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<H2><A NAME="SECTION004110000000000000000"></A>
<BR>
2.1.1 Truth Value Testing 
</H2>

<P>
Any object can be tested for truth value, for use in an <tt class=keyword>if</tt> or
<tt class=keyword>while</tt> condition or as operand of the Boolean operations below.
The following values are considered false:
<a name="l2h-10"></a>
<a name="l2h-11"></a>
<a name="l2h-12"></a>
<a name="l2h-13"></a>
<a name="l2h-14"></a>

<P>

<UL><LI><tt>None</tt>	<a name="l2h-9"></a>
<P>

<LI>zero of any numeric type, e.g., <tt>0</tt>, <tt>0L</tt>, <tt>0.0</tt>.

<P>

<LI>any empty sequence, e.g., <tt>''</tt>, <tt>()</tt>, <tt>[]</tt>.

<P>

<LI>any empty mapping, e.g., <tt>{}</tt>.

<P>

<LI>instances of user-defined classes, if the class defines a
	<tt class=method>__nonzero__()</tt> or <tt class=method>__len__()</tt> method, when that
	method returns zero.

<P>

</UL>

<P>
All other values are considered true -- so objects of many types are
always true.
<a name="l2h-15"></a>

<P>
Operations and built-in functions that have a Boolean result always
return <tt>0</tt> for false and <tt>1</tt> for true, unless otherwise
stated.  (Important exception: the Boolean operations
"<tt class=samp>or</tt>"<a name="l2h-16"></a> and "<tt class=samp>and</tt>"<a name="l2h-17"></a> always return one of
their operands.)

<P>
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