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Mon Jun 21 19:29:55 CEST 2010 · https://mail.python.org/pipermail/python-dev/2010-June/100793.html

At 05:49 PM 6/21/2010 +0100, Michael Foord wrote:
>Why is your proposed bstr wrapper not practical to implement outside 
>the core and use in your own libraries and frameworks?

__contains__ doesn't have a converse operation, so you can't code a 
type that works around this (Python 3.1 shown):

 >>> from os.path import join
 >>> join(b'x','y')
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "c:\Python31\lib\ntpath.py", line 161, in join
     if b[:1] in seps:
TypeError: Type str doesn't support the buffer API
 >>> join('y',b'x')
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "c:\Python31\lib\ntpath.py", line 161, in join
     if b[:1] in seps:
TypeError: 'in <string>' requires string as left operand, not bytes

IOW, only one of these two cases can be worked around by using a bstr 
(or ebytes) that doesn't have support from the core string type.

I'm not sure if the "in" operator is the only case where implementing 
such a type would fail, but it's the most obvious one.  String 
formatting, of both the % and .format() varieties is 
another.  (__rmod__ doesn't help if your bytes object is one of 
several data items in a tuple or dict -- the common case for % formatting.)

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