Why doesn't join() call str() on its arguments?

[Jeff Shannon]

Roy Smith wrote:

>>>What I can't find an explanation for is why str.join() doesn't
>>>automatically call str() on its arguments, so that e.g.
>>>str.join([1,2,4,5]) would yield "1245", and ditto for e.g.
>>>user-defined classes that have a __str__() defined.
>>
>>That would be the wrong thing to do when the arguments are unicodes.
> 
> Why would it be wrong?  I ask this with honest naivete, being quite 
> ignorant of unicode issues.

As someone else demonstrated earlier...

 >>> str(u'ü')
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
UnicodeError: ASCII encoding error: ordinal not in range(128)
 >>>

Using str() on a unicode object works... IF all of the unicode 
characters are also in the ASCII charset.  But if you're using 
non-ASCII unicode characters (and there's no point to using Unicode 
unless you are, or might be), then str() will throw an exception.

The Effbot mentioned a join() implementation that would be smart 
enough to do the right thing in this case, but it's not as simple as 
just implicitly calling str().

Jeff Shannon
Technician/Programmer
Credit International