From time to time we see advice to use 'is' when testing against None in
Python. I was reminded of that by this checkin (which I should say I don't
think is actually a problem)
- self.default = default or update # default value for field
+ self.default = default==None and update or default
At any rate, I got bitten the other day, and I thought I'd pass on the
experience just because it's a real-world example of (sometimes) a real need to
use 'is'. I had a variable that defaulted to None, but could be bound to a
library object (I don't remember just now what the object was).
I had written:
if variable:
blah
This often (mostly?) works, though of course it lumps None along with anything
else that evaluates to False. However, in this particular case, I got an
exception, because the bound object did not have a __nonzero__ method, which is
what this test wants to see.
Changing to:
if variable is None
blah
fixed the problem.
As a relative novice Python handler, I'm trying to develop the habit of using
'is' to test (None, True, False).
