How can I create customized classes that have similar properties as 'str'?

[samwyse]

On Nov 24, 10:35 am, Licheng Fang <fanglich... at gmail.com> wrote:
> Thanks. Then, is there a way to make python treat all strings this
> way, or any other kind of immutable objects?

The word generally used is 'atom' when referring to strings that are
set up such that 'a == b' implies 'a is b'.  This is usually an
expensive process, since you don't want to do it to strings that are,
e.g., read from a file.  Yes, it could be done only for string
constants, and some languages (starting with LISP) do this, but that
isn't what you (or most people) want.  Whether you realize it or not,
you want control over the process; in your example, you don't want to
do it for the lines read from your file, just the trigrams.

The example that I gave does exactly that.  It adds a fixed amount of
storage for each string that you 'intern' (the usual name given to the
process of generating such a string.  Let's look at my code again:

>>> store = {}
>>> def atom(str):
        global store
        if str not in store:
                store[str] = str
        return store[str]

Each string passed to 'atom' already exists.  We look to see if copy
already exists; if so we can discard the latest instance and use that
copy henceforth.  If a copy does not exist, we save the string inside
'store'.  Since the string already exists, we're just increasing its
reference count by two (so it won't be reference counted) and
increasing the size of 'store' by (an amortized) pair of pointers to
that same string.