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<TITLE>Truth Value Testing -- Python library reference</TITLE>
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<H2>2.1.1. Truth Value Testing</H2>
Any object can be tested for truth value, for use in an <CODE>if</CODE> or
<CODE>while</CODE> condition or as operand of the Boolean operations below.
The following values are considered false:
<UL>
<LI>• <CODE>None</CODE>
<LI>• zero of any numeric type, e.g., <CODE>0</CODE>, <CODE>0L</CODE>, <CODE>0.0</CODE>.
<P>
<LI>• any empty sequence, e.g., <CODE>''</CODE>, <CODE>()</CODE>, <CODE>[]</CODE>.
<P>
<LI>• any empty mapping, e.g., <CODE>{}</CODE>.
<P>
<LI>• instances of user-defined classes, if the class defines a
<CODE>__nonzero__()</CODE> or <CODE>__len__()</CODE> method, when that
method returns zero.
<P>
</UL>
All other values are considered true --- so objects of many types are
always true.
Operations and built-in functions that have a Boolean result always
return <CODE>0</CODE> for false and <CODE>1</CODE> for true, unless otherwise
stated. (Important exception: the Boolean operations `<SAMP>or</SAMP>' and
`<SAMP>and</SAMP>' always return one of their operands.)
<P>