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<H1><A NAME="SECTION005900000000000000000">
3.9 <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>.


<P>
The module defines the following functions:

<P>
<dl><dt><b><a name='l2h-385'><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-386'><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 (innermost 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-387'><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-388'><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-389'><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-390'><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-391'><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-392'><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-393'><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-394'><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-395'><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-396'><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-397'><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>

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