Using String Methods In Jump Tables

[Chris Rebert]

On Thu, Aug 19, 2010 at 4:27 PM, Tim Daneliuk <tundra at tundraware.com> wrote:
> Problem:
>
>  Given tuples in the form (key, string), use 'key' to determine
>  what string method to apply to the string:
>
>    key           operation
>    -----------------------
>
>     l            lower()
>     u            upper()
>     t            title()
>     ...
>
> Commentary:
>
>   Easy, right?  Well ... except that I would really, really like
>   to avoid cascading ifs or eval based solutions.  I'd like to implement
>   this as a jump table dictionary:
>
>     jt = { 'l' : lower_function reference,
>            'u' : upper_function reference,
>             ...
>          }
>
>   So I could then do this:
>
>     string = jt[key](string)
>
> But There's A Problem:
>
>   I tried to do this:
>
>     jt = {'l', "".lower,
>           'u', "".upper,
>           ...
>          }
>
>   You can get away with this because all string objects appear to point to common
>   method objects.  That is,: id("a".lower) == id("b".lower)
>
>   HOWEVER, you cannot then do this:
>
>     string = jt[key](string)
>
>   Why?  Because the methods of a string, while common to all strings
>   *do not accept an arg*.  They are implemented to "know" about the
>   string instance they "belong to" (that contains them).

Right; by looking the method up on an instance you get a "bound"
method that is "bound" to the instance you got it from and already has
self filled-in.

> (Forgive me here, I am probably not using some of the OO arcana properly...)
>
<snip>
>
>   How do you get a reference to a method found in one object instance, but
>   actually apply it to another instance of the same class?  I'm guessing this may
>   involve fiddling with some of the internal __ variables, but I'm not
>   quite sure where to go next.

You must access the method from the class rather than the instance;
this produces an "unbound" method that is not bound to a specific
instance and does not have self already filled-in.
In your dictionary, replace "".lower with str.lower, and so on for the
other methods.

Cheers,
Chris
--
http://blog.rebertia.com