[Python-Dev] bool(container) [was bool(iter([])) changedbetween 2.3 and 2.4]

Michael Chermside mcherm at mcherm.com
Fri Sep 30 18:23:10 CEST 2005

Jim Jewett writes:
> Python doesn't worry about a precise boolean object, it
> distinguishes between something and nothing.
> Is there anything left?" is a pretty good analogy for iterators.
> A Queue.Queue is always true.  Should I submit a bug patch

I would have phrased this very differently. I would have said
it as follows:

  All Python objects can be converted to a boolean value. Most
  objects are True, except for False (of course), None, any number
  equal to 0, an empty string (""), and the empty containers
  (), [], and {}. User defined objects can override __len__ or
  __nonzero__ to return False; everything else is True.

I don't believe that there is an overarching rule that all "containers"
must be false when empty. I'm not sure there is even a clear
definition of "container" that would satisfy everyone. It makes
complete sense to me that empty iterators and Queue.Queues are True...
it follows the general rule that everything is true except the above
(short) list of objects and user-defined (or library) classes that
want to mimic the behavior of one of these or have some obvious
meaning for non-true.

-- Michael Chermside

More information about the Python-Dev mailing list