<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>3.12 traceback -- Print or retrieve a stack traceback</title>
<META NAME="description" CONTENT="3.12 traceback -- Print or retrieve a stack traceback">
<META NAME="keywords" CONTENT="lib">
<META NAME="resource-type" CONTENT="document">
<META NAME="distribution" CONTENT="global">
<meta http-equiv="Content-Type" content="text/html; charset=">
<link rel="STYLESHEET" href="lib.css">
<link rel="first" href="lib.html">
<link rel="contents" href="contents.html" title="Contents">
<link rel="index" href="genindex.html" title="Index">
<LINK REL="next" href="module-linecache.html">
<LINK REL="previous" href="module-inspect.html">
<LINK REL="up" href="python.html">
<LINK REL="next" href="traceback-example.html">
</head>
<body>
<DIV CLASS="navigation">
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td><A href="inspect-stack.html"><img src="../icons/previous.gif"
border="0" height="32"
alt="Previous Page" width="32"></A></td>
<td><A href="python.html"><img src="../icons/up.gif"
border="0" height="32"
alt="Up One Level" width="32"></A></td>
<td><A href="traceback-example.html"><img src="../icons/next.gif"
border="0" height="32"
alt="Next Page" width="32"></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td><A href="contents.html"><img src="../icons/contents.gif"
border="0" height="32"
alt="Contents" width="32"></A></td>
<td><a href="modindex.html" title="Module Index"><img src="../icons/modules.gif"
border="0" height="32"
alt="Module Index" width="32"></a></td>
<td><A href="genindex.html"><img src="../icons/index.gif"
border="0" height="32"
alt="Index" width="32"></A></td>
</tr></table>
<b class="navlabel">Previous:</b> <a class="sectref" href="inspect-stack.html">3.11.4 The interpreter stack</A>
<b class="navlabel">Up:</b> <a class="sectref" href="python.html">3. Python Runtime Services</A>
<b class="navlabel">Next:</b> <a class="sectref" href="traceback-example.html">3.12.1 Traceback Example</A>
<br><hr>
</DIV>
<!--End of Navigation Panel-->
<H1><A NAME="SECTION0051200000000000000000">
3.12 <tt class="module">traceback</tt> --
Print or retrieve a stack traceback</A>
</H1>
<P>
<P>
This module provides a standard interface to extract, format and print
stack traces of Python programs. It exactly mimics the behavior of
the Python interpreter when it prints a stack trace. This is useful
when you want to print stack traces under program control, e.g. in a
``wrapper'' around the interpreter.
<P>
The module uses traceback objects -- this is the object type
that is stored in the variables <code>sys.exc_traceback</code> and
<code>sys.last_traceback</code> and returned as the third item from
<tt class="function">sys.exc_info()</tt>.
<a name="l2h-505"> </a>
<P>
The module defines the following functions:
<P>
<dl><dt><b><a name="l2h-492"><tt class="function">print_tb</tt></a></b>(<var>traceback</var><big>[</big><var>, limit</var><big>[</big><var>, file</var><big>]</big><big>]</big>)
<dd>
Print up to <var>limit</var> stack trace entries from <var>traceback</var>. If
<var>limit</var> is omitted or <code>None</code>, all entries are printed.
If <var>file</var> is omitted or <code>None</code>, the output goes to
<code>sys.stderr</code>; otherwise it should be an open file or file-like
object to receive the output.
</dl>
<P>
<dl><dt><b><a name="l2h-493"><tt class="function">print_exception</tt></a></b>(<var>type, value, traceback</var><big>[</big><var>,
limit</var><big>[</big><var>, file</var><big>]</big><big>]</big>)
<dd>
Print exception information and up to <var>limit</var> stack trace entries
from <var>traceback</var> to <var>file</var>.
This differs from <tt class="function">print_tb()</tt> in the
following ways: (1) if <var>traceback</var> is not <code>None</code>, it prints a
header "<tt class="samp">Traceback (most recent call last):</tt>"; (2) it prints the
exception <var>type</var> and <var>value</var> after the stack trace; (3) if
<var>type</var> is <tt class="exception">SyntaxError</tt> and <var>value</var> has the appropriate
format, it prints the line where the syntax error occurred with a
caret indicating the approximate position of the error.
</dl>
<P>
<dl><dt><b><a name="l2h-494"><tt class="function">print_exc</tt></a></b>(<big>[</big><var>limit</var><big>[</big><var>, file</var><big>]</big><big>]</big>)
<dd>
This is a shorthand for `<code>print_exception(sys.exc_type,</code>
<code>sys.exc_value,</code> <code>sys.exc_traceback,</code> <var>limit</var><code>,</code>
<var>file</var><code>)</code>'. (In fact, it uses <code>sys.exc_info()</code> to
retrieve the same information in a thread-safe way.)
</dl>
<P>
<dl><dt><b><a name="l2h-495"><tt class="function">print_last</tt></a></b>(<big>[</big><var>limit</var><big>[</big><var>, file</var><big>]</big><big>]</big>)
<dd>
This is a shorthand for `<code>print_exception(sys.last_type,</code>
<code>sys.last_value,</code> <code>sys.last_traceback,</code> <var>limit</var><code>,</code>
<var>file</var><code>)</code>'.
</dl>
<P>
<dl><dt><b><a name="l2h-496"><tt class="function">print_stack</tt></a></b>(<big>[</big><var>f</var><big>[</big><var>, limit</var><big>[</big><var>, file</var><big>]</big><big>]</big><big>]</big>)
<dd>
This function prints a stack trace from its invocation point. The
optional <var>f</var> argument can be used to specify an alternate stack
frame to start. The optional <var>limit</var> and <var>file</var> arguments have the
same meaning as for <tt class="function">print_exception()</tt>.
</dl>
<P>
<dl><dt><b><a name="l2h-497"><tt class="function">extract_tb</tt></a></b>(<var>traceback</var><big>[</big><var>, limit</var><big>]</big>)
<dd>
Return a list of up to <var>limit</var> ``pre-processed'' stack trace
entries extracted from the traceback object <var>traceback</var>. It is
useful for alternate formatting of stack traces. If <var>limit</var> is
omitted or <code>None</code>, all entries are extracted. A
``pre-processed'' stack trace entry is a quadruple (<var>filename</var>,
<var>line number</var>, <var>function name</var>, <var>text</var>) representing
the information that is usually printed for a stack trace. The
<var>text</var> is a string with leading and trailing whitespace
stripped; if the source is not available it is <code>None</code>.
</dl>
<P>
<dl><dt><b><a name="l2h-498"><tt class="function">extract_stack</tt></a></b>(<big>[</big><var>f</var><big>[</big><var>, limit</var><big>]</big><big>]</big>)
<dd>
Extract the raw traceback from the current stack frame. The return
value has the same format as for <tt class="function">extract_tb()</tt>. The
optional <var>f</var> and <var>limit</var> arguments have the same meaning as
for <tt class="function">print_stack()</tt>.
</dl>
<P>
<dl><dt><b><a name="l2h-499"><tt class="function">format_list</tt></a></b>(<var>list</var>)
<dd>
Given a list of tuples as returned by <tt class="function">extract_tb()</tt> or
<tt class="function">extract_stack()</tt>, return a list of strings ready for
printing. Each string in the resulting list corresponds to the item
with the same index in the argument list. Each string ends in a
newline; the strings may contain internal newlines as well, for those
items whose source text line is not <code>None</code>.
</dl>
<P>
<dl><dt><b><a name="l2h-500"><tt class="function">format_exception_only</tt></a></b>(<var>type, value</var>)
<dd>
Format the exception part of a traceback. The arguments are the
exception type and value such as given by <code>sys.last_type</code> and
<code>sys.last_value</code>. The return value is a list of strings, each
ending in a newline. Normally, the list contains a single string;
however, for <code>SyntaxError</code> exceptions, it contains several lines
that (when printed) display detailed information about where the
syntax error occurred. The message indicating which exception
occurred is the always last string in the list.
</dl>
<P>
<dl><dt><b><a name="l2h-501"><tt class="function">format_exception</tt></a></b>(<var>type, value, tb</var><big>[</big><var>, limit</var><big>]</big>)
<dd>
Format a stack trace and the exception information. The arguments
have the same meaning as the corresponding arguments to
<tt class="function">print_exception()</tt>. The return value is a list of strings,
each ending in a newline and some containing internal newlines. When
these lines are concatenated and printed, exactly the same text is
printed as does <tt class="function">print_exception()</tt>.
</dl>
<P>
<dl><dt><b><a name="l2h-502"><tt class="function">format_tb</tt></a></b>(<var>tb</var><big>[</big><var>, limit</var><big>]</big>)
<dd>
A shorthand for <code>format_list(extract_tb(<var>tb</var>, <var>limit</var>))</code>.
</dl>
<P>
<dl><dt><b><a name="l2h-503"><tt class="function">format_stack</tt></a></b>(<big>[</big><var>f</var><big>[</big><var>, limit</var><big>]</big><big>]</big>)
<dd>
A shorthand for <code>format_list(extract_stack(<var>f</var>, <var>limit</var>))</code>.
</dl>
<P>
<dl><dt><b><a name="l2h-504"><tt class="function">tb_lineno</tt></a></b>(<var>tb</var>)
<dd>
This function returns the current line number set in the traceback
object. This is normally the same as the <code><var>tb</var>.tb_lineno</code>
field of the object, but when optimization is used (the -O flag) this
field is not updated correctly; this function calculates the correct
value.
</dl>
<P>
<p><hr>
<!--Table of Child-Links-->
<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a>
<UL CLASS="ChildLinks">
<LI><A href="traceback-example.html">3.12.1 Traceback Example</a>
</ul>
<!--End of Table of Child-Links-->
<DIV CLASS="navigation">
<p><hr>
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td><A href="inspect-stack.html"><img src="../icons/previous.gif"
border="0" height="32"
alt="Previous Page" width="32"></A></td>
<td><A href="python.html"><img src="../icons/up.gif"
border="0" height="32"
alt="Up One Level" width="32"></A></td>
<td><A href="traceback-example.html"><img src="../icons/next.gif"
border="0" height="32"
alt="Next Page" width="32"></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td><A href="contents.html"><img src="../icons/contents.gif"
border="0" height="32"
alt="Contents" width="32"></A></td>
<td><a href="modindex.html" title="Module Index"><img src="../icons/modules.gif"
border="0" height="32"
alt="Module Index" width="32"></a></td>
<td><A href="genindex.html"><img src="../icons/index.gif"
border="0" height="32"
alt="Index" width="32"></A></td>
</tr></table>
<b class="navlabel">Previous:</b> <a class="sectref" href="inspect-stack.html">3.11.4 The interpreter stack</A>
<b class="navlabel">Up:</b> <a class="sectref" href="python.html">3. Python Runtime Services</A>
<b class="navlabel">Next:</b> <a class="sectref" href="traceback-example.html">3.12.1 Traceback Example</A>
<hr>
<span class="release-info">Release 2.2, documentation updated on December 21, 2001.</span>
</DIV>
<!--End of Navigation Panel-->
<ADDRESS>
See <i><a href="about.html">About this document...</a></i> for information on suggesting changes.
</ADDRESS>
</BODY>
</HTML>
