[Python-Dev] Backporting PEP 3101 to 2.6

[Eric Smith]

Eric Smith wrote:
>> Guido van Rossum wrote:
>>> For data types whose output uses only ASCII, would it be acceptable if
>>> they always returned an 8-bit string and left it up to the caller to
>>> convert it to Unicode? This would apply to all numeric types. (The
>>> date/time types have a strftime() style API which means the user must
>>> be able to specifiy Unicode.)

I'm finally getting around to finishing this up.  The approach I've 
taken for int, long, and float, is that they take either unicode or str 
format specifiers, and always return str results.  The builtin format() 
deals with converting str to unicode, if the format specifier was 
originally unicode.  This all works great.  It allows me to easily 
implement both ''.format and u''.format taking int, long, and float 
parameters.

I'm now working on datetime.  The __format__ method is really just a 
wrapper around strftime.  I was assuming (or rather hoping) that 
strftime does the right thing with unicode and str (unicode in = unicode 
out, str in = str out).  But it turns out strftime doesn't accept unicode:

$ ./python
Python 2.6a0 (trunk:60845M, Feb 15 2008, 21:09:57)
[GCC 4.1.2 20070626 (Red Hat 4.1.2-13)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> import datetime
 >>> datetime.date.today().strftime('%y')
'08'
 >>> datetime.date.today().strftime(u'%y')
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: strftime() argument 1 must be str, not unicode

As part of this task, I'm really not up to the job of changing strftime 
to support both str and unicode inputs.  So I think I'll put all of the 
__format__ code in place to support it if and when strftime supports 
unicode.  In the meantime, it won't be possible for u''.format to work 
with datetime objects.

 >>> 'year: {0:%y}'.format(datetime.date.today())
'year: 08'
 >>> u'year: {0:%y}'.format(datetime.date.today())
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: strftime() argument 1 must be str, not unicode

The bad error message is a result of __format__ passing on unicode to 
strftime.

There are, of course, various ugly ways to work around this involving 
nested format calls.

Maybe I'll extend strftime to unicode for the PyCon sprint.

Eric.