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Code Editor : traceback.html

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<title>28.10. traceback — Print or retrieve a stack traceback &#8212; Python 2.7.18 documentation</title>
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 <section id="module-traceback">
<h1>28.10. <a class="reference internal" href="#module-traceback" title="traceback: Print or retrieve a stack traceback."><code class="xref py py-mod docutils literal notranslate">traceback</code></a> — Print or retrieve a stack traceback<a class="headerlink" href="#module-traceback" title="Permalink to this headline">¶</a></h1>
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.
The module uses traceback objects — this is the object type that is stored in
the variables <a class="reference internal" href="sys.html#sys.exc_traceback" title="sys.exc_traceback"><code class="xref py py-data docutils literal notranslate">sys.exc_traceback</code></a> (deprecated) and
<a class="reference internal" href="sys.html#sys.last_traceback" title="sys.last_traceback"><code class="xref py py-data docutils literal notranslate">sys.last_traceback</code></a> and returned as the third item from
<a class="reference internal" href="sys.html#sys.exc_info" title="sys.exc_info"><code class="xref py py-func docutils literal notranslate">sys.exc_info()</code></a>.
The module defines the following functions:
<dl class="py function">
<dt class="sig sig-object py" id="traceback.print_tb">
traceback.print_tb(tb[, limit[, file]])<a class="headerlink" href="#traceback.print_tb" title="Permalink to this definition">¶</a></dt>
<dd>Print up to limit stack trace entries from the traceback object tb. If
limit is omitted or <code class="docutils literal notranslate">None</code>, all entries are printed. If file is omitted
or <code class="docutils literal notranslate">None</code>, the output goes to <code class="docutils literal notranslate">sys.stderr</code>; otherwise it should be an
open file or file-like object to receive the output.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.print_exception">
traceback.print_exception(etype, value, tb[, limit[, file]])<a class="headerlink" href="#traceback.print_exception" title="Permalink to this definition">¶</a></dt>
<dd>Print exception information and up to limit stack trace entries from the
traceback tb to file. This differs from <a class="reference internal" href="#traceback.print_tb" title="traceback.print_tb"><code class="xref py py-func docutils literal notranslate">print_tb()</code></a> in the following
ways: (1) if tb is not <code class="docutils literal notranslate">None</code>, it prints a header <code class="docutils literal notranslate">Traceback (most
recent call last):</code>; (2) it prints the exception etype and value after
the stack trace; (3) if etype is <code class="xref py py-exc docutils literal notranslate">SyntaxError</code> and value has the
appropriate format, it prints the line where the syntax error occurred with a
caret indicating the approximate position of the error.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.print_exc">
traceback.print_exc([limit[, file]])<a class="headerlink" href="#traceback.print_exc" title="Permalink to this definition">¶</a></dt>
<dd>This is a shorthand for <code class="docutils literal notranslate">print_exception(sys.exc_type, sys.exc_value,
sys.exc_traceback, limit, file)</code>. (In fact, it uses <a class="reference internal" href="sys.html#sys.exc_info" title="sys.exc_info"><code class="xref py py-func docutils literal notranslate">sys.exc_info()</code></a> to
retrieve the same information in a thread-safe way instead of using the
deprecated variables.)
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.format_exc">
traceback.format_exc([limit])<a class="headerlink" href="#traceback.format_exc" title="Permalink to this definition">¶</a></dt>
<dd>This is like <code class="docutils literal notranslate">print_exc(limit)</code> but returns a string instead of printing to
a file.
<div class="versionadded">
New in version 2.4.
</div>
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.print_last">
traceback.print_last([limit[, file]])<a class="headerlink" href="#traceback.print_last" title="Permalink to this definition">¶</a></dt>
<dd>This is a shorthand for <code class="docutils literal notranslate">print_exception(sys.last_type, sys.last_value,
sys.last_traceback, limit, file)</code>. In general it will work only after
an exception has reached an interactive prompt (see <a class="reference internal" href="sys.html#sys.last_type" title="sys.last_type"><code class="xref py py-data docutils literal notranslate">sys.last_type</code></a>).
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.print_stack">
traceback.print_stack([f[, limit[, file]]])<a class="headerlink" href="#traceback.print_stack" title="Permalink to this definition">¶</a></dt>
<dd>This function prints a stack trace from its invocation point. The optional
f argument can be used to specify an alternate stack frame to start. The
optional limit and file arguments have the same meaning as for
<a class="reference internal" href="#traceback.print_exception" title="traceback.print_exception"><code class="xref py py-func docutils literal notranslate">print_exception()</code></a>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.extract_tb">
traceback.extract_tb(tb[, limit])<a class="headerlink" href="#traceback.extract_tb" title="Permalink to this definition">¶</a></dt>
<dd>Return a list of up to limit “pre-processed” stack trace entries extracted
from the traceback object tb. It is useful for alternate formatting of
stack traces. If limit is omitted or <code class="docutils literal notranslate">None</code>, all entries are extracted.
A “pre-processed” stack trace entry is a 4-tuple (filename, line number,
function name*, text) representing the information that is usually printed
for a stack trace. The text is a string with leading and trailing
whitespace stripped; if the source is not available it is <code class="docutils literal notranslate">None</code>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.extract_stack">
traceback.extract_stack([f[, limit]])<a class="headerlink" href="#traceback.extract_stack" title="Permalink to this definition">¶</a></dt>
<dd>Extract the raw traceback from the current stack frame. The return value has
the same format as for <a class="reference internal" href="#traceback.extract_tb" title="traceback.extract_tb"><code class="xref py py-func docutils literal notranslate">extract_tb()</code></a>. The optional f and limit
arguments have the same meaning as for <a class="reference internal" href="#traceback.print_stack" title="traceback.print_stack"><code class="xref py py-func docutils literal notranslate">print_stack()</code></a>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.format_list">
traceback.format_list(extracted_list)<a class="headerlink" href="#traceback.format_list" title="Permalink to this definition">¶</a></dt>
<dd>Given a list of tuples as returned by <a class="reference internal" href="#traceback.extract_tb" title="traceback.extract_tb"><code class="xref py py-func docutils literal notranslate">extract_tb()</code></a> or
<a class="reference internal" href="#traceback.extract_stack" title="traceback.extract_stack"><code class="xref py py-func docutils literal notranslate">extract_stack()</code></a>, 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 class="docutils literal notranslate">None</code>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.format_exception_only">
traceback.format_exception_only(etype, value)<a class="headerlink" href="#traceback.format_exception_only" title="Permalink to this definition">¶</a></dt>
<dd>Format the exception part of a traceback. The arguments are the exception
type, etype and value such as given by <code class="docutils literal notranslate">sys.last_type</code> and
<code class="docutils literal notranslate">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 class="xref py py-exc docutils literal notranslate">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.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.format_exception">
traceback.format_exception(etype, value, tb[, limit])<a class="headerlink" href="#traceback.format_exception" title="Permalink to this definition">¶</a></dt>
<dd>Format a stack trace and the exception information. The arguments have the
same meaning as the corresponding arguments to <a class="reference internal" href="#traceback.print_exception" title="traceback.print_exception"><code class="xref py py-func docutils literal notranslate">print_exception()</code></a>. 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 <a class="reference internal" href="#traceback.print_exception" title="traceback.print_exception"><code class="xref py py-func docutils literal notranslate">print_exception()</code></a>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.format_tb">
traceback.format_tb(tb[, limit])<a class="headerlink" href="#traceback.format_tb" title="Permalink to this definition">¶</a></dt>
<dd>A shorthand for <code class="docutils literal notranslate">format_list(extract_tb(tb, limit))</code>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.format_stack">
traceback.format_stack([f[, limit]])<a class="headerlink" href="#traceback.format_stack" title="Permalink to this definition">¶</a></dt>
<dd>A shorthand for <code class="docutils literal notranslate">format_list(extract_stack(f, limit))</code>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="traceback.tb_lineno">
traceback.tb_lineno(tb)<a class="headerlink" href="#traceback.tb_lineno" title="Permalink to this definition">¶</a></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 <a class="reference internal" href="../using/cmdline.html#cmdoption-O"><code class="xref std std-option docutils literal notranslate">-O</code></a> flag was passed to Python the <code class="docutils literal notranslate">tb.tb_lineno</code> was not
updated correctly. This function has no use in versions past 2.3.
</dd></dl>

<section id="traceback-examples">
<h2>28.10.1. Traceback Examples<a class="headerlink" href="#traceback-examples" title="Permalink to this headline">¶</a></h2>
This simple example implements a basic read-eval-print loop, similar to (but
less useful than) the standard Python interactive interpreter loop. For a more
complete implementation of the interpreter loop, refer to the <a class="reference internal" href="code.html#module-code" title="code: Facilities to implement read-eval-print loops."><code class="xref py py-mod docutils literal notranslate">code</code></a>
module.
<div class="highlight-default notranslate"><div class="highlight"><pre>import sys, traceback

def run_user_code(envdir):
 source = raw_input(&quot;&gt;&gt;&gt; &quot;)
 try:
 exec source in envdir
 except:
 print &quot;Exception in user code:&quot;
 print &#39;-&#39;*60
 traceback.print_exc(file=sys.stdout)
 print &#39;-&#39;*60

envdir = {}
while 1:
 run_user_code(envdir)
</pre></div>
</div>
The following example demonstrates the different ways to print and format the
exception and traceback:
<div class="highlight-default notranslate"><div class="highlight"><pre>import sys, traceback

def lumberjack():
 bright_side_of_death()

def bright_side_of_death():
 return tuple()[0]

try:
 lumberjack()
except IndexError:
 exc_type, exc_value, exc_traceback = sys.exc_info()
 print &quot;*** print_tb:&quot;
 traceback.print_tb(exc_traceback, limit=1, file=sys.stdout)
 print &quot;*** print_exception:&quot;
 traceback.print_exception(exc_type, exc_value, exc_traceback,
 limit=2, file=sys.stdout)
 print &quot;*** print_exc:&quot;
 traceback.print_exc()
 print &quot;*** format_exc, first and last line:&quot;
 formatted_lines = traceback.format_exc().splitlines()
 print formatted_lines[0]
 print formatted_lines[-1]
 print &quot;*** format_exception:&quot;
 print repr(traceback.format_exception(exc_type, exc_value,
 exc_traceback))
 print &quot;*** extract_tb:&quot;
 print repr(traceback.extract_tb(exc_traceback))
 print &quot;*** format_tb:&quot;
 print repr(traceback.format_tb(exc_traceback))
 print &quot;*** tb_lineno:&quot;, exc_traceback.tb_lineno
</pre></div>
</div>
The output for the example would look similar to this:
<div class="highlight-default notranslate"><div class="highlight"><pre>*** print_tb:
 File &quot;&lt;doctest...&gt;&quot;, line 10, in &lt;module&gt;
 lumberjack()
*** print_exception:
Traceback (most recent call last):
 File &quot;&lt;doctest...&gt;&quot;, line 10, in &lt;module&gt;
 lumberjack()
 File &quot;&lt;doctest...&gt;&quot;, line 4, in lumberjack
 bright_side_of_death()
IndexError: tuple index out of range
*** print_exc:
Traceback (most recent call last):
 File &quot;&lt;doctest...&gt;&quot;, line 10, in &lt;module&gt;
 lumberjack()
 File &quot;&lt;doctest...&gt;&quot;, line 4, in lumberjack
 bright_side_of_death()
IndexError: tuple index out of range
*** format_exc, first and last line:
Traceback (most recent call last):
IndexError: tuple index out of range
*** format_exception:
[&#39;Traceback (most recent call last):\n&#39;,
 &#39; File &quot;&lt;doctest...&gt;&quot;, line 10, in &lt;module&gt;\n lumberjack()\n&#39;,
 &#39; File &quot;&lt;doctest...&gt;&quot;, line 4, in lumberjack\n bright_side_of_death()\n&#39;,
 &#39; File &quot;&lt;doctest...&gt;&quot;, line 7, in bright_side_of_death\n return tuple()[0]\n&#39;,
 &#39;IndexError: tuple index out of range\n&#39;]
*** extract_tb:
[(&#39;&lt;doctest...&gt;&#39;, 10, &#39;&lt;module&gt;&#39;, &#39;lumberjack()&#39;),
 (&#39;&lt;doctest...&gt;&#39;, 4, &#39;lumberjack&#39;, &#39;bright_side_of_death()&#39;),
 (&#39;&lt;doctest...&gt;&#39;, 7, &#39;bright_side_of_death&#39;, &#39;return tuple()[0]&#39;)]
*** format_tb:
[&#39; File &quot;&lt;doctest...&gt;&quot;, line 10, in &lt;module&gt;\n lumberjack()\n&#39;,
 &#39; File &quot;&lt;doctest...&gt;&quot;, line 4, in lumberjack\n bright_side_of_death()\n&#39;,
 &#39; File &quot;&lt;doctest...&gt;&quot;, line 7, in bright_side_of_death\n return tuple()[0]\n&#39;]
*** tb_lineno: 10
</pre></div>
</div>
The following example shows the different ways to print and format the stack:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import traceback
&gt;&gt;&gt; def another_function():
... lumberstack()
...
&gt;&gt;&gt; def lumberstack():
... traceback.print_stack()
... print repr(traceback.extract_stack())
... print repr(traceback.format_stack())
...
&gt;&gt;&gt; another_function()
 File &quot;&lt;doctest&gt;&quot;, line 10, in &lt;module&gt;
 another_function()
 File &quot;&lt;doctest&gt;&quot;, line 3, in another_function
 lumberstack()
 File &quot;&lt;doctest&gt;&quot;, line 6, in lumberstack
 traceback.print_stack()
[(&#39;&lt;doctest&gt;&#39;, 10, &#39;&lt;module&gt;&#39;, &#39;another_function()&#39;),
 (&#39;&lt;doctest&gt;&#39;, 3, &#39;another_function&#39;, &#39;lumberstack()&#39;),
 (&#39;&lt;doctest&gt;&#39;, 7, &#39;lumberstack&#39;, &#39;print repr(traceback.extract_stack())&#39;)]
[&#39; File &quot;&lt;doctest&gt;&quot;, line 10, in &lt;module&gt;\n another_function()\n&#39;,
 &#39; File &quot;&lt;doctest&gt;&quot;, line 3, in another_function\n lumberstack()\n&#39;,
 &#39; File &quot;&lt;doctest&gt;&quot;, line 8, in lumberstack\n print repr(traceback.format_stack())\n&#39;]
</pre></div>
</div>
This last example demonstrates the final few formatting functions:
<div class="highlight-pycon3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import traceback
&gt;&gt;&gt; traceback.format_list([(&#39;spam.py&#39;, 3, &#39;&lt;module&gt;&#39;, &#39;spam.eggs()&#39;),
... (&#39;eggs.py&#39;, 42, &#39;eggs&#39;, &#39;return &quot;bacon&quot;&#39;)])
[&#39; File &quot;spam.py&quot;, line 3, in &lt;module&gt;\n spam.eggs()\n&#39;,
 &#39; File &quot;eggs.py&quot;, line 42, in eggs\n return &quot;bacon&quot;\n&#39;]
&gt;&gt;&gt; an_error = IndexError(&#39;tuple index out of range&#39;)
&gt;&gt;&gt; traceback.format_exception_only(type(an_error), an_error)
[&#39;IndexError: tuple index out of range\n&#39;]
</pre></div>
</div>
</section>
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