This is only true for string literals. Strings constructed at runtime may (and often will) end up as separate objects:

    >>> a = "this is a long sentence in a string literal"
    >>> b = "this is a long" + " sentence in a " + "string literal"
    >>> a is b
    False
    >>> id(a)
    4386376272
    >>> id(b)
    4387721264
    >>> a == b
    True