Question

I read somewhere (an SO post, I think, and probably somewhere else, too), that Python automatically references single character strings, so not only does 'a' == 'a', but 'a' is 'a'.

However, I can’t remember reading if this is guaranteed behavior in Python, or is it just implementation specific?

Bonus points for official sources.

Answer 1

It’s implementation specific. It’s difficult to tell, because (as the reference says):

… for immutable types, operations that compute new values may actually return a reference to any existing object with the same type and value, while for mutable objects this is not allowed.

The interpreter’s pretty good about ensuring they’re identical, but it doesn’t always work:

x = u'a'
y = u'abc'[:1]
print x == y, x is y

Run on CPython 2.6, this gives True False.