python-peps/pep-0414.txt

PEP: 414
Title: Explicit Unicode Literal for Python 3.3
Version: $Revision$
Last-Modified: $Date$
Author: Armin Ronacher <armin.ronacher@active-4.com>,
        Nick Coghlan <ncoghlan@gmail.com>
Status: Accepted
Type: Standards Track
Content-Type: text/x-rst
Created: 15-Feb-2012
Post-History: 28-Feb-2012, 04-Mar-2012
Resolution: http://mail.python.org/pipermail/python-dev/2012-February/116995.html


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" | "ur" | "UR" | "Ur" | "uR"

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"""

Combination of the unicode prefix with the raw string prefix will also be
supported, just as it was in Python 2.

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


Author's Note
=============

This PEP was originally written by Armin Ronacher, and directly reflected his
feelings regarding his personal experiences porting Unicode aware Python
applications to Python 3. Guido's approval was given based on Armin's version
of the PEP.

The currently published version has been rewritten by Nick Coghlan to address
the concerns of those who felt that Armin's experience did not accurately
reflect the *typical* experience of porting to Python 3, but rather only
related to a specific subset of porting activities that were not well served
by the existing set of porting tools.

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. After all, developers are people first,
coders second.


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 may be feasible to support it as a third
party import hook. While such an import hook will impose a small import time
overhead, and will require additional steps from each application that needs it
to get the hook in place, it would allow applications that target Python 3.2
to use libraries and frameworks that may otherwise only run on Python 3.3+.

This approach may prove useful, for example, for applications that wish to
target Python 3 for the Ubuntu LTS release that ships with Python 2.7 and 3.2.

If such an import hook becomes available, this PEP will be updated to include
a reference to it.


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 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 be eventually eliminated
from Python 3, regardless of the current acceptance of this PEP.


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

Native strings are a useful concept because there are some APIs and internal
operations that are designed primarily to work with native strings. They often
don't support ``unicode`` in Python 2 or support ``bytes`` in Python 3 (at
least, not without needing additional encoding information and/or imposing
constraints that don't apply to the native string variants).

Some example of such interfaces are:

* Python identifiers (dict keys, class names, module names, import paths)
* 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
* ``""``: native string
* ``b""``: binary data

In Python 3, the native strings are not distinguished from any other text
strings:

* ``""``: text string
* ``""``: native string
* ``b""``: binary data

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+. If the import hook approach works out as planned, it may even be
supported in Python 3.1 and 3.2. A bit more effort could likely adapt the hook
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 sucessfully
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 succesfully *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.


References
==========

.. [1] Python-Modernize
   (http://github.com/mitsuhiko/python-modernize)

.. [2] Porting to Python 3: Migration Strategies
   (http://python3porting.com/strategies.html)

.. [3] Porting Python 2 Code to Python 3
   (http://docs.python.org/howto/pyporting.html)

Copyright
=========

This document has been placed in the public domain.


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