[BangPypers] ElementTree object instance evaluates to False?

Noufal Ibrahim noufal at gmail.com
Tue Dec 6 06:45:03 CET 2011

Vikas BN <vikas.bn at gmail.com> writes:


>     Anyone else encountered this or even have any insights regarding
> this behaviour? I'm using Python2.7.2 on Linux
>     if that matters.

ElementTree's `Element` objects behave this way. The "booleanness" is
not conformant to the rest of Python.

I don't know if or when it's been fixed but here's how to do it.

The method you need to work with is `__nonzero__`[1] if you want `bool`
to work with your classes. 

With Python 2.7.2, The `Element` class defined in
/usr/lib/python2.7/xml/etree/ElementTree.py has

    def __nonzero__(self):
            "The behavior of this method will change in future versions.  "
            "Use specific 'len(elem)' or 'elem is not None' test instead.",
            FutureWarning, stacklevel=2
        return len(self._children) != 0 # emulate old behaviour, for now

So, it will evaluate to False only if the number of children is

Annoying but when I stumbled on this, it did make me aware of how
consistent Python libraries generally are so that this stuck out. 


[1] http://docs.python.org/reference/datamodel.html#object.__nonzero__


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