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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 | ||
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160 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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