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<div class="section" id="module-traceback">
<span id="traceback-print-or-retrieve-a-stack-traceback"></span><h1>27.9. <a class="reference internal" href="#module-traceback" title="traceback: Print or retrieve a stack traceback."><tt class="xref py py-mod docutils literal"><span class="pre">traceback</span></tt></a> — Print or retrieve a stack traceback<a class="headerlink" href="#module-traceback" title="Permalink to this headline">¶</a></h1>
<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>
<p id="index-0">The module uses traceback objects — this is the object type that is stored in
the <a class="reference internal" href="sys.html#sys.last_traceback" title="sys.last_traceback"><tt class="xref py py-data docutils literal"><span class="pre">sys.last_traceback</span></tt></a> variable and returned as the third item from
<a class="reference internal" href="sys.html#sys.exc_info" title="sys.exc_info"><tt class="xref py py-func docutils literal"><span class="pre">sys.exc_info()</span></tt></a>.</p>
<p>The module defines the following functions:</p>
<dl class="function">
<dt id="traceback.print_tb">
<tt class="descclassname">traceback.</tt><tt class="descname">print_tb</tt><big>(</big><em>traceback</em>, <em>limit=None</em>, <em>file=None</em><big>)</big><a class="headerlink" href="#traceback.print_tb" title="Permalink to this definition">¶</a></dt>
<dd><p>Print up to <em>limit</em> stack trace entries from <em>traceback</em>. If <em>limit</em> is omitted
or <tt class="xref docutils literal"><span class="pre">None</span></tt>, all entries are printed. If <em>file</em> is omitted or <tt class="xref docutils literal"><span class="pre">None</span></tt>, the
output goes to <tt class="docutils literal"><span class="pre">sys.stderr</span></tt>; otherwise it should be an open file or file-like
object to receive the output.</p>
</dd></dl>
<dl class="function">
<dt id="traceback.print_exception">
<tt class="descclassname">traceback.</tt><tt class="descname">print_exception</tt><big>(</big><em>type</em>, <em>value</em>, <em>traceback</em>, <em>limit=None</em>, <em>file=None</em>, <em>chain=True</em><big>)</big><a class="headerlink" href="#traceback.print_exception" title="Permalink to this definition">¶</a></dt>
<dd><p>Print exception information and up to <em>limit</em> stack trace entries from
<em>traceback</em> to <em>file</em>. This differs from <a class="reference internal" href="#traceback.print_tb" title="traceback.print_tb"><tt class="xref py py-func docutils literal"><span class="pre">print_tb()</span></tt></a> in the following
ways:</p>
<ul class="simple">
<li>if <em>traceback</em> is not <tt class="xref docutils literal"><span class="pre">None</span></tt>, it prints a header <tt class="docutils literal"><span class="pre">Traceback</span> <span class="pre">(most</span> <span class="pre">recent</span>
<span class="pre">call</span> <span class="pre">last):</span></tt></li>
<li>it prints the exception <em>type</em> and <em>value</em> after the stack trace</li>
<li>if <em>type</em> is <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></tt></a> and <em>value</em> has the appropriate format, it
prints the line where the syntax error occurred with a caret indicating the
approximate position of the error.</li>
</ul>
<p>If <em>chain</em> is true (the default), then chained exceptions (the
<tt class="xref py py-attr docutils literal"><span class="pre">__cause__</span></tt> or <tt class="xref py py-attr docutils literal"><span class="pre">__context__</span></tt> attributes of the exception) will be
printed as well, like the interpreter itself does when printing an unhandled
exception.</p>
</dd></dl>
<dl class="function">
<dt id="traceback.print_exc">
<tt class="descclassname">traceback.</tt><tt class="descname">print_exc</tt><big>(</big><em>limit=None</em>, <em>file=None</em>, <em>chain=True</em><big>)</big><a class="headerlink" href="#traceback.print_exc" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a shorthand for <tt class="docutils literal"><span class="pre">print_exception(*sys.exc_info())</span></tt>.</p>
</dd></dl>
<dl class="function">
<dt id="traceback.print_last">
<tt class="descclassname">traceback.</tt><tt class="descname">print_last</tt><big>(</big><em>limit=None</em>, <em>file=None</em>, <em>chain=True</em><big>)</big><a class="headerlink" href="#traceback.print_last" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a shorthand for <tt class="docutils literal"><span class="pre">print_exception(sys.last_type,</span> <span class="pre">sys.last_value,</span>
<span class="pre">sys.last_traceback,</span> <span class="pre">limit,</span> <span class="pre">file)</span></tt>. 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"><tt class="xref py py-data docutils literal"><span class="pre">sys.last_type</span></tt></a>).</p>
</dd></dl>
<dl class="function">
<dt id="traceback.print_stack">
<tt class="descclassname">traceback.</tt><tt class="descname">print_stack</tt><big>(</big><em>f=None</em>, <em>limit=None</em>, <em>file=None</em><big>)</big><a class="headerlink" href="#traceback.print_stack" title="Permalink to this definition">¶</a></dt>
<dd><p>This function prints a stack trace from its invocation point. The optional <em>f</em>
argument can be used to specify an alternate stack frame to start. The optional
<em>limit</em> and <em>file</em> arguments have the same meaning as for
<a class="reference internal" href="#traceback.print_exception" title="traceback.print_exception"><tt class="xref py py-func docutils literal"><span class="pre">print_exception()</span></tt></a>.</p>
</dd></dl>
<dl class="function">
<dt id="traceback.extract_tb">
<tt class="descclassname">traceback.</tt><tt class="descname">extract_tb</tt><big>(</big><em>traceback</em>, <em>limit=None</em><big>)</big><a class="headerlink" href="#traceback.extract_tb" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a list of up to <em>limit</em> “pre-processed” stack trace entries extracted
from the traceback object <em>traceback</em>. It is useful for alternate formatting of
stack traces. If <em>limit</em> is omitted or <tt class="xref docutils literal"><span class="pre">None</span></tt>, all entries are extracted. A
“pre-processed” stack trace entry is a quadruple (<em>filename</em>, <em>line number</em>,
<em>function name</em>, <em>text</em>) representing the information that is usually printed
for a stack trace. The <em>text</em> is a string with leading and trailing whitespace
stripped; if the source is not available it is <tt class="xref docutils literal"><span class="pre">None</span></tt>.</p>
</dd></dl>
<dl class="function">
<dt id="traceback.extract_stack">
<tt class="descclassname">traceback.</tt><tt class="descname">extract_stack</tt><big>(</big><em>f=None</em>, <em>limit=None</em><big>)</big><a class="headerlink" href="#traceback.extract_stack" title="Permalink to this definition">¶</a></dt>
<dd><p>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"><tt class="xref py py-func docutils literal"><span class="pre">extract_tb()</span></tt></a>. The optional <em>f</em> and <em>limit</em>
arguments have the same meaning as for <a class="reference internal" href="#traceback.print_stack" title="traceback.print_stack"><tt class="xref py py-func docutils literal"><span class="pre">print_stack()</span></tt></a>.</p>
</dd></dl>
<dl class="function">
<dt id="traceback.format_list">
<tt class="descclassname">traceback.</tt><tt class="descname">format_list</tt><big>(</big><em>list</em><big>)</big><a class="headerlink" href="#traceback.format_list" title="Permalink to this definition">¶</a></dt>
<dd><p>Given a list of tuples as returned by <a class="reference internal" href="#traceback.extract_tb" title="traceback.extract_tb"><tt class="xref py py-func docutils literal"><span class="pre">extract_tb()</span></tt></a> or
<a class="reference internal" href="#traceback.extract_stack" title="traceback.extract_stack"><tt class="xref py py-func docutils literal"><span class="pre">extract_stack()</span></tt></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 <tt class="xref docutils literal"><span class="pre">None</span></tt>.</p>
</dd></dl>
<dl class="function">
<dt id="traceback.format_exception_only">
<tt class="descclassname">traceback.</tt><tt class="descname">format_exception_only</tt><big>(</big><em>type</em>, <em>value</em><big>)</big><a class="headerlink" href="#traceback.format_exception_only" title="Permalink to this definition">¶</a></dt>
<dd><p>Format the exception part of a traceback. The arguments are the exception type
and value such as given by <tt class="docutils literal"><span class="pre">sys.last_type</span></tt> and <tt class="docutils literal"><span class="pre">sys.last_value</span></tt>. The return
value is a list of strings, each ending in a newline. Normally, the list
contains a single string; however, for <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></tt></a> 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.</p>
</dd></dl>
<dl class="function">
<dt id="traceback.format_exception">
<tt class="descclassname">traceback.</tt><tt class="descname">format_exception</tt><big>(</big><em>type</em>, <em>value</em>, <em>tb</em>, <em>limit=None</em>, <em>chain=True</em><big>)</big><a class="headerlink" href="#traceback.format_exception" title="Permalink to this definition">¶</a></dt>
<dd><p>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"><tt class="xref py py-func docutils literal"><span class="pre">print_exception()</span></tt></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"><tt class="xref py py-func docutils literal"><span class="pre">print_exception()</span></tt></a>.</p>
</dd></dl>
<dl class="function">
<dt id="traceback.format_exc">
<tt class="descclassname">traceback.</tt><tt class="descname">format_exc</tt><big>(</big><em>limit=None</em>, <em>chain=True</em><big>)</big><a class="headerlink" href="#traceback.format_exc" title="Permalink to this definition">¶</a></dt>
<dd><p>This is like <tt class="docutils literal"><span class="pre">print_exc(limit)</span></tt> but returns a string instead of printing to a
file.</p>
</dd></dl>
<dl class="function">
<dt id="traceback.format_tb">
<tt class="descclassname">traceback.</tt><tt class="descname">format_tb</tt><big>(</big><em>tb</em>, <em>limit=None</em><big>)</big><a class="headerlink" href="#traceback.format_tb" title="Permalink to this definition">¶</a></dt>
<dd><p>A shorthand for <tt class="docutils literal"><span class="pre">format_list(extract_tb(tb,</span> <span class="pre">limit))</span></tt>.</p>
</dd></dl>
<dl class="function">
<dt id="traceback.format_stack">
<tt class="descclassname">traceback.</tt><tt class="descname">format_stack</tt><big>(</big><em>f=None</em>, <em>limit=None</em><big>)</big><a class="headerlink" href="#traceback.format_stack" title="Permalink to this definition">¶</a></dt>
<dd><p>A shorthand for <tt class="docutils literal"><span class="pre">format_list(extract_stack(f,</span> <span class="pre">limit))</span></tt>.</p>
</dd></dl>
<div class="section" id="traceback-examples">
<span id="traceback-example"></span><h2>27.9.1. Traceback Examples<a class="headerlink" href="#traceback-examples" title="Permalink to this headline">¶</a></h2>
<p>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."><tt class="xref py py-mod docutils literal"><span class="pre">code</span></tt></a>
module.</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sys</span><span class="o">,</span> <span class="nn">traceback</span>
<span class="k">def</span> <span class="nf">run_user_code</span><span class="p">(</span><span class="n">envdir</span><span class="p">):</span>
<span class="n">source</span> <span class="o">=</span> <span class="nb">input</span><span class="p">(</span><span class="s">">>> "</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">exec</span><span class="p">(</span><span class="n">source</span><span class="p">,</span> <span class="n">envdir</span><span class="p">)</span>
<span class="k">except</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"Exception in user code:"</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"-"</span><span class="o">*</span><span class="mi">60</span><span class="p">)</span>
<span class="n">traceback</span><span class="o">.</span><span class="n">print_exc</span><span class="p">(</span><span class="n">file</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"-"</span><span class="o">*</span><span class="mi">60</span><span class="p">)</span>
<span class="n">envdir</span> <span class="o">=</span> <span class="p">{}</span>
<span class="k">while</span> <span class="k">True</span><span class="p">:</span>
<span class="n">run_user_code</span><span class="p">(</span><span class="n">envdir</span><span class="p">)</span>
</pre></div>
</div>
<p>The following example demonstrates the different ways to print and format the
exception and traceback:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sys</span><span class="o">,</span> <span class="nn">traceback</span>
<span class="k">def</span> <span class="nf">lumberjack</span><span class="p">():</span>
<span class="n">bright_side_of_death</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">bright_side_of_death</span><span class="p">():</span>
<span class="k">return</span> <span class="nb">tuple</span><span class="p">()[</span><span class="mi">0</span><span class="p">]</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">lumberjack</span><span class="p">()</span>
<span class="k">except</span> <span class="ne">IndexError</span><span class="p">:</span>
<span class="n">exc_type</span><span class="p">,</span> <span class="n">exc_value</span><span class="p">,</span> <span class="n">exc_traceback</span> <span class="o">=</span> <span class="n">sys</span><span class="o">.</span><span class="n">exc_info</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"*** print_tb:"</span><span class="p">)</span>
<span class="n">traceback</span><span class="o">.</span><span class="n">print_tb</span><span class="p">(</span><span class="n">exc_traceback</span><span class="p">,</span> <span class="n">limit</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">file</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"*** print_exception:"</span><span class="p">)</span>
<span class="n">traceback</span><span class="o">.</span><span class="n">print_exception</span><span class="p">(</span><span class="n">exc_type</span><span class="p">,</span> <span class="n">exc_value</span><span class="p">,</span> <span class="n">exc_traceback</span><span class="p">,</span>
<span class="n">limit</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">file</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"*** print_exc:"</span><span class="p">)</span>
<span class="n">traceback</span><span class="o">.</span><span class="n">print_exc</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"*** format_exc, first and last line:"</span><span class="p">)</span>
<span class="n">formatted_lines</span> <span class="o">=</span> <span class="n">traceback</span><span class="o">.</span><span class="n">format_exc</span><span class="p">()</span><span class="o">.</span><span class="n">splitlines</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="n">formatted_lines</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span>
<span class="nb">print</span><span class="p">(</span><span class="n">formatted_lines</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">])</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"*** format_exception:"</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="nb">repr</span><span class="p">(</span><span class="n">traceback</span><span class="o">.</span><span class="n">format_exception</span><span class="p">(</span><span class="n">exc_type</span><span class="p">,</span> <span class="n">exc_value</span><span class="p">,</span>
<span class="n">exc_traceback</span><span class="p">)))</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"*** extract_tb:"</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="nb">repr</span><span class="p">(</span><span class="n">traceback</span><span class="o">.</span><span class="n">extract_tb</span><span class="p">(</span><span class="n">exc_traceback</span><span class="p">)))</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"*** format_tb:"</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="nb">repr</span><span class="p">(</span><span class="n">traceback</span><span class="o">.</span><span class="n">format_tb</span><span class="p">(</span><span class="n">exc_traceback</span><span class="p">)))</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"*** tb_lineno:"</span><span class="p">,</span> <span class="n">exc_traceback</span><span class="o">.</span><span class="n">tb_lineno</span><span class="p">)</span>
</pre></div>
</div>
<p>The output for the example would look similar to this:</p>
<div class="highlight-none"><div class="highlight"><pre>*** print_tb:
File "<doctest...>", line 10, in <module>
lumberjack()
*** print_exception:
Traceback (most recent call last):
File "<doctest...>", line 10, in <module>
lumberjack()
File "<doctest...>", line 4, in lumberjack
bright_side_of_death()
IndexError: tuple index out of range
*** print_exc:
Traceback (most recent call last):
File "<doctest...>", line 10, in <module>
lumberjack()
File "<doctest...>", 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:
['Traceback (most recent call last):\n',
' File "<doctest...>", line 10, in <module>\n lumberjack()\n',
' File "<doctest...>", line 4, in lumberjack\n bright_side_of_death()\n',
' File "<doctest...>", line 7, in bright_side_of_death\n return tuple()[0]\n',
'IndexError: tuple index out of range\n']
*** extract_tb:
[('<doctest...>', 10, '<module>', 'lumberjack()'),
('<doctest...>', 4, 'lumberjack', 'bright_side_of_death()'),
('<doctest...>', 7, 'bright_side_of_death', 'return tuple()[0]')]
*** format_tb:
[' File "<doctest...>", line 10, in <module>\n lumberjack()\n',
' File "<doctest...>", line 4, in lumberjack\n bright_side_of_death()\n',
' File "<doctest...>", line 7, in bright_side_of_death\n return tuple()[0]\n']
*** tb_lineno: 10
</pre></div>
</div>
<p>The following example shows the different ways to print and format the stack:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">traceback</span>
<span class="gp">>>> </span><span class="k">def</span> <span class="nf">another_function</span><span class="p">():</span>
<span class="gp">... </span> <span class="n">lumberstack</span><span class="p">()</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="k">def</span> <span class="nf">lumberstack</span><span class="p">():</span>
<span class="gp">... </span> <span class="n">traceback</span><span class="o">.</span><span class="n">print_stack</span><span class="p">()</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="nb">repr</span><span class="p">(</span><span class="n">traceback</span><span class="o">.</span><span class="n">extract_stack</span><span class="p">()))</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="nb">repr</span><span class="p">(</span><span class="n">traceback</span><span class="o">.</span><span class="n">format_stack</span><span class="p">()))</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">another_function</span><span class="p">()</span>
<span class="go"> File "<doctest>", line 10, in <module></span>
<span class="go"> another_function()</span>
<span class="go"> File "<doctest>", line 3, in another_function</span>
<span class="go"> lumberstack()</span>
<span class="go"> File "<doctest>", line 6, in lumberstack</span>
<span class="go"> traceback.print_stack()</span>
<span class="go">[('<doctest>', 10, '<module>', 'another_function()'),</span>
<span class="go"> ('<doctest>', 3, 'another_function', 'lumberstack()'),</span>
<span class="go"> ('<doctest>', 7, 'lumberstack', 'print(repr(traceback.extract_stack()))')]</span>
<span class="go">[' File "<doctest>", line 10, in <module>\n another_function()\n',</span>
<span class="go"> ' File "<doctest>", line 3, in another_function\n lumberstack()\n',</span>
<span class="go"> ' File "<doctest>", line 8, in lumberstack\n print(repr(traceback.format_stack()))\n']</span>
</pre></div>
</div>
<p>This last example demonstrates the final few formatting functions:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">traceback</span>
<span class="gp">>>> </span><span class="n">traceback</span><span class="o">.</span><span class="n">format_list</span><span class="p">([(</span><span class="s">'spam.py'</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="s">'<module>'</span><span class="p">,</span> <span class="s">'spam.eggs()'</span><span class="p">),</span>
<span class="gp">... </span> <span class="p">(</span><span class="s">'eggs.py'</span><span class="p">,</span> <span class="mi">42</span><span class="p">,</span> <span class="s">'eggs'</span><span class="p">,</span> <span class="s">'return "bacon"'</span><span class="p">)])</span>
<span class="go">[' File "spam.py", line 3, in <module>\n spam.eggs()\n',</span>
<span class="go"> ' File "eggs.py", line 42, in eggs\n return "bacon"\n']</span>
<span class="gp">>>> </span><span class="n">an_error</span> <span class="o">=</span> <span class="ne">IndexError</span><span class="p">(</span><span class="s">'tuple index out of range'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">traceback</span><span class="o">.</span><span class="n">format_exception_only</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="n">an_error</span><span class="p">),</span> <span class="n">an_error</span><span class="p">)</span>
<span class="go">['IndexError: tuple index out of range\n']</span>
</pre></div>
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