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<H1><A NAME="SECTION0051200000000000000000">
3.12 <tt class="module">traceback</tt> --
Print or retrieve a stack traceback</A>
</H1>
<P>
<A NAME="module-traceback"><!--z--></A>
<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, such as 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> (deprecated) and
<code>sys.last_traceback</code> and returned as the third item from
<tt class="function">sys.exc_info()</tt>.
<a id='l2h-596'><!--x--></a>
<P>
The module defines the following functions:
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-597' class="function">print_tb</tt></b>(</nobr></td>
<td><var>traceback</var><big>[</big><var>, limit</var><big>[</big><var>, file</var><big>]</big><big>]</big>)</td></tr></table></dt>
<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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-598' class="function">print_exception</tt></b>(</nobr></td>
<td><var>type, value, traceback</var><big>[</big><var>,
limit</var><big>[</big><var>, file</var><big>]</big><big>]</big>)</td></tr></table></dt>
<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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-599' class="function">print_exc</tt></b>(</nobr></td>
<td><big>[</big><var>limit</var><big>[</big><var>, file</var><big>]</big><big>]</big>)</td></tr></table></dt>
<dd>
This is a shorthand for <code>print_exception(sys.exc_type,
sys.exc_value, sys.exc_traceback, <var>limit</var>, <var>file</var>)</code>. (In
fact, it uses <tt class="function">sys.exc_info()</tt> to retrieve the same
information in a thread-safe way instead of using the deprecated
variables.)
</dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-600' class="function">print_last</tt></b>(</nobr></td>
<td><big>[</big><var>limit</var><big>[</big><var>, file</var><big>]</big><big>]</big>)</td></tr></table></dt>
<dd>
This is a shorthand for <code>print_exception(sys.last_type,
sys.last_value, sys.last_traceback, <var>limit</var>, <var>file</var>)</code>.
</dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-601' class="function">print_stack</tt></b>(</nobr></td>
<td><big>[</big><var>f</var><big>[</big><var>, limit</var><big>[</big><var>, file</var><big>]</big><big>]</big><big>]</big>)</td></tr></table></dt>
<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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-602' class="function">extract_tb</tt></b>(</nobr></td>
<td><var>traceback</var><big>[</big><var>, limit</var><big>]</big>)</td></tr></table></dt>
<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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-603' class="function">extract_stack</tt></b>(</nobr></td>
<td><big>[</big><var>f</var><big>[</big><var>, limit</var><big>]</big><big>]</big>)</td></tr></table></dt>
<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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-604' class="function">format_list</tt></b>(</nobr></td>
<td><var>list</var>)</td></tr></table></dt>
<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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-605' class="function">format_exception_only</tt></b>(</nobr></td>
<td><var>type, value</var>)</td></tr></table></dt>
<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 <tt class="exception">SyntaxError</tt> 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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-606' class="function">format_exception</tt></b>(</nobr></td>
<td><var>type, value, tb</var><big>[</big><var>, limit</var><big>]</big>)</td></tr></table></dt>
<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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-607' class="function">format_tb</tt></b>(</nobr></td>
<td><var>tb</var><big>[</big><var>, limit</var><big>]</big>)</td></tr></table></dt>
<dd>
A shorthand for <code>format_list(extract_tb(<var>tb</var>, <var>limit</var>))</code>.
</dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-608' class="function">format_stack</tt></b>(</nobr></td>
<td><big>[</big><var>f</var><big>[</big><var>, limit</var><big>]</big><big>]</big>)</td></tr></table></dt>
<dd>
A shorthand for <code>format_list(extract_stack(<var>f</var>, <var>limit</var>))</code>.
</dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-609' class="function">tb_lineno</tt></b>(</nobr></td>
<td><var>tb</var>)</td></tr></table></dt>
<dd>
This function returns the current line number set in the traceback
object. This function was necessary because in versions of Python
prior to 2.3 when the <b class="programopt">-O</b> flag was passed to Python the
<code><var>tb</var>.tb_lineno</code> was not updated correctly. This function
has no use in versions past 2.3.
</dl>
<P>
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