PEP: Generalised String Coercion
nas at arctrix.com
Sat Aug 6 12:23:42 CEST 2005
The title is perhaps a little too grandiose but it's the best I
could think of. The change is really not large. Personally, I
would be happy enough if only %s was changed and the built-in was
not added. Please comment.
Title: Generalised String Coercion
Version: $Revision: 1.2 $
Last-Modified: $Date: 2005/08/06 04:05:48 $
Author: Neil Schemenauer <nas at arctrix.com>
Type: Standards Track
This PEP proposes the introduction of a new built-in function,
text(), that provides a way of generating a string representation
of an object without forcing the result to be a particular string
type. In addition, the behavior %s format specifier would be
changed to call text() on the argument. These two changes would
make it easier to write library code that can be used by
applications that use only the str type and by others that also
use the unicode type.
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
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:
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()
Using str() makes the code not Unicode-safe. Replacing a str()
call with a unicode() call makes the code not str-stable. Using a
string format almost accomplishes the goal but not quite.
Consider the following code:
return '%s' % obj
It behaves as desired except if 'obj' is not a basestring instance
and needs to return a Unicode representation of itself. In that
case, the string format will attempt to coerce the result of
__str__ to a str instance. Defining a __unicode__ method does not
help since it will only be called if the right-hand operand is a
unicode instance. Using a unicode instance for the right-hand
operand does not work because the function is no longer str-stable
(i.e. it will coerce everything to unicode).
A Python implementation of the text() built-in follows:
"""Return a nice string representation of the object. The
return value is a basestring instance.
if isinstance(s, basestring):
r = s.__str__()
if not isinstance(r, basestring):
raise TypeError('__str__ returned non-string')
Note that it is currently possible, although not very useful, to
write __str__ methods that return unicode instances.
The %s format specifier for str objects would be changed to call
text() on the argument. Currently it calls str() unless the
argument is a unicode instance (in which case the object is
substituted as is and the % operation returns a unicode instance).
The following function would be added to the C API and would be the
equivalent of the text() function:
PyObject *PyObject_Text(PyObject *o);
A reference implementation is available on Sourceforge  as a
The change to the %s format specifier would result in some %
operations returning a unicode instance rather than raising a
UnicodeDecodeError exception. It seems unlikely that the change
would break currently working code.
Rather than adding the text() built-in, if PEP 246 were
implemented then adapt(s, basestring) could be equivalent to
text(s). The advantage would be one less built-in function. The
problem is that PEP 246 is not implemented.
Fredrik Lundh has suggested  that perhaps a new slot should be
added (e.g. __text__), that could return any kind of string that's
compatible with Python's text model. That seems like an
attractive idea but many details would still need to be worked
Instead of providing the text() built-in, the %s format specifier
could be changed and a string format could be used instead of
calling text(). However, it seems like the operation is important
enough to justify a built-in.
Instead of providing the text() built-in, the basestring type
could be changed to provide the same functionality. That would
possibly be confusing behaviour for an abstract base type.
Some people have suggested  that an easier migration path would
be to change the default encoding to be UTF-8. Code that is not
Unicode safe would then encode Unicode strings as UTF-8 and
operate on them as str instances, rather than raising a
UnicodeDecodeError exception. Other code would assume that str
instances were encoded using UTF-8 and decode them if necessary.
While that solution may work for some applications, it seems
unsuitable as a general solution. For example, some applications
get string data from many different sources and assuming that all
str instances were encoded using UTF-8 could easily introduce
This document has been placed in the public domain.
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