Way for see if dict has a key

[Paul McGuire]

"Michele Petrazzo" <michele.petrazzo at TOGLIunipex.it> wrote in message
news:zZ7pg.21792$zy5.445667 at twister1.libero.it...
> > when we expect that the key will most often be in my_dict so that
> > exception will be raised rarely
>
> I didn't thought this because if I think that a key aren't in a dict, I
> use dict.get(key, default)
>

Another factor to consider is if "default" is not something simple like 0 or
None, but is an object constructed and initialized with some expensive
initialization code (like access to a database).  In this case:

    dict.get(key, ExpensiveObjectToCreate())

always creates the default value, and if the key does not exist, then
promptly drops it on the floor.

In such a case you'd be better off with one of the "check for existence"
patterns, which only creates the default value if you *know* you're going to
need it.

-- Paul