A matter of style: class.foo = bar vs. class.set_foo(bar)

echuck at mindspring.com echuck at mindspring.com
Tue Nov 14 05:50:33 CET 2000


In article <8ujrso02bef at news1.newsguy.com>,
  "Alex Martelli" <aleaxit at yahoo.com> wrote:
> <echuck at mindspring.com> wrote in message
news:8ujpft$4at$1 at nnrp1.deja.com...
>     [snip]
> > The disadvantage of:
> >   obj.foo = 5
> >
> > is that in the future if you want to take some action when foo is
set,
> > like asserting that it's in range or not None, then you're out of
luck.
>
> Naah -- you just have to add a __setattr__ method (or a clause to
> your existing __setattr__ method); or, with suitable metaclasses
> (such as those in py_cpp), you even get to define a specific
> __setattr__foo method to catch the specific setting of obj.foo only.

Although it's technically feasible to do a __setattr__, what would the
results look like for foo, bar, a, b, c?

   if name=="foo":
      do_this()
   elif name=="bar":
      do_that()
   elif name=="...

You get the drift. This is really inefficient. Every time you add a new
attribute you increase this linear search and pile on more code in
__setattr__. I suppose you could do this:

   methodName = "set_"+name
   method = getattr(self, methodName, None)
   if method:
      method(name, value)
   else:
      self.__dict__[name] = value

But it feels silly to make setattr to call set_foo() for you...maybe
I'm just not use to it.


I can't say I'm familiar with the metaclasses you refer to. Care to
expand with an example in this context?


> You may still choose, as a matter of style, to impose the use of
> .setFoo or whatever, but it's also a possibility, of equal functional
> ability, to let client-code use the more-natural-to-some syntax
>     obj.foo = 5
> rather than
>     obj.setFoo(5)
>
> Worst case, you'll trade off a tiny bit more of programming in
> the class (if you're not using suitable metaclasses) for a bit
> more convenience for the client!  Remember that, if you let
> the assignment operator be used by the client, he then can,
> "for free" (no extra coding needed on your part...):
>     obj.foo += 5
> and
>     obj.foo *= 2
> etc etc, rather than having to write
>     obj.setFoo(obj.getFoo() * 2)
> etc.  It's surely a feasible style choice to enable this.

Well, I don't use "get", so it would be shorter. :-)

Also, my attributes are hardly ever numeric. They are usually other
objects, strings, lists, dictionaries, etc.


-Chuck
--
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