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<title>Selecting Pretty-Printers (Debugging with GDB)</title>

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<a name="Selecting-Pretty_002dPrinters"></a>
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Next: <a href="Writing-a-Pretty_002dPrinter.html#Writing-a-Pretty_002dPrinter" accesskey="n" rel="next">Writing a Pretty-Printer</a>, Previous: <a href="Pretty-Printing-API.html#Pretty-Printing-API" accesskey="p" rel="prev">Pretty Printing API</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Selecting-Pretty_002dPrinters-1"></a>
<h4 class="subsubsection">23.2.2.6 Selecting Pretty-Printers</h4>
<a name="index-selecting-python-pretty_002dprinters"></a>

<p><small>GDB</small> provides several ways to register a pretty-printer:
globally, per program space, and per objfile.  When choosing how to
register your pretty-printer, a good rule is to register it with the
smallest scope possible: that is prefer a specific objfile first, then
a program space, and only register a printer globally as a last
resort.
</p>
<a name="index-gdb_002epretty_005fprinters"></a>
<dl>
<dt><a name="index-gdb_002epretty_005fprinters-1"></a>Variable: <strong>gdb.pretty_printers</strong></dt>
<dd><p>The Python list <code>gdb.pretty_printers</code> contains an array of
functions or callable objects that have been registered via addition
as a pretty-printer.  Printers in this list are called <code>global</code>
printers, they&rsquo;re available when debugging all inferiors.
</p></dd></dl>

<p>Each <code>gdb.Progspace</code> contains a <code>pretty_printers</code> attribute.
Each <code>gdb.Objfile</code> also contains a <code>pretty_printers</code>
attribute.
</p>
<p>Each function on these lists is passed a single <code>gdb.Value</code>
argument and should return a pretty-printer object conforming to the
interface definition above (see <a href="Pretty-Printing-API.html#Pretty-Printing-API">Pretty Printing API</a>).  If a function
cannot create a pretty-printer for the value, it should return
<code>None</code>.
</p>
<p><small>GDB</small> first checks the <code>pretty_printers</code> attribute of each
<code>gdb.Objfile</code> in the current program space and iteratively calls
each enabled lookup routine in the list for that <code>gdb.Objfile</code>
until it receives a pretty-printer object.
If no pretty-printer is found in the objfile lists, <small>GDB</small> then
searches the pretty-printer list of the current program space,
calling each enabled function until an object is returned.
After these lists have been exhausted, it tries the global
<code>gdb.pretty_printers</code> list, again calling each enabled function until an
object is returned.
</p>
<p>The order in which the objfiles are searched is not specified.  For a
given list, functions are always invoked from the head of the list,
and iterated over sequentially until the end of the list, or a printer
object is returned.
</p>
<p>For various reasons a pretty-printer may not work.
For example, the underlying data structure may have changed and
the pretty-printer is out of date.
</p>
<p>The consequences of a broken pretty-printer are severe enough that
<small>GDB</small> provides support for enabling and disabling individual
printers.  For example, if <code>print frame-arguments</code> is on,
a backtrace can become highly illegible if any argument is printed
with a broken printer.
</p>
<p>Pretty-printers are enabled and disabled by attaching an <code>enabled</code>
attribute to the registered function or callable object.  If this attribute
is present and its value is <code>False</code>, the printer is disabled, otherwise
the printer is enabled.
</p>
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Next: <a href="Writing-a-Pretty_002dPrinter.html#Writing-a-Pretty_002dPrinter" accesskey="n" rel="next">Writing a Pretty-Printer</a>, Previous: <a href="Pretty-Printing-API.html#Pretty-Printing-API" accesskey="p" rel="prev">Pretty Printing API</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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