Different storage position of equal strings with special characters [duplicate]

Question

I am new at python and I'm currently exploring some of its core functionalities.

Could you explain me why the following example always return false in case of a string with special characters:

>>> a="x"
>>> b="x"
>>> a is b
True
>>> a="xxx"
>>> b="xxx"
>>> a is b
True
>>> a="xü"
>>> b="xü"
>>> a is b
False
>>> a="ü"
>>> b="ü"
>>> a is b
True
>>> #strange: with one special character it works as expected

I understand that the storage positions are different for strings with special characters on each assignment, I already checked it with the id() function but for which reason python handles strings in this unconsistent way?

Answer 1

Python (the reference implementation at least) has a cache for small integers and strings. I guess unicode strings outside the ASCII range are bigger than the cache threshold (internally unicode is stored using 16 or 32 bit wide characters, UCS-2 or UCS-4) and so they are not cached.

[edit]

Found a more complete answer at: About the changing id of a Python immutable string

Se also: http://www.laurentluce.com/posts/python-string-objects-implementation/