PEP 414 – Explicit Unicode Literal for Python 3.3

PEP: 414
Title: Explicit Unicode Literal for Python 3.3
Author: Armin Ronacher <armin.ronacher at active-4.com>, Nick Coghlan <ncoghlan at gmail.com>
Status: Final
Type: Standards Track
Created: 15-Feb-2012
Post-History: 28-Feb-2012, 04-Mar-2012
Resolution: https://mail.python.org/pipermail/python-dev/2012-February/116995.html

Abstract
BDFL Pronouncement
Proposal
Exclusion of “Raw” Unicode Literals
Author’s Note
Rationale
Common Objections
References
Copyright

Abstract

This document proposes the reintegration of an explicit unicode literal from Python 2.x to the Python 3.x language specification, in order to reduce the volume of changes needed when porting Unicode-aware Python 2 applications to Python 3.

BDFL Pronouncement

This PEP has been formally accepted for Python 3.3:

I’m accepting the PEP. It’s about as harmless as they come. Make it so.

Proposal

This PEP proposes that Python 3.3 restore support for Python 2’s Unicode literal syntax, substantially increasing the number of lines of existing Python 2 code in Unicode aware applications that will run without modification on Python 3.

Specifically, the Python 3 definition for string literal prefixes will be expanded to allow:

"u" | "U"

in addition to the currently supported:

"r" | "R"

The following will all denote ordinary Python 3 strings:

'text'
"text"
'''text'''
"""text"""
u'text'
u"text"
u'''text'''
u"""text"""
U'text'
U"text"
U'''text'''
U"""text"""

No changes are proposed to Python 3’s actual Unicode handling, only to the acceptable forms for string literals.

Exclusion of “Raw” Unicode Literals

Python 2 supports a concept of “raw” Unicode literals that don’t meet the conventional definition of a raw string: \uXXXX and \UXXXXXXXX escape sequences are still processed by the compiler and converted to the appropriate Unicode code points when creating the associated Unicode objects.

Python 3 has no corresponding concept - the compiler performs no preprocessing of the contents of raw string literals. This matches the behaviour of 8-bit raw string literals in Python 2.

Since such strings are rarely used and would be interpreted differently in Python 3 if permitted, it was decided that leaving them out entirely was a better choice. Code which uses them will thus still fail immediately on Python 3 (with a Syntax Error), rather than potentially producing different output.

To get equivalent behaviour that will run on both Python 2 and Python 3, either an ordinary Unicode literal can be used (with appropriate additional escaping within the string), or else string concatenation or string formatting can be combine the raw portions of the string with those that require the use of Unicode escape sequences.

Note that when using from __future__ import unicode_literals in Python 2, the nominally “raw” Unicode string literals will process \uXXXX and \UXXXXXXXX escape sequences, just like Python 2 strings explicitly marked with the “raw Unicode” prefix.

Author’s Note

This PEP was originally written by Armin Ronacher, and Guido’s approval was given based on that version.

The currently published version has been rewritten by Nick Coghlan to include additional historical details and rationale that were taken into account when Guido made his decision, but were not explicitly documented in Armin’s version of the PEP.

Readers should be aware that many of the arguments in this PEP are not technical ones. Instead, they relate heavily to the social and personal aspects of software development.

Rationale

With the release of a Python 3 compatible version of the Web Services Gateway Interface (WSGI) specification (PEP 3333) for Python 3.2, many parts of the Python web ecosystem have been making a concerted effort to support Python 3 without adversely affecting their existing developer and user communities.

One major item of feedback from key developers in those communities, including Chris McDonough (WebOb, Pyramid), Armin Ronacher (Flask, Werkzeug), Jacob Kaplan-Moss (Django) and Kenneth Reitz (requests) is that the requirement to change the spelling of every Unicode literal in an application (regardless of how that is accomplished) is a key stumbling block for porting efforts.

In particular, unlike many of the other Python 3 changes, it isn’t one that framework and library authors can easily handle on behalf of their users. Most of those users couldn’t care less about the “purity” of the Python language specification, they just want their websites and applications to work as well as possible.

While it is the Python web community that has been most vocal in highlighting this concern, it is expected that other highly Unicode aware domains (such as GUI development) may run into similar issues as they (and their communities) start making concerted efforts to support Python 3.

Common Objections

Complaint: This PEP may harm adoption of Python 3.2

This complaint is interesting, as it carries within it a tacit admission that this PEP will make it easier to port Unicode aware Python 2 applications to Python 3.

There are many existing Python communities that are prepared to put up with the constraints imposed by the existing suite of porting tools, or to update their Python 2 code bases sufficiently that the problems are minimised.

This PEP is not for those communities. Instead, it is designed specifically to help people that don’t want to put up with those difficulties.

However, since the proposal is for a comparatively small tweak to the language syntax with no semantic changes, it is feasible to support it as a third party import hook. While such an import hook imposes some import time overhead, and requires additional steps from each application that needs it to get the hook in place, it allows applications that target Python 3.2 to use libraries and frameworks that would otherwise only run on Python 3.3+ due to their use of unicode literal prefixes.

One such import hook project is Vinay Sajip’s uprefix 4.

For those that prefer to translate their code in advance rather than converting on the fly at import time, Armin Ronacher is working on a hook that runs at install time rather than during import 5.

Combining the two approaches is of course also possible. For example, the import hook could be used for rapid edit-test cycles during local development, but the install hook for continuous integration tasks and deployment on Python 3.2.

The approaches described in this section may prove useful, for example, for applications that wish to target Python 3 on the Ubuntu 12.04 LTS release, which will ship with Python 2.7 and 3.2 as officially supported Python versions.

Complaint: Python 3 shouldn’t be made worse just to support porting from Python 2

This is indeed one of the key design principles of Python 3. However, one of the key design principles of Python as a whole is that “practicality beats purity”. If we’re going to impose a significant burden on third party developers, we should have a solid rationale for doing so.

In most cases, the rationale for backwards incompatible Python 3 changes are either to improve code correctness (for example, stricter default separation of binary and text data and integer division upgrading to floats when necessary), reduce typical memory usage (for example, increased usage of iterators and views over concrete lists), or to remove distracting nuisances that make Python code harder to read without increasing its expressiveness (for example, the comma based syntax for naming caught exceptions). Changes backed by such reasoning are not going to be reverted, regardless of objections from Python 2 developers attempting to make the transition to Python 3.

In many cases, Python 2 offered two ways of doing things for historical reasons. For example, inequality could be tested with both != and <> and integer literals could be specified with an optional L suffix. Such redundancies have been eliminated in Python 3, which reduces the overall size of the language and improves consistency across developers.

In the original Python 3 design (up to and including Python 3.2), the explicit prefix syntax for unicode literals was deemed to fall into this category, as it is completely unnecessary in Python 3. However, the difference between those other cases and unicode literals is that the unicode literal prefix is not redundant in Python 2 code: it is a programmatically significant distinction that needs to be preserved in some fashion to avoid losing information.

While porting tools were created to help with the transition (see next section) it still creates an additional burden on heavy users of unicode strings in Python 2, solely so that future developers learning Python 3 don’t need to be told “For historical reasons, string literals may have an optional u or U prefix. Never use this yourselves, it’s just there to help with porting from an earlier version of the language.”

Plenty of students learning Python 2 received similar warnings regarding string exceptions without being confused or irreparably stunted in their growth as Python developers. It will be the same with this feature.

This point is further reinforced by the fact that Python 3 still allows the uppercase variants of the B and R prefixes for bytes literals and raw bytes and string literals. If the potential for confusion due to string prefix variants is that significant, where was the outcry asking that these redundant prefixes be removed along with all the other redundancies that were eliminated in Python 3?

Just as support for string exceptions was eliminated from Python 2 using the normal deprecation process, support for redundant string prefix characters (specifically, B, R, u, U) may eventually be eliminated from Python 3, regardless of the current acceptance of this PEP. However, such a change will likely only occur once third party libraries supporting Python 2.7 is about as common as libraries supporting Python 2.2 or 2.3 is today.

Complaint: The WSGI “native strings” concept is an ugly hack

One reason the removal of unicode literals has provoked such concern amongst the web development community is that the updated WSGI specification had to make a few compromises to minimise the disruption for existing web servers that provide a WSGI-compatible interface (this was deemed necessary in order to make the updated standard a viable target for web application authors and web framework developers).

One of those compromises is the concept of a “native string”. WSGI defines three different kinds of string:

text strings: handled as unicode in Python 2 and str in Python 3
native strings: handled as str in both Python 2 and Python 3
binary data: handled as str in Python 2 and bytes in Python 3

Some developers consider WSGI’s “native strings” to be an ugly hack, as they are explicitly documented as being used solely for latin-1 decoded “text”, regardless of the actual encoding of the underlying data. Using this approach bypasses many of the updates to Python 3’s data model that are designed to encourage correct handling of text encodings. However, it generally works due to the specific details of the problem domain - web server and web framework developers are some of the individuals most aware of how blurry the line can get between binary data and text when working with HTTP and related protocols, and how important it is to understand the implications of the encodings in use when manipulating encoded text data. At the application level most of these details are hidden from the developer by the web frameworks and support libraries (both in Python 2 and in Python 3).

In practice, native strings are a useful concept because there are some APIs (both in the standard library and in third party frameworks and packages) and some internal interpreter details that are designed primarily to work with str. These components often don’t support unicode in Python 2 or bytes in Python 3, or, if they do, require additional encoding details and/or impose constraints that don’t apply to the str variants.

Some example of interfaces that are best handled by using actual str instances are:

Python identifiers (as attributes, dict keys, class names, module names, import references, etc)
URLs for the most part as well as HTTP headers in urllib/http servers
WSGI environment keys and CGI-inherited values
Python source code for dynamic compilation and AST hacks
Exception messages
__repr__ return value
preferred filesystem paths
preferred OS environment

In Python 2.6 and 2.7, these distinctions are most naturally expressed as follows:

u"": text string (unicode)
"": native string (str)
b"": binary data (str, also aliased as bytes)

In Python 3, the latin-1 decoded native strings are not distinguished from any other text strings:

"": text string (str)
"": native string (str)
b"": binary data (bytes)

If from __future__ import unicode_literals is used to modify the behaviour of Python 2, then, along with an appropriate definition of n(), the distinction can be expressed as:

"": text string
n(""): native string
b"": binary data

(While n=str works for simple cases, it can sometimes have problems due to non-ASCII source encodings)

In the common subset of Python 2 and Python 3 (with appropriate specification of a source encoding and definitions of the u() and b() helper functions), they can be expressed as:

u(""): text string
"": native string
b(""): binary data

That last approach is the only variant that supports Python 2.5 and earlier.

Of all the alternatives, the format currently supported in Python 2.6 and 2.7 is by far the cleanest approach that clearly distinguishes the three desired kinds of behaviour. With this PEP, that format will also be supported in Python 3.3+. It will also be supported in Python 3.1 and 3.2 through the use of import and install hooks. While it is significantly less likely, it is also conceivable that the hooks could be adapted to allow the use of the b prefix on Python 2.5.

Complaint: The existing tools should be good enough for everyone

A commonly expressed sentiment from developers that have already successfully ported applications to Python 3 is along the lines of “if you think it’s hard, you’re doing it wrong” or “it’s not that hard, just try it!”. While it is no doubt unintentional, these responses all have the effect of telling the people that are pointing out inadequacies in the current porting toolset “there’s nothing wrong with the porting tools, you just suck and don’t know how to use them properly”.

These responses are a case of completely missing the point of what people are complaining about. The feedback that resulted in this PEP isn’t due to people complaining that ports aren’t possible. Instead, the feedback is coming from people that have successfully completed ports and are objecting that they found the experience thoroughly unpleasant for the class of application that they needed to port (specifically, Unicode aware web frameworks and support libraries).

This is a subjective appraisal, and it’s the reason why the Python 3 porting tools ecosystem is a case where the “one obvious way to do it” philosophy emphatically does not apply. While it was originally intended that “develop in Python 2, convert with 2to3, test both” would be the standard way to develop for both versions in parallel, in practice, the needs of different projects and developer communities have proven to be sufficiently diverse that a variety of approaches have been devised, allowing each group to select an approach that best fits their needs.

Lennart Regebro has produced an excellent overview of the available migration strategies 2, and a similar review is provided in the official porting guide 3. (Note that the official guidance has softened to “it depends on your specific situation” since Lennart wrote his overview).

However, both of those guides are written from the founding assumption that all of the developers involved are already committed to the idea of supporting Python 3. They make no allowance for the social aspects of such a change when you’re interacting with a user base that may not be especially tolerant of disruptions without a clear benefit, or are trying to persuade Python 2 focused upstream developers to accept patches that are solely about improving Python 3 forward compatibility.

With the current porting toolset, every migration strategy will result in changes to every Unicode literal in a project. No exceptions. They will be converted to either an unprefixed string literal (if the project decides to adopt the unicode_literals import) or else to a converter call like u("text").

If the unicode_literals import approach is employed, but is not adopted across the entire project at the same time, then the meaning of a bare string literal may become annoyingly ambiguous. This problem can be particularly pernicious for aggregated software, like a Django site - in such a situation, some files may end up using the unicode_literals import and others may not, creating definite potential for confusion.

While these problems are clearly solvable at a technical level, they’re a completely unnecessary distraction at the social level. Developer energy should be reserved for addressing real technical difficulties associated with the Python 3 transition (like distinguishing their 8-bit text strings from their binary data). They shouldn’t be punished with additional code changes (even automated ones) solely due to the fact that they have already explicitly identified their Unicode strings in Python 2.

Armin Ronacher has created an experimental extension to 2to3 which only modernizes Python code to the extent that it runs on Python 2.7 or later with support from the cross-version compatibility six library. This tool is available as python-modernize 1. Currently, the deltas generated by this tool will affect every Unicode literal in the converted source. This will create legitimate concerns amongst upstream developers asked to accept such changes, and amongst framework users being asked to change their applications.

However, by eliminating the noise from changes to the Unicode literal syntax, many projects could be cleanly and (comparatively) non-controversially made forward compatible with Python 3.3+ just by running python-modernize and applying the recommended changes.