bool(x) may sometimes do more than x is None. If x is not None, then truth value is determined by calling __nonzero__ or __len__. For an ordinary sequence type

bool(x) may sometimes do more than x is None. If x is not None, then truth value is determined by calling __nonzero__ or __len__. For an ordinary sequence type that's fine, but some years ago I had code like this: if not self.db: self.db = bsddb.hashopen(...). I just couldn't find out why my process spent valuable seconds apparently reading in the entire bsddb database into memory at random times -- but that's because bool(self.db) above turned into len(self.db.keys()) != 0