicit choices (such as publication, licensing, language

of documentation, and language of identifiers). It should always be the choice

of the author to make these decisions - not the choice of the language

designers.

In particular, projects wishing to have wide usage probably might want to

establish a policy that all identifiers, comments, and documentation is written

in English (see the GNU coding style guide for an example of such a policy).

Restricting the language to ASCII-only identifiers does not enforce comments and

documentation to be English, or the identifiers actually to be English words, so

an additional policy is necessary, anyway.

Specification of Language Changes

The syntax of identifiers in Python will be based on the `Unicode standard annex

UAX-31 <https://www.unicode.org/reports/tr31/>`__, with elaboration and changes

as defined below.

Within the ASCII range (U+0001..U+007F), the valid characters for identifiers

are the same as in Python 2.5. This specification only introduces additional

characters from outside the ASCII range. For other characters, the

classification uses the version of the Unicode Character Database as included in

the ``unicodedata`` module.

The identifier syntax is ``<XID_Start> <XID_Continue>*``.

The exact specification of what characters have the XID_Start or

XID_Continue properties can be found in the `DerivedCoreProperties

file <https://www.unicode.org/Public/4.1.0/ucd/DerivedCoreProperties.txt>`__

of the Unicode data in use by Python (4.1 at the time this

PEP was written). For reference, the construction rules

for these sets are given below. The XID_* properties are derived

from ID_Start/ID_Continue, which are derived themselves.

``ID_Start`` is defined as all characters having one of the general

categories uppercase letters (Lu), lowercase letters (Ll), titlecase

letters (Lt), modifier letters (Lm), other letters (Lo), letter

numbers (Nl), the underscore, and characters carrying the

Other_ID_Start property. ``XID_Start`` then closes this set under

normalization, by removing all characters whose NFKC normalization

is not of the form ID_Start ID_Continue* anymore.

``ID_Continue`` is defined as all characters in ``ID_Start``, plus

nonspacing marks (Mn), spacing combining marks (Mc), decimal number

(Nd), connector punctuations (Pc), and characters carrying the

Other_ID_Continue property. Again, ``XID_Continue`` closes this set

under NFKC-normalization; it also adds U+00B7 to support Catalan.

All identifiers are converted into the normal form NFKC while parsing;

comparison of identifiers is based on NFKC.

A non-normative HTML file listing all valid identifier characters for

Unicode 4.1 can be found at

https://web.archive.org/web/20081016132748/http://www.dcl.hpi.uni-potsdam.de/home/loewis/table-3131.html.

Policy Specification

As an addition to the Python Coding style, the following policy is

prescribed: All identifiers in the Python standard library MUST use

ASCII-only identifiers, and SHOULD use English words wherever feasible

(in many cases, abbreviations and technical terms are used which

aren't English). In addition, string literals and comments must also

be in ASCII. The only exceptions are (a) test cases testing the

non-ASCII features, and (b) names of authors. Authors whose names are

not based on the Latin alphabet MUST provide a Latin transliteration

of their names.

As an option, this specification can be applied to Python 2.x. In

that case, ASCII-only identifiers would continue to be represented as

byte string objects in namespace dictionaries; identifiers with

non-ASCII characters would be represented as Unicode strings.

Implementation

The following changes will need to be made to the parser:

1. If a non-ASCII character is found in the UTF-8 representation of

the source code, a forward scan is made to find the first ASCII

non-identifier character (e.g. a space or punctuation character)

2. The entire UTF-8 string is passed to a function to normalize the

string to NFKC, and then verify that it follows the identifier

syntax. No such callout is made for pure-ASCII identifiers, which

continue to be parsed the way they are today. The Unicode database

must start including the Other_ID_{Start|Continue} property.

3. If this specification is implemented for 2.x, reflective libraries

(such as pydoc) must be verified to continue to work when Unicode

strings appear in ``__dict__`` slots as keys.

Open Issues

John Nagle suggested consideration of `Unicode Technical Standard #39

<https://www.unicode.org/reports/tr39/>`__,

which discusses security mechanisms for Unicode identifiers.

It's not clear how that can precisely apply to this PEP; possible

consequences are

* warn about characters listed as "restricted" in xidmodifications.txt

* warn about identifiers using mixed scripts

* somehow perform Confusable Detection

In the latter two approaches, it's not clear how precisely the

algorithm should work. For mixed scripts, certain kinds of mixing

should probably allowed - are these the "Common" and "Inherited"

scripts mentioned in section 5? For Confusable Detection, it seems one

needs two identifiers to compare them for confusion - is it possible

to somehow apply it to a single identifier only, and warn?

In follow-up discussion, it turns out that John Nagle actually

meant to suggest `UTR#36 <https://www.unicode.org/reports/tr36/>`__,

level "Highly Restrictive".

Several people suggested to allow and ignore formatting control

characters (general category Cf), as is done in Java, JavaScript, and

C#. It's not clear whether this would improve things (it might

for RTL languages); if there is a need, these can be added

later.

Some people would like to see an option on selecting support

for this PEP at run-time; opinions vary on what precisely

that option should be, and what precisely its default value

should be. `Guido van Rossum commented

<https://mail.python.org/pipermail/python-3000/2007-May/007925.html>`__

that a global flag passed to the interpreter is not acceptable, as it would

apply to all modules.

Discussion

`Ka-Ping Yee summarizes discussion and further objection

<https://mail.python.org/pipermail/python-3000/2007-June/008161.html>`__

as such:

A. Should identifiers be allowed to contain any Unicode letter?

Drawbacks of allowing non-ASCII identifiers wholesale:

1. Python will lose the ability to make a reliable round trip to

a human-readable display on screen or on paper.

2. Python will become vulnerable to a new class of security exploits;

code and submitted patches will be much harder to inspect.

3. Humans will no longer be able to validate Python syntax.

4. Unicode is young; its problems are not yet well understood and

solved; tool support is weak.

5. Languages with non-ASCII identifiers use different character sets

and normalization schemes; :pep:`3131`'s choices are non-obvious.

6. The Unicode bidi algorithm yields an extremely confusing display

order for RTL text when digits or operators are nearby.

B. Should the default behaviour accept only ASCII identifiers, or

should it accept identifiers containing non-ASCII characters?

Arguments for ASCII only by default:

1. Non-ASCII identifiers by default makes common practice/assumptions

subtly/unknowingly wrong; rarely wrong is worse than obviously wrong.

2. Better to raise a warning than to fail silently when encountering

a probably unexpected situation.

3. All of current usage is ASCII-only; the vast majority of future

usage will be ASCII-only.

3. It is the pockets of Unicode adoption that are parochial, not the

ASCII advocates.

4. Python should audit for ASCII-only identifiers for the same

reasons that it audits for tab-space consistency

5. Incremental change is safer.

6. An ASCII-only default favors open-source development and sharing

of source code.

7. Existing projects won't have to waste any brainpower worrying

about the implications of Unicode identifiers.

C. Should non-ASCII identifiers be optional?

Various voices in support of a flag (although there's been debate

over which should be the default, no one seems to be saying that

there shouldn't be an off switch)

D. Should the identifier character set be configurable?

Various voices proposing and supporting a selectable character set,

so that users can get all the benefits of using their own language

without the drawbacks of confusable/unfamiliar characters

E. Which identifier characters should be allowed?

1. What to do about bidi format control characters?

2. What about other ID_Continue characters? What about characters

that look like punctuation? What about other recommendations

in UTS #39? What about mixed-script identifiers?

F. Which normalization form should be used, NFC or NFKC?

G. Should source code be required to be in normalized form?

Copyright

This document has been placed in the public domain.