[Python-Dev] itertools addition: getitem()

[Walter Dörwald]

Guido van Rossum wrote:
> On 7/8/07, Georg Brandl <g.brandl at gmx.net> wrote:
>> Guido van Rossum schrieb:
>>> How important is it to have the default in this API? __getitem__()
>>> doesn't have a default; instead, there's a separate API get() that
>>> provides a default (and I find defaulting to None more manageable than
>>> the "_default = object()" pattern).

Of course it isn't implemented this way in the C version.

>> getattr() has a default too, while __getattr__ hasn't...
> 
> Fair enough.
> 
> But I still want to hear of a practical use case for the default here.

In most cases

    foo = getitem(iterable, 0, None)
    if foo is not None:
       ...

is simpler than:

    try:
       foo = getitem(iterable, 0)
    except IndexError:
       pass
    else:
       ...

Here is a use case from one of my "import XML into the database" scripts:

    compid = getitem(root[ns.Company_company_id], 0, None)
    if compid:
       compid = int(compid)

The expression root[ns.company_id] returns an iterator that produces all 
children of the root node that are of the element type company_id. If 
there is a company_id its content will be turned into an int, if not 
None will be used.

Servus,
    Walter