Location of python string class in the source code

You might mean this.

class MyCharacter( object ):
    def __init__( self, aString ):
        self.value= ord(aString[0])
    def __add__( self, other ):
        return MyCharacter( chr(self.value + other) )
    def __str__( self ):
        return chr( self.value )

It works like this.

>>> c= MyCharacter( "ABC" )
>>> str(c+2)
'C'
>>> str(c+3)
'D'
>>> c= c+1
>>> str(c)
'B'

It's documented here. The main implementation is in Objects/stringobject.c. Subclassing str probably isn't what you want, though. I would tend to prefer composition here; have an object with a string field, and special behavior.