PEP218: Representing the empty set

[Peter Hansen]

Paul Rubin wrote:
> 
> Peter Hansen <peter at engcorp.com> writes:
> > Maybe it's nothing more than this distinction:  dict() is a call which
> > *returns* an empty dictionary.  {} *is* an empty dictionary.
> 
> I don't understand in what ways these are really distinct.
> 
>     a = []
>     for i in range(2):
>       a.append({})
>     print (a[0] is a[1])
> 
> prints 0, if that helps.  Clearly "a = {}" and "a = dict()" both run
> some code that creates a new empty dictionary.  It's not the case that
> "a = {}" sets a to an empty dict created at compile time or anything
> like that.

I agree.  In that example, either is equally readable.

I think the example I would have in mind is a comparison:

if a == []:
    # do something
    if b != {}:
        a.append(b)

or whatever.

I think the difference lies in ones intent.  If the empty item is
going to be kept around and used, then a constructor approach seems
appropriate, maybe even arguably more readable.  If the empty item
is merely a temporary, to be used and discarded, then the apparent
extra overhead of the call, and the less succinct syntax, lead me to
think the short form is preferable.

-Peter