class with __len__ member fools boolean usage "if x:" ; bad coding style?

george young gry at ll.mit.edu
Sun Jun 27 00:44:36 CEST 2004


"John Roth" <newsgroups at jhrothjr.com> wrote in message news:<10dpicb9rjib2bb at news.supernews.com>...
> "george young" <gry at ll.mit.edu> wrote in message
> news:78b6a744.0406250737.310f31da at posting.google.com...
> > [Python 2.3.3, x86 linux]
> > I had developed the habit of using the neat python form:
> >    if someinstance:
> >       someinstance.memb()
> >
> > because it seems cleaner than "if someinstance is not None".
> > {please no flames about "is not None" vs. "!= None" ...}
> >
> > This seemed like a good idea at the time :().  Twice, recently,
> > however, as my
> > app grew, I thought, hmm... it would make things clearer if I gave
> > this container class a __len__ member and maybe a __getitem__. Great,
> > code looks
> > nicer now... crash,crash, many expletives deleted...
> >
> > Its great to be able to say containerinstance[seq] instead of
> > containerinstance.steps[seq], but I've also changed the semantics of
> > (containerinstance) in a boolean context.  My app breaks only in the
> > seldom case that the container is empty.
> >
> > Obviously I know how to fix the code, but I'm wondering if this isn't
> > a message
> > that "if containerinstance:" is not a good coding practice.
> 
> Almost. The message is that testing for None, however
> you're doing it, is a Code Smell in the sense defined in
> the Refactoring book. If some attribute is supposed to have
> a Foo object, then it should have a Foo or a subclass of
> Foo, not None.
> 
> Sometimes there's no way around it, but whenever you find
> yourself testing for None, consider using a Null Object instead.
> A Null Object is a subclass of the normal object you would
> be expecting, but one that has methods and attributes that
> handle the exceptional case cleanly.
> 
> Of course, there are a couple of very pretty idioms for
> handling optional parameters that depend on tests for None,
> but they're definitely special cases, and they also break if the
> real parameter can be False.

Null Object seems like a perfect fit for this.  I was unaware of it.
I read the original GOF book, but not much since then on patterns.
Thnks very much!

-- George



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