Extending built-in objects/classes

[Jon Clements]

Hi All,

I've reached the point in using Python where projects, instead of being
like 'batch scripts', are becoming more like 'proper' programs.

Therefore, I'm re-designing most of these and have found things in
common which I can use classes for. As I'm only just starting to get
into classes, I see that new style classes are thte way to go, so will
be using those. I come from a C++ background, and understand I need to
adjust my thinking in certain ways - I have read
http://www.geocities.com/foetsch/python/new_style_classes.htm.


As a really simple class, I've decided to make a 'str' to include a
'substr' function. Yes, I know this can be done using slicing, and
effectively this is what substr would do: something like;

class mystr(str):
    """" My rather rubbish but trying to be simple custom string class
"""
    def substr(self,start,length,pad=False):
        """
        Return str of (up to) _length_ chars, starting at _start_ which
is 1 offset based.
        If pad is True, ensure _length_ chars is returned by padding
with trailing whitespace.
        """"
        return self.<what>[ (start-1): (start-1)+length ]

Ignore the fact pad isn't implemented...

<what> should be the actual string value of the string object: How do I
work out what this is?
Secondly, I'm not 100% sure what I need for the __init__; is str's
__init__ implicitly called, or do I need to call str's __init__ in
mystr's (I seem to remember seeing some code which did this, as well as
calling super()).

Any critiscm is appreciated.

Many thanks,

Jon.