Preferred Python idiom for handling non-existing dictionary keys and why?

[Tim Peters]

[Skip Montanaro]
> d.setdefault() never made any sense to me (IOW, to use it I always
> had to look it up).  The semantics of what it does just never stick
> in my brain.

Mentally change the name to getorset() and I bet it will be easier.
d.getorset(x, default) gets the value of d[x] if there is one already, or
sets d[x] to default and returns that.  I think Guido would have changed the
name to something sane, except he was pissed at me that day <wink>.

> Consequently, even though it's less efficient I generally write such
> loops like this:
>
>     d = {}
>     for (key, val) in some_items:
>         lst = d.get(key) or []
>         lst.append(val)
>         d[key] = lst

If d[key] originally has a false value (0 or [] or {} or None or ...), this
will end up replacing it.  It works in the specific context of this "build a
list" loop, but in the general case can screw up.