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<H1><A NAME="SECTION008200000000000000000"><!--x--></A><A NAME="assert"><!--z--></A>
<BR>
6.2 Assert statements
</H1>
<P>
Assert statements<a id='l2h-456'><!--x--></a> are a convenient way to insert
debugging assertions<a id='l2h-457'><!--x--></a> into a program:
<P>
<dl><dd class="grammar">
<div class="productions">
<table cellpadding="2">
<tr valign="baseline">
<td><code><a id='tok-assert_stmt'>assert_stmt</a></code></td>
<td> ::= </td>
<td><code>"assert" <a href="Booleans.html#tok-expression">expression</a> ["," <a href="Booleans.html#tok-expression">expression</a>]</code></td></tr>
</table>
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<a class="grammar-footer"
href="grammar.txt" type="text/plain"
>Download entire grammar as text.</a>
</dd></dl>
<P>
The simple form, "<tt class="samp">assert expression</tt>", is equivalent to
<P>
<div class="verbatim"><pre>
if __debug__:
if not expression: raise AssertionError
</pre></div>
<P>
The extended form, "<tt class="samp">assert expression1, expression2</tt>", is
equivalent to
<P>
<div class="verbatim"><pre>
if __debug__:
if not expression1: raise AssertionError, expression2
</pre></div>
<P>
These equivalences assume that <code>__debug__</code><a id='l2h-459'><!--x--></a> and
<tt class="exception">AssertionError</tt><a id='l2h-458'><!--x--></a> refer to the built-in
variables with those names. In the current implementation, the
built-in variable <code>__debug__</code> is 1 under normal circumstances, 0
when optimization is requested (command line option -O). The current
code generator emits no code for an assert statement when optimization
is requested at compile time. Note that it is unnecessary to include
the source code for the expression that failed in the error message;
it will be displayed as part of the stack trace.
<P>
Assignments to <code>__debug__</code> are illegal. The value for the
built-in variable is determined when the interpreter starts.
<P>
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<span class="release-info">Release 2.3.4, documentation updated on May 20, 2004.</span>
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