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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"></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-605' xml:id='l2h-605'></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-606' xml:id='l2h-606' class="function">print_tb</tt></b>(</nobr></td>
  <td><var>traceback</var><big>[</big><var>, limit</var><big>[</big><var>, file</var><big>]</big><var></var><big>]</big><var></var>)</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-607' xml:id='l2h-607' 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><var></var><big>]</big><var></var>)</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-608' xml:id='l2h-608' class="function">print_exc</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>limit</var><big>[</big><var>, file</var><big>]</big><var></var><big>]</big><var></var>)</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-609' xml:id='l2h-609' class="function">format_exc</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>limit</var><big>[</big><var>, file</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
This is like <code>print_exc(<var>limit</var>)</code> but returns a string
instead of printing to a file.

<span class="versionnote">New in version 2.4.</span>

</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-610' xml:id='l2h-610' class="function">print_last</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>limit</var><big>[</big><var>, file</var><big>]</big><var></var><big>]</big><var></var>)</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-611' xml:id='l2h-611' class="function">print_stack</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>f</var><big>[</big><var>, limit</var><big>[</big><var>, file</var><big>]</big><var></var><big>]</big><var></var><big>]</big><var></var>)</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-612' xml:id='l2h-612' class="function">extract_tb</tt></b>(</nobr></td>
  <td><var>traceback</var><big>[</big><var>, limit</var><big>]</big><var></var>)</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-613' xml:id='l2h-613' class="function">extract_stack</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>f</var><big>[</big><var>, limit</var><big>]</big><var></var><big>]</big><var></var>)</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-614' xml:id='l2h-614' 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-615' xml:id='l2h-615' 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-616' xml:id='l2h-616' class="function">format_exception</tt></b>(</nobr></td>
  <td><var>type, value, tb</var><big>[</big><var>, limit</var><big>]</big><var></var>)</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-617' xml:id='l2h-617' class="function">format_tb</tt></b>(</nobr></td>
  <td><var>tb</var><big>[</big><var>, limit</var><big>]</big><var></var>)</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-618' xml:id='l2h-618' class="function">format_stack</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>f</var><big>[</big><var>, limit</var><big>]</big><var></var><big>]</big><var></var>)</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-619' xml:id='l2h-619' 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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	54
	55	<H1><A NAME="SECTION0051200000000000000000">
	56	3.12 <tt class="module">traceback</tt> --
	57	Print or retrieve a stack traceback</A>
	58	</H1>
	59
	60	<P>
	61	<A NAME="module-traceback"></A>
	62
	63	<P>
	64	This module provides a standard interface to extract, format and print
65	stack traces of Python programs. It exactly mimics the behavior of
66	the Python interpreter when it prints a stack trace. This is useful
67	when you want to print stack traces under program control, such as in a
68	``wrapper'' around the interpreter.
69
70	<P>
71	The module uses traceback objects -- this is the object type that is
72	stored in the variables <code>sys.exc_traceback</code> (deprecated) and
73	<code>sys.last_traceback</code> and returned as the third item from
74	<tt class="function">sys.exc_info()</tt>.
75	<a id='l2h-605' xml:id='l2h-605'></a>
76	<P>
77	The module defines the following functions:
78
79	<P>
80	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
81	<td><nobr><b><tt id='l2h-606' xml:id='l2h-606' class="function">print_tb</tt></b>(</nobr></td>
82	<td><var>traceback</var><big>[</big><var>, limit</var><big>[</big><var>, file</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
83	<dd>
84	Print up to <var>limit</var> stack trace entries from <var>traceback</var>. If
85	<var>limit</var> is omitted or <code>None</code>, all entries are printed.
86	If <var>file</var> is omitted or <code>None</code>, the output goes to
87	<code>sys.stderr</code>; otherwise it should be an open file or file-like
88	object to receive the output.
89	</dl>
90
91	<P>
92	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
93	<td><nobr><b><tt id='l2h-607' xml:id='l2h-607' class="function">print_exception</tt></b>(</nobr></td>
94	<td><var>type, value, traceback</var><big>[</big><var>,
95	limit</var><big>[</big><var>, file</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
96	<dd>
97	Print exception information and up to <var>limit</var> stack trace entries
98	from <var>traceback</var> to <var>file</var>.
99	This differs from <tt class="function">print_tb()</tt> in the
100	following ways: (1) if <var>traceback</var> is not <code>None</code>, it prints a
101	header "<tt class="samp">Traceback (most recent call last):</tt>"; (2) it prints the
102	exception <var>type</var> and <var>value</var> after the stack trace; (3) if
103	<var>type</var> is <tt class="exception">SyntaxError</tt> and <var>value</var> has the
104	appropriate format, it prints the line where the syntax error occurred
105	with a caret indicating the approximate position of the error.
106	</dl>
107
108	<P>
109	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
110	<td><nobr><b><tt id='l2h-608' xml:id='l2h-608' class="function">print_exc</tt></b>(</nobr></td>
111	<td><var></var><big>[</big><var>limit</var><big>[</big><var>, file</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
112	<dd>
113	This is a shorthand for <code>print_exception(sys.exc_type,
114	sys.exc_value, sys.exc_traceback, <var>limit</var>, <var>file</var>)</code>. (In
115	fact, it uses <tt class="function">sys.exc_info()</tt> to retrieve the same
116	information in a thread-safe way instead of using the deprecated
117	variables.)
118	</dl>
119
120	<P>
121	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
122	<td><nobr><b><tt id='l2h-609' xml:id='l2h-609' class="function">format_exc</tt></b>(</nobr></td>
123	<td><var></var><big>[</big><var>limit</var><big>[</big><var>, file</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
124	<dd>
125	This is like <code>print_exc(<var>limit</var>)</code> but returns a string
126	instead of printing to a file.
127
128	<span class="versionnote">New in version 2.4.</span>
129
130	</dl>
131
132	<P>
133	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
134	<td><nobr><b><tt id='l2h-610' xml:id='l2h-610' class="function">print_last</tt></b>(</nobr></td>
135	<td><var></var><big>[</big><var>limit</var><big>[</big><var>, file</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
136	<dd>
137	This is a shorthand for <code>print_exception(sys.last_type,
138	sys.last_value, sys.last_traceback, <var>limit</var>, <var>file</var>)</code>.
139	</dl>
140
141	<P>
142	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
143	<td><nobr><b><tt id='l2h-611' xml:id='l2h-611' class="function">print_stack</tt></b>(</nobr></td>
144	<td><var></var><big>[</big><var>f</var><big>[</big><var>, limit</var><big>[</big><var>, file</var><big>]</big><var></var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
145	<dd>
146	This function prints a stack trace from its invocation point. The
147	optional <var>f</var> argument can be used to specify an alternate stack
148	frame to start. The optional <var>limit</var> and <var>file</var> arguments have the
149	same meaning as for <tt class="function">print_exception()</tt>.
150	</dl>
151
152	<P>
153	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
154	<td><nobr><b><tt id='l2h-612' xml:id='l2h-612' class="function">extract_tb</tt></b>(</nobr></td>
155	<td><var>traceback</var><big>[</big><var>, limit</var><big>]</big><var></var>)</td></tr></table></dt>
156	<dd>
157	Return a list of up to <var>limit</var> ``pre-processed'' stack trace
158	entries extracted from the traceback object <var>traceback</var>. It is
159	useful for alternate formatting of stack traces. If <var>limit</var> is
160	omitted or <code>None</code>, all entries are extracted. A
161	``pre-processed'' stack trace entry is a quadruple (<var>filename</var>,
162	<var>line number</var>, <var>function name</var>, <var>text</var>) representing
163	the information that is usually printed for a stack trace. The
164	<var>text</var> is a string with leading and trailing whitespace
165	stripped; if the source is not available it is <code>None</code>.
166	</dl>
167
168	<P>
169	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
170	<td><nobr><b><tt id='l2h-613' xml:id='l2h-613' class="function">extract_stack</tt></b>(</nobr></td>
171	<td><var></var><big>[</big><var>f</var><big>[</big><var>, limit</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
172	<dd>
173	Extract the raw traceback from the current stack frame. The return
174	value has the same format as for <tt class="function">extract_tb()</tt>. The
175	optional <var>f</var> and <var>limit</var> arguments have the same meaning as
176	for <tt class="function">print_stack()</tt>.
177	</dl>
178
179	<P>
180	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
181	<td><nobr><b><tt id='l2h-614' xml:id='l2h-614' class="function">format_list</tt></b>(</nobr></td>
182	<td><var>list</var>)</td></tr></table></dt>
183	<dd>
184	Given a list of tuples as returned by <tt class="function">extract_tb()</tt> or
185	<tt class="function">extract_stack()</tt>, return a list of strings ready for
186	printing. Each string in the resulting list corresponds to the item
187	with the same index in the argument list. Each string ends in a
188	newline; the strings may contain internal newlines as well, for those
189	items whose source text line is not <code>None</code>.
190	</dl>
191
192	<P>
193	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
194	<td><nobr><b><tt id='l2h-615' xml:id='l2h-615' class="function">format_exception_only</tt></b>(</nobr></td>
195	<td><var>type, value</var>)</td></tr></table></dt>
196	<dd>
197	Format the exception part of a traceback. The arguments are the
198	exception type and value such as given by <code>sys.last_type</code> and
199	<code>sys.last_value</code>. The return value is a list of strings, each
200	ending in a newline. Normally, the list contains a single string;
201	however, for <tt class="exception">SyntaxError</tt> exceptions, it contains several
202	lines that (when printed) display detailed information about where the
203	syntax error occurred. The message indicating which exception
204	occurred is the always last string in the list.
205	</dl>
206
207	<P>
208	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
209	<td><nobr><b><tt id='l2h-616' xml:id='l2h-616' class="function">format_exception</tt></b>(</nobr></td>
210	<td><var>type, value, tb</var><big>[</big><var>, limit</var><big>]</big><var></var>)</td></tr></table></dt>
211	<dd>
212	Format a stack trace and the exception information. The arguments
213	have the same meaning as the corresponding arguments to
214	<tt class="function">print_exception()</tt>. The return value is a list of strings,
215	each ending in a newline and some containing internal newlines. When
216	these lines are concatenated and printed, exactly the same text is
217	printed as does <tt class="function">print_exception()</tt>.
218	</dl>
219
220	<P>
221	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
222	<td><nobr><b><tt id='l2h-617' xml:id='l2h-617' class="function">format_tb</tt></b>(</nobr></td>
223	<td><var>tb</var><big>[</big><var>, limit</var><big>]</big><var></var>)</td></tr></table></dt>
224	<dd>
225	A shorthand for <code>format_list(extract_tb(<var>tb</var>, <var>limit</var>))</code>.
226	</dl>
227
228	<P>
229	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
230	<td><nobr><b><tt id='l2h-618' xml:id='l2h-618' class="function">format_stack</tt></b>(</nobr></td>
231	<td><var></var><big>[</big><var>f</var><big>[</big><var>, limit</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
232	<dd>
233	A shorthand for <code>format_list(extract_stack(<var>f</var>, <var>limit</var>))</code>.
234	</dl>
235
236	<P>
237	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
238	<td><nobr><b><tt id='l2h-619' xml:id='l2h-619' class="function">tb_lineno</tt></b>(</nobr></td>
239	<td><var>tb</var>)</td></tr></table></dt>
240	<dd>
241	This function returns the current line number set in the traceback
242	object. This function was necessary because in versions of Python
243	prior to 2.3 when the <b class="programopt">-O</b> flag was passed to Python the
244	<code><var>tb</var>.tb_lineno</code> was not updated correctly. This function
245	has no use in versions past 2.3.
246	</dl>
247
248	<P>
249
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