Revised PEP 349: Allow str() to return unicode strings

[Neil Schemenauer]

[Please mail followups to py********@pyth on.org.]

The PEP has been rewritten based on a suggestion by Guido to change
str() rather than adding a new built-in function. Based on my
testing, I believe the idea is feasible. It would be helpful if
people could test the patched Python with their own applications and
report any incompatibiliti es.
PEP: 349
Title: Allow str() to return unicode strings
Version: $Revision: 1.3 $
Last-Modified: $Date: 2005/08/22 21:12:08 $
Author: Neil Schemenauer <na*@arctrix.co m>
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('__st r__ 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(P yObject *);

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_de code() requires a str instance and the
email.Header.de code_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(head er, 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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[wolf]

neil,

i just intended to worry that returning a unicode object from ``str()``
would break assumptions about the way that 'type definers' like
``str()``, ``int()``, ``float()`` and so on work, but i quickly
realized that e.g. ``int()`` does return a long where appropriate!
since the principle works there one may surmise it will also work for
``str()`` in the long run.

one point i don't seem to understand right now is why it says in the
function definition::

if type(s) is str or type(s) is unicode:
...

instead of using ``isinstance()` `.

Testing for ``type()`` means that instances of derived classes (that
may or may not change nothing or almost nothing to the underlying
class) when passed to a function that uses ``str()`` will behave in a
different way!

isn't it more realistic and commonplace to assume that derivatives of a
class do fulfill the requirements of the underlying class? -- which may
turn out to be wrong! but still...

the code as it stands means i have to remember that *in this special
case only* (when deriving from ``unicode``), i have to add a
``__str__()`` method myself that simply returns ``self``.

then of course, one could change ``unicode.__str __()`` to return
``self``, itself, which should work. but then, why so complicated?

i suggest to change said line to::

if isinstance( s, ( str, unicode ) ):
...

any objections?

_wolf

[Dieter Maurer]

Neil Schemenauer <na*@arctrix.co m> writes on Mon, 22 Aug 2005 15:31:42 -0600:

...
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_de code() requires a str instance and the
email.Header.de code_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(head er, unicode):
header = header.encode(' ascii')
Note, that this is not equivalent to the old "str(header )":

"str(header )" used Python's "default encoding" while the
new code uses 'ascii'.

The new code might be more correct than the old one has been.

...
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.

Can we get a new builtin with the exact same behaviour as
the current "str" which can be used when we do require an "str"
(and cannot use a "unicode").

Dieter