On Aug 20, 12:27 am, Tim Daneliuk <tun...@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).
>
> (Forgive me here, I am probably not using some of the OO arcana properly...)
>
> I realize that the string module has many of the same functions that
> are found as methods on strings. But it doesn't have all of them -
> the .title() method is notably absent. If this were not the case,
> I could happily do things like 'l' : string.lower and my approach
> would work fine.
>
> Even if I could use the string module, I'd still be curious though:
>
> 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.
>
> As I said, I know I could do this as a set of cascading ifs or even as an
> eval, but I'm loathe to use such approaches. I like jump tables as a
> structural construct because they are easy to understand and maintain. I
> also realize that what I'm asking may be violating some deeply held notion
> of OO purity, but, well, now I'm just curious if there is a way to do this
I think the solution to what you are asking for is to use getattr(),
eg:
>>> ops={'u':'upper','l':'lower'}
>>> s='hello world'
>>> getattr( s, ops['u'] )()
'HELLO WORLD'
>>>
For info on getattr() see:
http://docs.python.org/library/functions.html#getattr
or
http://effbot.org/zone/python-getattr.htm
Best wishes,
John
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