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<h5 class="subsubsection">23.2.2.5 Pretty Printing API</h5>

<p>An example output is provided (see <a href="Pretty-Printing.html#Pretty-Printing">Pretty Printing</a>).

   <p>A pretty-printer is just an object that holds a value and implements a
specific interface, defined here.

<div class="defun">
&mdash; Operation on pretty printer: <b>children</b> (<var>self</var>)<var><a name="index-children-on-pretty-printer-1694"></a></var><br>
<blockquote><p><span class="sc">gdb</span> will call this method on a pretty-printer to compute the
children of the pretty-printer's value.

        <p>This method must return an object conforming to the Python iterator
protocol.  Each item returned by the iterator must be a tuple holding
two elements.  The first element is the &ldquo;name&rdquo; of the child; the
second element is the child's value.  The value can be any Python
object which is convertible to a <span class="sc">gdb</span> value.

        <p>This method is optional.  If it does not exist, <span class="sc">gdb</span> will act
as though the value has no children. 
</p></blockquote></div>

<div class="defun">
&mdash; Operation on pretty printer: <b>display_hint</b> (<var>self</var>)<var><a name="index-display_005fhint-on-pretty-printer-1695"></a></var><br>
<blockquote><p>The CLI may call this method and use its result to change the
formatting of a value.  The result will also be supplied to an MI
consumer as a &lsquo;<samp><span class="samp">displayhint</span></samp>&rsquo; attribute of the variable being
printed.

        <p>This method is optional.  If it does exist, this method must return a
string.

        <p>Some display hints are predefined by <span class="sc">gdb</span>:

          <dl>
<dt>&lsquo;<samp><span class="samp">array</span></samp>&rsquo;<dd>Indicate that the object being printed is &ldquo;array-like&rdquo;.  The CLI
uses this to respect parameters such as <code>set print elements</code> and
<code>set print array</code>.

          <br><dt>&lsquo;<samp><span class="samp">map</span></samp>&rsquo;<dd>Indicate that the object being printed is &ldquo;map-like&rdquo;, and that the
children of this value can be assumed to alternate between keys and
values.

          <br><dt>&lsquo;<samp><span class="samp">string</span></samp>&rsquo;<dd>Indicate that the object being printed is &ldquo;string-like&rdquo;.  If the
printer's <code>to_string</code> method returns a Python string of some
kind, then <span class="sc">gdb</span> will call its internal language-specific
string-printing function to format the string.  For the CLI this means
adding quotation marks, possibly escaping some characters, respecting
<code>set print elements</code>, and the like. 
</dl>
        </p></blockquote></div>

<div class="defun">
&mdash; Operation on pretty printer: <b>to_string</b> (<var>self</var>)<var><a name="index-to_005fstring-on-pretty-printer-1696"></a></var><br>
<blockquote><p><span class="sc">gdb</span> will call this method to display the string
representation of the value passed to the object's constructor.

        <p>When printing from the CLI, if the <code>to_string</code> method exists,
then <span class="sc">gdb</span> will prepend its result to the values returned by
<code>children</code>.  Exactly how this formatting is done is dependent on
the display hint, and may change as more hints are added.  Also,
depending on the print settings (see <a href="Print-Settings.html#Print-Settings">Print Settings</a>), the CLI may
print just the result of <code>to_string</code> in a stack trace, omitting
the result of <code>children</code>.

        <p>If this method returns a string, it is printed verbatim.

        <p>Otherwise, if this method returns an instance of <code>gdb.Value</code>,
then <span class="sc">gdb</span> prints this value.  This may result in a call to
another pretty-printer.

        <p>If instead the method returns a Python value which is convertible to a
<code>gdb.Value</code>, then <span class="sc">gdb</span> performs the conversion and prints
the resulting value.  Again, this may result in a call to another
pretty-printer.  Python scalars (integers, floats, and booleans) and
strings are convertible to <code>gdb.Value</code>; other types are not.

        <p>Finally, if this method returns <code>None</code> then no further operations
are peformed in this method and nothing is printed.

        <p>If the result is not one of these types, an exception is raised. 
</p></blockquote></div>

   <p><span class="sc">gdb</span> provides a function which can be used to look up the
default pretty-printer for a <code>gdb.Value</code>:

   <p><a name="index-gdb_002edefault_005fvisualizer-1697"></a>

<div class="defun">
&mdash; Function: <b>default_visualizer</b><var> value<a name="index-default_005fvisualizer-1698"></a></var><br>
<blockquote><p>This function takes a <code>gdb.Value</code> object as an argument.  If a
pretty-printer for this value exists, then it is returned.  If no such
printer exists, then this returns <code>None</code>. 
</p></blockquote></div>

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