Trivial performance questions

[Brian Patterson]

I have noticed in the book of words that hasattr works by calling getattr
and raising an exception if no such attribute exists.  If I need the value
in any case, am I better off using getattr within a try statement myself, or
is there some clever implementation enhancement which makes this a bad idea?

i.e. should I prefer:
    if hasattr(self,"datum"):
        datum=getattr("datum")
    else:
        datum=None
        self.datum=None

over:
    try:
        datum=self.getattr("datum")
    except:
        self.datum=None
        datum=None

The concept of deliberately raising an error is still foreign to this python
newbie, but I really like the trapping facilities.  I just worry about the
performance implications and memory usage of such things, especially since
I'm writing for Zope.

And while I'm here:  Is there a difference in performance when checking:
    datum is None
over:
    datum == None

and similarly:
    if x is None or y is None:
or:
    if None in (x,y):

I appreciate that these are trivial in the extreme, but I seem to be writing
dozens of them, and I may as well use the right one and squeeze what
performance I can.

Many thanks,
Christopher Boomer.