python-peps/pep-0349.txt

PEP: 349
Title: Allow str() to return unicode strings
Version: $Revision$
Last-Modified: $Date$
Author: Neil Schemenauer <nas@arctrix.com>
Status: Draft
Type: Standards Track
Content-Type: text/plain
Created: 02-Aug-2005
Post-History: 06-Aug-2005
Python-Version: 2.5


Abstract

    This PEP proposes to change the str() built-in function so that it
    can return unicode strings.  This change would make it easier to
    write code that works with either string type and would also make
    some existing code handle unicode strings.  The C function
    PyObject_Str() would remain unchanged and the function
    PyString_New() would be added instead.


Rationale

    Python has had a Unicode string type for some time now but use of
    it is not yet widespread.  There is a large amount of Python code
    that assumes that string data is represented as str instances.
    The long term plan for Python is to phase out the str type and use
    unicode for all string data.  Clearly, a smooth migration path
    must be provided.

    We need to upgrade existing libraries, written for str instances,
    to be made capable of operating in an all-unicode string world.
    We can't change to an all-unicode world until all essential
    libraries are made capable for it.  Upgrading the libraries in one
    shot does not seem feasible.  A more realistic strategy is to
    individually make the libraries capable of operating on unicode
    strings while preserving their current all-str environment
    behaviour.

    First, we need to be able to write code that can accept unicode
    instances without attempting to coerce them to str instances.  Let
    us label such code as Unicode-safe.  Unicode-safe libraries can be
    used in an all-unicode world.

    Second, we need to be able to write code that, when provided only
    str instances, will not create unicode results.  Let us label such
    code as str-stable.  Libraries that are str-stable can be used by
    libraries and applications that are not yet Unicode-safe.
    
    Sometimes it is simple to write code that is both str-stable and
    Unicode-safe.  For example, the following function just works:

        def appendx(s):
            return s + 'x'

    That's not too surprising since the unicode type is designed to
    make the task easier.  The principle is that when str and unicode
    instances meet, the result is a unicode instance.  One notable
    difficulty arises when code requires a string representation of an
    object; an operation traditionally accomplished by using the str()
    built-in function.
    
    Using the current str() function makes the code not Unicode-safe.
    Replacing a str() call with a unicode() call makes the code not
    str-stable.  Changing str() so that it could return unicode
    instances would solve this problem.  As a further benefit, some code
    that is currently not Unicode-safe because it uses str() would
    become Unicode-safe.


Specification

    A Python implementation of the str() built-in follows:

        def str(s):
            """Return a nice string representation of the object.  The
            return value is a str or unicode instance.
            """
            if type(s) is str or type(s) is unicode:
                return s
            r = s.__str__()
            if not isinstance(r, (str, unicode)):
                raise TypeError('__str__ returned non-string')
            return r
            
    The following function would be added to the C API and would be the
    equivalent to the str() built-in (ideally it be called PyObject_Str,
    but changing that function could cause a massive number of
    compatibility problems):

        PyObject *PyString_New(PyObject *);

    A reference implementation is available on Sourceforge [1] as a
    patch.

                
Backwards Compatibility

    Some code may require that str() returns a str instance.  In the
    standard library, only one such case has been found so far.  The
    function email.header_decode() requires a str instance and the
    email.Header.decode_header() function tries to ensure this by
    calling str() on its argument.  The code was fixed by changing
    the line "header = str(header)" to:

        if isinstance(header, unicode):
            header = header.encode('ascii')

    Whether this is truly a bug is questionable since decode_header()
    really operates on byte strings, not character strings.  Code that
    passes it a unicode instance could itself be considered buggy.


Alternative Solutions

    A new built-in function could be added instead of changing str().
    Doing so would introduce virtually no backwards compatibility
    problems.  However, since the compatibility problems are expected to
    rare, changing str() seems preferable to adding a new built-in.

    The basestring type could be changed to have the proposed behaviour,
    rather than changing str().  However, that would be confusing
    behaviour for an abstract base type.


References

    [1] http://www.python.org/sf/1266570


Copyright

    This document has been placed in the public domain.



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