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<h5 class="subsubsection">23.2.2.6 Selecting Pretty-Printers</h5>

<p><a name="index-selecting-python-pretty_002dprinters-2010"></a>
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're available when debugging all inferiors. 
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>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><span class="sc">gdb</span> 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, <span class="sc">gdb</span> 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>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>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the pretty-printer is out of date.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>The consequences of a broken pretty-printer are severe enough that
<span class="sc">gdb</span> 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>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.

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