invalid conversion xml.etree.ElementTree.Element object to boolean

[xaka]

BPO	3195
Nosy	@pitrou

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

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2008-06-25.12:27:49.772>
created_at = <Date 2008-06-25.09:09:15.257>
labels = ['expert-XML', 'invalid']
title = 'invalid conversion xml.etree.ElementTree.Element object to boolean'
updated_at = <Date 2008-06-25.12:27:49.761>
user = 'https://bugs.python.org/xaka'

bugs.python.org fields:

activity = <Date 2008-06-25.12:27:49.761>
actor = 'benjamin.peterson'
assignee = 'none'
closed = True
closed_date = <Date 2008-06-25.12:27:49.772>
closer = 'benjamin.peterson'
components = ['XML']
creation = <Date 2008-06-25.09:09:15.257>
creator = 'xaka'
dependencies = []
files = []
hgrepos = []
issue_num = 3195
keywords = []
message_count = 3.0
messages = ['68720', '68721', '68723']
nosy_count = 3.0
nosy_names = ['pitrou', 'mishok13', 'xaka']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue3195'
versions = ['Python 2.5']

[xaka]

Python 2.5.2 (r252:60911, May  7 2008, 15:19:09)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from xml.etree.ElementTree import Element
>>> e = Element('Test', {'attr' : 'value'})
>>> b = not e
>>> print b
True

Why i'm getting True here instead of False?
Because of this i can not do:
if not e:
# here some logic
pass

[mishok13]

To quote Python Library Reference, paragraph 3.1:
"""Any object can be tested for truth value, for use in an if or while
condition or as operand of the Boolean operations below. The following
values are considered false:
[skipped]

• instances of user-defined classes, if the class defines a
  __nonzero__() or __len__() method, when that method returns the integer
  zero or bool value False.
  """
  And back to python console:
  In [112]: from xml.etree.ElementTree import Element

In [113]: e = Element('foo', {'key': 'value'})

In [114]: len(e)
Out[114]: 0

In [115]: not e
Out[115]: True

This is because Element is just a container and acts more like a list.
So, if you actually append items to this list-like structure, you'll get
this:

In [116]: e.append(Element('bar', {42: 1337}))

In [117]: e.append(Element('baz', {'whatever': 'wherever'}))

In [118]: len(e)
Out[118]: 2

In [119]: not e
Out[119]: False

In conclusion, this just doesn't look like a bug to me. You could try
using "if e is not None" form.

[pitrou]

Indeed, this is not a bug, although it can be misleading.
Generally, if you test for None, it is better to write "if x is None",
it is at the same time more accurate, more explicit for someone reading
your code, and also executes faster than "if not x".