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Date: Fri, 25 Nov 2011 19:46:17 +1100
Subject: Re: Strange result ffor object to bool
From: Chris Angelico <rosuav@gmail.com>
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:16205

On Fri, Nov 25, 2011 at 5:52 PM, ZhouPeng <zpengxen@gmail.com> wrote:
> I am sure listen is not None and can be accessed properly.
>
> But print bool(listen) is False
> if not listen =A0is True

Casting something to boolean follows strict rules derived from the
notion that emptiness is false and nonemptiness is true. For instance:

>>> bool(""),bool([]),bool({}),bool(0)
(False, False, False, False)

Details here: http://docs.python.org/library/stdtypes.html

Chris Angelico