3 ms·

> if x is None: ... > if items is None: ... None's truth value is false, so the above are equivalent to: if x: ... if items: ... Empty sequences and mapp

> if x is None: ... > if items is None: ... None's truth value is false, so the above are equivalent to: if x: ... if items: ... Empty sequences and mappings are also considered false, so you don't need to if len(items): ... Instead, you should if items: ... On a side note, does it annoy the hell out of anyone else that a sequence's length is len(foo) instead of foo.length() (or size, count, etc)?

sah 18y ago
There's a reason why "if x is None" is idiomatic when "if not x" is shorter: >>> x = '' >>> if x is None: ... print 'x is None' ... >>> if not x: ... print 'x is empty' ... x is empty
- yogipatel 18y ago
  Good point. I was only thinking of sequence types, and failed to see that it would break with strings. I also somehow forgot to include "not" in the if statements! Pretty sad for a first post...
deleted 18y ago
[deleted]
Erwin 18y ago
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
etal 18y ago
Most of the builtins have a corresponding __magic_attribute__ for objects that support it: bool -> foo.__bool__, len -> foo.__len__, iter -> foo.__iter__. This is usually because some syntactic sugar or other built-in function/method relies on it being there. Example: "for x in foo" relies on foo having __iter__ (or an equivalent, maybe).