On Tue, Feb 10, 2009 at 11:15 AM, Josh Dukes <josh.du...@microvu.com> wrote: > quite simply...what??? > > In [108]: bool([ x for x in range(10) if False ]) > Out[108]: False
This evaluates the list comprehension and creates an empty list, which is considered boolean False by Python. > In [109]: bool( x for x in range(10) if False ) > Out[109]: True Whereas this creates a /generator object/, whose inner expression is *not evaluated until specifically required* (e.g. by for-looping over the generator object). Generators don't have a specially defined truthiness critera (it's impossible to define generally -- consider something like `x for x in all_integers if f(x)`, for a complicated f(x), which would require a solution to the halting problem to know in advance if it will have a non-zero length), so they end up using the default behavior for objects with otherwise undefined boolean truth, which is to consider them True. Cheers, Chris -- Follow the path of the Iguana... http://rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
