lambda

[Just]

In article <slrncunejk.36o.apardon at rcpc42.vub.ac.be>,
 Antoon Pardon <apardon at forel.vub.ac.be> wrote:

> Op 2005-01-17, Steve Holden schreef <steve at holdenweb.com>:

> > There you go with the minutiae again. How about:
> >
> > "Don't use mutables as hash keys"?
> 
> That sounds too dogmatic to my ears. I also find it
> too selective. The problem with mutables as dictionary
> keys is not specific to dictionaries. Everywhere you
> have mutables in a container, it is possible that
> mutating the object in the container will cause
> problem.

The main difference is: if you mutate a dict key you *always* have a 
problem. So don't do that. Mutating (say) a list item *only* is a 
problem if you (say) need that list to remain sorted. Lists don't 
magically remain sorted, so people generally sort it before they do some 
operation that expects a sorted list.

> Heck even using mutables as arguments can
> cause trouble. Why else the specific advice against
> 
>   def foo(p = [])
> 
> type of arguments. So should we adopt the principles:
> 
>   Don't use mutables in containers

Nonsense.

>   Don't use mutables as default values for parameters

That's good advice in general.

>   Don't use mutables as arguments.

Nonsense.

>   Don't assign one mutable to an other.

Nonsense. Some newbies get surprised by Python's assignment-doesn't-copy 
semantics, but it's such basic knowledge (as well as a useful feature) 
that I simply don't understand you saying this.

> I don't see a big difference between these principles
> and the hash key principle,

Than you haven't looked hard enough.

> so in the end may be we
> should just stick with the more general principle:
> 
>   Don't use mutables!
> 
> 
> and be done with it.

Ok, so you're indeed a troll.

Just