A test using in format "chr(x) in <string>" raises a TypeError if "x" is in range 128-255 (i.e. non-ascii) and string is unicode. This happens even if the unicode string contains only ascii data as the example below demonstrates.

Python 2.5.1 (r251:54863, May  2 2007, 16:56:35) 
[GCC 4.1.2 (Ubuntu 4.1.2-0ubuntu4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> chr(127) in 'hello'
False
>>> chr(128) in 'hello'
False
>>> chr(127) in u'hi'
False
>>> chr(128) in u'hi'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'in <string>' requires string as left operand

This can cause pretty nasty and hard-to-debug bugs in code using "in <string>" format if e.g. user provided data is converted to unicode internally. Most other string operations work nicely between normal and unicode strings and I'd say simply returning False in this situation would be ok too. Issuing a warning similarly as below might be a good idea also.  

>>> chr(128) == u''
__main__:1: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal

Finally, the error message is somewhat misleading since the left operand is definitely a string.

>>> type(chr(128))
<type 'str'>

A real life example of code where this problem exist is telnetlib. I'll submit a separate bug about it as that problem can obviously be fixed in the library itself.

"Most other string operations work nicely between normal and unicode strings"

Nope.  You *always* get errors if you mix Unicode with NON-ASCII data (unless you've messed up the system's default encoding, which is a bad thing to do if you care about portability).  Some examples:

>>> chr(128) + u"foo"
UnicodeDecodeError: 'ascii' codec can't decode byte 0x80 in position 0: ordinal not in range(128)
>>> u"foo".find(chr(128))
UnicodeDecodeError: 'ascii' codec can't decode byte 0x80 in position 0: ordinal not in range(128)

etc.  If there's a bug here, it's that you get a TypeError instead of a ValueError subclass.

Fredrik, you are obviously correct that most operations between normal and unicode strings don't work if the normal string contains non-ascii data.

I still do think that a UnicodeWarning like you get from "chr(128) == u'foo'" would be nicer than an exception and prevent problems like the one in telnetlib [1]. If an exception is raised I don't care too much about its type but a better message would make debugging possible problems easier.

[1] https://sourceforge.net/tracker/index.php?func=detail&aid=1772794&group_id=5470&atid=105470

assigning all open telnetlib items to myself

I don't think we'll do anything about this message in 2.x -- in 3.x you get a clear TypeError anyway if you mix str and bytes.

I'm not sure that I'd consider:

>>> 'abc' in b'abcde'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Type str doesn't support the buffer API

a clear error message :)  It certainly isn't as bad as the 2.x message, though.

Ah. I tried the other combination :)