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<title>5.10 Boolean operations</title>
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<H1><A NAME="SECTION0071000000000000000000"></A><A NAME="Booleans"></A>
<BR>
5.10 Boolean operations
</H1>
<a id='l2h-439' xml:id='l2h-439'></a>
<P>
Boolean operations have the lowest priority of all Python operations:

<P>
<dl><dd class="grammar">
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    <td><a id='tok-expression' xml:id='tok-expression'>expression</a></td>
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    <td><a id='tok-or_test' xml:id='tok-or_test'>or_test</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="Booleans.html#tok-and_test">and_test</a> | <a class='grammartoken' href="Booleans.html#tok-or_test">or_test</a> "or" <a class='grammartoken' href="Booleans.html#tok-and_test">and_test</a></td></tr>
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    <td><a id='tok-and_test' xml:id='tok-and_test'>and_test</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="Booleans.html#tok-not_test">not_test</a> | <a class='grammartoken' href="Booleans.html#tok-and_test">and_test</a> "and" <a class='grammartoken' href="Booleans.html#tok-not_test">not_test</a></td></tr>
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    <td><a id='tok-not_test' xml:id='tok-not_test'>not_test</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="comparisons.html#tok-comparison">comparison</a> | "not" <a class='grammartoken' href="Booleans.html#tok-not_test">not_test</a></td></tr>
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<a class="grammar-footer"
  href="grammar.txt" type="text/plain"
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</dd></dl>

<P>
In the context of Boolean operations, and also when expressions are
used by control flow statements, the following values are interpreted
as false: <code>False</code>, <code>None</code>, numeric zero of all types, and empty
strings and containers (including strings, tuples, lists, dictionaries,
sets and frozensets).  All other values are interpreted as true.

<P>
The operator <tt class="keyword">not</tt> yields <code>True</code> if its argument is false,
<code>False</code> otherwise.
<a id='l2h-440' xml:id='l2h-440'></a>
<P>
The expression <code><var>x</var> and <var>y</var></code> first evaluates <var>x</var>; if
<var>x</var> is false, its value is returned; otherwise, <var>y</var> is
evaluated and the resulting value is returned.
<a id='l2h-441' xml:id='l2h-441'></a>
<P>
The expression <code><var>x</var> or <var>y</var></code> first evaluates <var>x</var>; if
<var>x</var> is true, its value is returned; otherwise, <var>y</var> is
evaluated and the resulting value is returned.
<a id='l2h-442' xml:id='l2h-442'></a>
<P>
(Note that neither <tt class="keyword">and</tt> nor <tt class="keyword">or</tt> restrict the value
and type they return to <code>False</code> and <code>True</code>, but rather return the
last evaluated argument.
This is sometimes useful, e.g., if <code>s</code> is a string that should be
replaced by a default value if it is empty, the expression
<code>s or 'foo'</code> yields the desired value.  Because <tt class="keyword">not</tt> has to
invent a value anyway, it does not bother to return a value of the
same type as its argument, so e.g., <code>not 'foo'</code> yields <code>False</code>,
not <code>''</code>.)

<P>

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