Code design for a sub-class of built-ins

[Alex Martelli]

Steven D'Aprano <steve at REMOVETHIScyber.com.au> wrote:
   ...
> The obvious problem is, I have to create a custom method for every string
> method -- and if strings gain any new methods in some future version of
> Python, my subclass won't exhibit the correct behaviour.

As others already suggested, automating such decoration is pretty easy;
you can do it with either a custom metaclass or a simple post-processing
of your class in a loop.  Untested details below, but the general idea
would be something like:

class SDAstr(str):
    magic = 'whatever'

def _SDAdecorate(methodname, strmethod):
    def decorated(self, *a, **k):
        if self == self.magic: print "whatever!"
        return strmethod(self, *a, **k)
    setattr(SDAstr, methodname, decorated)

for methodname, strmethod in \
    inspect.getmembers(str, inspect.ismethoddescriptor):
        _SDAdecorate(methodname, strmethod)

and variants thereof (to provide more accurate metainformation with the
decorated methods -- name, docstring, signature, whatever; and/or to
encapsulate things in a custom metaclass; whatever).


> class MyInt(int):
>     """Like an int but with special behaviour."""
>     def __init__(self, value, extra):
   ...
> Looks like my __init__ isn't even being called here. Why not, and how do I
> fix this?

Override __new__, which is what invariably gets called (before __init__,
which only gets called if __new__ returns an instance of the class that
you're instantiating).


Alex