Add __bool__ to None

[rhettinger]

BPO	12647
Nosy	@brettcannon, @rhettinger, @pitrou, @vstinner, @durban, @ericsnowcurrently
Files	none_bool.diff: Py3.3 patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2011-07-28.16:56:42.947>
created_at = <Date 2011-07-27.19:26:16.149>
labels = ['interpreter-core']
title = 'Add __bool__ to None'
updated_at = <Date 2011-07-28.16:56:42.946>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2011-07-28.16:56:42.946>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2011-07-28.16:56:42.947>
closer = 'rhettinger'
components = ['Interpreter Core']
creation = <Date 2011-07-27.19:26:16.149>
creator = 'rhettinger'
dependencies = []
files = ['22781']
hgrepos = []
issue_num = 12647
keywords = ['patch']
message_count = 7.0
messages = ['141269', '141279', '141280', '141281', '141282', '141283', '141308']
nosy_count = 7.0
nosy_names = ['brett.cannon', 'rhettinger', 'pitrou', 'vstinner', 'daniel.urban', 'python-dev', 'eric.snow']
pr_nums = []
priority = 'low'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue12647'
versions = ['Python 3.3']

[rhettinger]

Currently bool(None) returns False only because it has a special case in PyObject_IsTrue(). All other types can become False only if they implement __bool__() or __len__().

I propose removing the special case branch and instead adding __bool__ to None. This will simplify the explanation of what bool() does to: "bool(x) always returns True unless the object defines __bool__() to return False or defines __len__() to return a non-zero value".

The removal of the special case will slightly slow down tests for "if None", and it will slightly speed-up tests for "if x" where x is something other than True, False, or None.

[brettcannon]

It all sounds good to me. Less magic behind the scenes is good.

[ericsnowcurrently]

And this doesn't impact "if x is None", so +1.

[pitrou]

I'm not sure why the special case shouldn't be retained. It's obviously there for speed reasons.

[vstinner]

I'm not sure why the special case shouldn't be retained.

We can keep the fast path in PyObject_IsTrue but add a __bool__ method to NoneType.

[pitrou]

> I'm not sure why the special case shouldn't be retained.

We can keep the fast path in PyObject_IsTrue but add a __bool__ method to NoneType.

That's what I meant.

[python-dev]

New changeset ccce01988603 by Raymond Hettinger in branch 'default':
bpo-12647: Add __bool__() method to the None object.
http://hg.python.org/cpython/rev/ccce01988603