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<H2><A NAME="SECTION004310000000000000000"></A><A NAME="truth"></A>
<BR>
2.3.1 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 id='l2h-91' xml:id='l2h-91'></a><a id='l2h-92' xml:id='l2h-92'></a><a id='l2h-93' xml:id='l2h-93'></a><a id='l2h-94' xml:id='l2h-94'></a><a id='l2h-101' xml:id='l2h-101'></a>

<P>

<UL>
<LI><code>None</code>
        <a id='l2h-96' xml:id='l2h-96'></a>
<P>
</LI>
<LI><code>False</code>
        <a id='l2h-98' xml:id='l2h-98'></a>
<P>
</LI>
<LI>zero of any numeric type, for example, <code>0</code>, <code>0L</code>,
        <code>0.0</code>, <code>0j</code>.

<P>
</LI>
<LI>any empty sequence, for example, <code>''</code>, <code>()</code>, <code>[]</code>.

<P>
</LI>
<LI>any empty mapping, for example, <code>{}</code>.

<P>
</LI>
<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 the integer zero or <tt class="class">bool</tt> value
        <code>False</code>.<A NAME="tex2html6"
  HREF="#foot3425"><SUP>2.5</SUP></A>
<P>
</LI>
</UL>

<P>
All other values are considered true -- so objects of many types are
always true.
<a id='l2h-102' xml:id='l2h-102'></a>

<P>
Operations and built-in functions that have a Boolean result always
return <code>0</code> or <code>False</code> for false and <code>1</code> or <code>True</code>
for true, unless otherwise stated.  (Important exception: the Boolean
operations "<tt class="samp">or</tt>"<a id='l2h-99' xml:id='l2h-99'></a> and "<tt class="samp">and</tt>"<a id='l2h-100' xml:id='l2h-100'></a> always
return one of their operands.)
<a id='l2h-103' xml:id='l2h-103'></a>

<P>
<BR><HR><H4>Footnotes</H4>
<DL>
<DT><A NAME="foot3425">...False.</A><A
 href="truth.html#tex2html6"><SUP>2.5</SUP></A></DT>
<DD>Additional 
information on these special methods may be found in the
<em class="citetitle"><a
 href="../ref/ref.html"
 title="Python Reference Manual"
 >Python Reference Manual</a></em>.

</DD>
</DL>
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920dae64 AT	1	<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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	3	<head>
	4	<link rel="STYLESHEET" href="lib.css" type='text/css' />
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	7	<link rel="first" href="lib.html" title='Python Library Reference' />
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	14	<link rel="parent" href="types.html" />
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	17	<title>2.3.1 Truth Value Testing</title>
	18	</head>
	19	<body>
	20	<DIV CLASS="navigation">
	21	<div id='top-navigation-panel' xml:id='top-navigation-panel'>
	22	<table align="center" width="100%" cellpadding="0" cellspacing="2">
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	54
	55	<H2><A NAME="SECTION004310000000000000000"></A><A NAME="truth"></A>
	56	<BR>
	57	2.3.1 Truth Value Testing
	58	</H2>
	59
	60	<P>
	61	Any object can be tested for truth value, for use in an <tt class="keyword">if</tt> or
	62	<tt class="keyword">while</tt> condition or as operand of the Boolean operations below.
	63	The following values are considered false:
	64	<a id='l2h-91' xml:id='l2h-91'></a><a id='l2h-92' xml:id='l2h-92'></a><a id='l2h-93' xml:id='l2h-93'></a><a id='l2h-94' xml:id='l2h-94'></a><a id='l2h-101' xml:id='l2h-101'></a>
65
66	<P>
67
68	<UL>
69	<LI><code>None</code>
70	<a id='l2h-96' xml:id='l2h-96'></a>
71	<P>
72	</LI>
73	<LI><code>False</code>
74	<a id='l2h-98' xml:id='l2h-98'></a>
75	<P>
76	</LI>
77	<LI>zero of any numeric type, for example, <code>0</code>, <code>0L</code>,
78	<code>0.0</code>, <code>0j</code>.
79
80	<P>
81	</LI>
82	<LI>any empty sequence, for example, <code>''</code>, <code>()</code>, <code>[]</code>.
83
84	<P>
85	</LI>
86	<LI>any empty mapping, for example, <code>{}</code>.
87
88	<P>
89	</LI>
90	<LI>instances of user-defined classes, if the class defines a
91	<tt class="method">__nonzero__()</tt> or <tt class="method">__len__()</tt> method, when that
92	method returns the integer zero or <tt class="class">bool</tt> value
93	<code>False</code>.<A NAME="tex2html6"
94	HREF="#foot3425"><SUP>2.5</SUP></A>
95	<P>
96	</LI>
97	</UL>
98
99	<P>
100	All other values are considered true -- so objects of many types are
101	always true.
102	<a id='l2h-102' xml:id='l2h-102'></a>
103
104	<P>
105	Operations and built-in functions that have a Boolean result always
106	return <code>0</code> or <code>False</code> for false and <code>1</code> or <code>True</code>
107	for true, unless otherwise stated. (Important exception: the Boolean
108	operations "<tt class="samp">or</tt>"<a id='l2h-99' xml:id='l2h-99'></a> and "<tt class="samp">and</tt>"<a id='l2h-100' xml:id='l2h-100'></a> always
109	return one of their operands.)
110	<a id='l2h-103' xml:id='l2h-103'></a>
111
112	<P>
113	<BR><HR><H4>Footnotes</H4>
114	<DL>
115	<DT><A NAME="foot3425">...False.</A><A
116	href="truth.html#tex2html6"><SUP>2.5</SUP></A></DT>
117	<DD>Additional
118	information on these special methods may be found in the
119	<em class="citetitle"><a
120	href="../ref/ref.html"
121	title="Python Reference Manual"
122	>Python Reference Manual</a></em>.
123
124	</DD>
125	</DL>
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