Characters contain themselves?

[WENDUM Denis 47.76.11 (agent)]

While testing recursive algoritms dealing with generic lists I stumbled 
on infinite loops which were triggered by the fact that (at least for my 
version of Pyton) characters contain themselves.See session:

system prompt% python
Python 2.3.5 (#2, Feb  9 2005, 00:38:15)
[GCC 3.3.5 (Debian 1:3.3.5-8)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
    ....
 >>> 'a' is 'a'
True
 >>> 'a' in 'a'
True
 >>> 'a' in ['a']
True
 >>>  ....

Leading to paradoxes and loops objects which contain themselves (and 
other kinds of monsters) are killed in set theory with the Axiom of 
Foundation:=)

But let's go back to more earthly matters. I couldn't find any clue in a 
python FAQ after having googled with the following "Python strings FAQ" 
about why this design choice and how to avoid falling in this trap 
without having to litter my code everywhere with tests for stringiness 
each time I process a generic list of items.

Any hints would be appreciated.