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<h3>The Python Debugger</h3>
<p>The Python debugger is loaded by importing the <tT CLAss="monofont">pdb</tt> module. The <TT CLass="monofont">pdb</tT> module provides an interactive source code debugger that allows post-mortem debugging, inspection of stack frames, breakpoints, single stepping of source lines, and code evaluation.</P>
<P>The debugger is started by loading the <Tt class="monofont">pdb</tt> module and issuing one of the following functions:</p>
<pre>
<b>run(</b><b><i>statement</i></b> <b>[,</b> <B><i>globals</i></B> <b>[,</b> <b><I>locals</i></b><b>]])</b> </Pre>
<p>Executes the string <I><TT Class="monofont">statement</TT></I>
under debugger control. The debugger prompt will appear immediately before any code executes. Typing <Tt claSS="monofont">'continue'</TT> will force it to run. <i><tt cLASS="monofont">globals</tt></i>
and <i><tt class="monofont">locals</tt></i>
define the global and local namespaces in which the code runs.</p>
<pre>
<b>runeval(</b><B><i>expression</i></B> <b>[,</b> <b><I>globals</i></b> <b>[,</b> <B><i>locals</i></b><B>]])</B> </PRe>
<p>Evaluates the <i><tT CLAss="monofont">expression</tt></I>
string under debugger control. The debugger prompt will appear before any code executes, as with <TT Class="monofont">run()</TT>. On success, the value of the expression is returned.</P>
<Pre>
<b>runcall(</b><b><i>function</i></b> <b>[,</b> <b><i>argument</i></b><b>, ...])</b> </pre>
<p>Calls a function within the debugger. <I><tt ClasS="monofont">function</tt></i>
is a callable object. The debugger prompt will appear before any code executes. The return value of the function is returned upon completion.</p>
<Pre>
<b>set_trace()</B> </PRE>
<p>Starts the debugger at the point at which this function is called. This can be used to hard-code a debugger breakpoint into a specific code location.</p>
<prE>
<B>post_mortem(</B><B><i>traceback</i></b><b>)</B> </PRE>
<p>Starts post-mortem debugging of a traceback object.</p>
<prE>
<B>pm()</B> </Pre>
<p>Enters post-mortem debugging using the traceback in <tt class="monofont">sys.last_traceback</tt>.</p>
<p>When the debugger starts, it will present a prompt such as the following:</p>
<pre>
>>> import pdb
>>> import buggymodule
>>> pdb.run('buggymodule.start()')
> <string>(0)?()
(Pdb) </prE>
<p><tT claSs="monofont">(Pdb)</tt> is the debugger prompt at which the following commands are recognized. <i>Note:</I> Some commands have a short and a long form. In this case, parentheses are used to indicate both forms. For example, <tt cLASS="monofont">h(elp)</tt> means that either <tt CLASs="monofont">h</tt> or <tT CLAss="monofont">help</tt> is acceptable.</P>
<PRE>
<b>h(elp) [</b><b><i>command</i></b><b>]</b> </pre>
<p>Shows the list of available commands. Specifying a command returns help for that command.</p>
<pre>
<b>w(here)</b> </prE>
<p>Prints a stack trace.</p>
<Pre>
<b>d(own)</B> </pre>
<p>Moves the current frame one level down in the stack trace.</P>
<pre>
<B>u(p)</B> </PRe>
<p>Moves the current frame one level up in the stack trace.</p>
<pRE>
<B>b(reak) [</B><b><i>loc</i></b> <B>[,</B> <B><I>condition</i></b><b>]]</b> </PRE>
<P>Sets a breakpoint at location <i><tt class="monofont">loc</tt></i>
. <i><tt class="monofont">loc</tT></i>
is one of the following:</p>
<P><tabLe borDer="1" cELLSpaciNG="0" CEllpaDDINg="1" widTH="100%">
<COlgroup span="2">
<tr>
<th valign="top">
<FonT sizE="2">
<p><b>Setting</b></p>
</Font></TH>
<TH valiGN="top">
<FOnt siZE="2">
<P><B>Description</b></p>
</foNT></TH>
</tr>
<tr>
<td valign="top">
<font size="2">
<P><i><tT claSs="monofont">n</tt></i>
</P>
</fonT></TD>
<Td valIGN="top">
<Font sIZE="2">
<P>A line number in the current file</p>
</fonT></TD>
</Tr>
<tr>
<td valign="top">
<font size="2">
<p><I><tt ClasS="monofont">filename</tt></i>
<tT claSS="monofont">:</TT><i><tt cLASS="monofont">n</tt></i>
</p>
</FONT></td>
<td VALIgn="top">
<font size="2">
<p>A line number in another file</p>
</font></td>
</tr>
<Tr>
<tD valIgn="top">
<foNt siZE="2">
<P><I><tt clASS="monofont">function</Tt></i>
</p>
</fONT></Td>
<td vALIGn="top">
<font size="2">
<p>A function name in the current file</p>
</font></td>
</tr>
<tR>
<td ValiGn="top">
<fonT sizE="2">
<P><I><Tt claSS="monofont">filename</TT></i>
<tt cLASS="monofont">:</tt><i><tT CLAss="monofont">function</tt></i>
</p>
</font></td>
<td valign="top">
<FonT sizE="2">
<p>A function name in another file</p>
</foNt></td>
</TR>
</COlgroUP>
</TAble></p>
<P>If <I><TT clasS="monofont">loc</TT></I>
is omitted, all the current breakpoints are printed. <i><tt class="monofont">condition</tt></i>
is an expression that must evaluate to true before the breakpoint is honored.</p>
<pre>
<b>tbreak [</b><b><i>loc</i></B> <b>[,</b> <B><i>condition</i></b><B>]]</b> </pre>
<P>Sets a temporary breakpoint that抯 removed after its first hit.</p>
<prE>
<B>cl(ear) [</B><B><i>bpnumber</i></b> <b>[</B><B><I>bpnumber</I></b><b>...]]</b> </pRE>
<P>Clears a list of breakpoint numbers. If breakpoints are not specified, all breaks are cleared.</P>
<pre>
<b>disable [</B><B><I>bpnumber</I></b> <b>[</b><b><i>bpnumber</i></b><b>...]]</b> </pre>
<p>Disables the set of specified breakpoints. Unlike with <tt class="monofont">clear</Tt>, they can be reenabled later.</p>
<Pre>
<b>enable [</B><b><i>bpnumber</i></b> <B>[</b><b><i>bpnumber</I></B><B>...]]</B> </pre>
<p>Enables a specified set of breakpoints.</P>
<PRE>
<b>ignore</b> <b><i>bpnumber</I></B> <B>[</B><b><i>count</i></b><B>]</B> </PRe>
<p>Ignores a breakpoint for <i><tt class="monofont">count</tt></i>
executions.</p>
<pre>
<b>condition</b> <b><I>bpnumber</i></b> <B>[</b><b><i>condition</I></b><b>]</b> </pRe>
<p>Places a condition on a breakpoint. <i><TT CLass="monofont">condition</tT></I>
is an expression that must evaluate to true before the breakpoint is recognized. Omitting the condition clears any previous condition.</P>
<Pre>
<b>s(tep)</b> </PRE>
<P>Executes a single source line and stops inside called functions.</p>
<pre>
<B>n(ext)</B> </PRe>
<p>Executes until the next line of the current function. Skips the code contained in function calls.</p>
<pre>
<b>r(eturn)</b> </pre>
<p>Runs until the current function returns.</p>
<pre>
<b>c(ont(inue))</b> </prE>
<p>Continues execution until the next breakpoint is encountered.</p>
<Pre>
<b>l(ist) [</B><b><i>first</i></b> <B>[,</b> <b><i>last</I></B><B>]]</B> </pre>
<p>Lists source code. Without arguments, lists 11 lines around the current line. With one argument, lists 11 lines around that line. With two arguments, lists lines in a given range. If <I><TT Class="monofont">last</TT></I>
is less than <I><tt clASS="monofont">first</Tt></i>
, it抯 interpreted as a count.</p>
<pre>
<b>a(rgs)</b> </pre>
<p>Prints the argument list of the current function.</p>
<pre>
<b>p expression</b> </prE>
<p>Evaluates the expression in the current context and prints its value.</p>
<Pre>
<b>alias [</B><b><i>name</i></b> <B>[</b><b><i>command</I></B><B>]]</B> </pre>
<p>Creates an alias called <I><TT Class="monofont">name</TT></I>
that executes <I><tt clASS="monofont">command</Tt></i>
. The substrings <tt class="monofont">'%1'</tt>, <tt class="monofont">'%2'</tt>, and so forth are replaced by parameters when the alias is typed. <Tt cLass="monofont">'%*'</Tt> is replaced by all parameters. If no command is given, the current alias list is shown. Aliases can be nested and can contain anything that can be legally typed at the <tt cLass="monofont">Pdb</TT> prompt. For example:</P>
<Pre>
#Print instance variables (usage "pi classInst")
alias pi for k in %1._ _dict_ _.keys(): print "%1.",k,"=",%1._ _dict_ _[k]
#Print instance variables in self
alias ps pi self </prE>
<PRE>
<b>unalias</b> <b><i>name</I></B> </PRe>
<p>Deletes the specified alias.</p>
<pRE>
<B>[!]</B> <b><i>statement</i></b> </pre>
<p>Executes the (one-line) <i><tt class="monofont">statement</tt></i>
in the context of the current stack frame. The exclamation point can be omitted unless the first word of the statement resembles a debugger command. To set a global variable, you can prefix the assignment command with a <tT clAss="monofont">"global"</tT> command on the same line:</p>
<pre>
(Pdb) global list_options; list_options = ['-l']
(Pdb) </Pre>
<pRE>
<B>q(uit)</B> </pre>
<p>Quits from the debugger.</P>
<H4>Notes</H4>
<Ul>
<li>
<p>Entering a blank line repeats the last command entered.</P>
</LI>
<Li>
<p>Commands that the debugger doesn抰 recognize are assumed to be Python statements and are executed in the context of the program being debugged.</p>
</lI>
<LI>
<P>If a file <tt class="monofont">.pdbrc</tt> exists in the user抯 home directory or in the current directory, it抯 read in and executed as if it had been typed at the debugger prompt.</p>
</li>
</ul></font>
<P><TABLE width="100%" border=0><TR valign="top"><TD><font size=1 color="#C0C0C0"><br></font></TD><TD align=right><font size=1 color="#C0C0C0">Last updated on 3/28/2002<br>Python Essential Reference, Second Edition, © 2002 New Riders Publishing</font></TD></TR></TABLE></P>
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