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<a name="Pretty-Printing-API"></a>
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<p>
Next: <a href="Selecting-Pretty_002dPrinters.html#Selecting-Pretty_002dPrinters" accesskey="n" rel="next">Selecting Pretty-Printers</a>, Previous: <a href="Types-In-Python.html#Types-In-Python" accesskey="p" rel="prev">Types In Python</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>
</div>
<hr>
<a name="Pretty-Printing-API-1"></a>
<h4 class="subsubsection">23.2.2.5 Pretty Printing API</h4>
<a name="index-python-pretty-printing-api"></a>

<p>A pretty-printer is just an object that holds a value and implements a
specific interface, defined here.  An example output is provided
(see <a href="Pretty-Printing.html#Pretty-Printing">Pretty Printing</a>).
</p>
<dl>
<dt><a name="index-pretty_005fprinter_002echildren"></a>Function: <strong>pretty_printer.children</strong> <em>(self)</em></dt>
<dd><p><small>GDB</small> will call this method on a pretty-printer to compute the
children of the pretty-printer&rsquo;s value.
</p>
<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&rsquo;s value.  The value can be any Python
object which is convertible to a <small>GDB</small> value.
</p>
<p>This method is optional.  If it does not exist, <small>GDB</small> will act
as though the value has no children.
</p>
<p>For efficiency, the <code>children</code> method should lazily compute its
results.  This will let <small>GDB</small> read as few elements as
necessary, for example when various print settings (see <a href="Print-Settings.html#Print-Settings">Print Settings</a>) or <code>-var-list-children</code> (see <a href="GDB_002fMI-Variable-Objects.html#GDB_002fMI-Variable-Objects">GDB/MI Variable Objects</a>) limit the number of elements to be displayed.
</p>
<p>Children may be hidden from display based on the value of &lsquo;<samp>set
print max-depth</samp>&rsquo; (see <a href="Print-Settings.html#Print-Settings">Print Settings</a>).
</p></dd></dl>

<dl>
<dt><a name="index-pretty_005fprinter_002edisplay_005fhint"></a>Function: <strong>pretty_printer.display_hint</strong> <em>(self)</em></dt>
<dd><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>displayhint</samp>&rsquo; attribute of the variable being
printed.
</p>
<p>This method is optional.  If it does exist, this method must return a
string or the special value <code>None</code>.
</p>
<p>Some display hints are predefined by <small>GDB</small>:
</p>
<dl compact="compact">
<dt>&lsquo;<samp>array</samp>&rsquo;</dt>
<dd><p>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>.
</p>
</dd>
<dt>&lsquo;<samp>map</samp>&rsquo;</dt>
<dd><p>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.
</p>
</dd>
<dt>&lsquo;<samp>string</samp>&rsquo;</dt>
<dd><p>Indicate that the object being printed is &ldquo;string-like&rdquo;.  If the
printer&rsquo;s <code>to_string</code> method returns a Python string of some
kind, then <small>GDB</small> 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.
</p></dd>
</dl>

<p>The special value <code>None</code> causes <small>GDB</small> to apply the default
display rules.
</p></dd></dl>

<dl>
<dt><a name="index-pretty_005fprinter_002eto_005fstring"></a>Function: <strong>pretty_printer.to_string</strong> <em>(self)</em></dt>
<dd><p><small>GDB</small> will call this method to display the string
representation of the value passed to the object&rsquo;s constructor.
</p>
<p>When printing from the CLI, if the <code>to_string</code> method exists,
then <small>GDB</small> 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>
<p>If this method returns a string, it is printed verbatim.
</p>
<p>Otherwise, if this method returns an instance of <code>gdb.Value</code>,
then <small>GDB</small> prints this value.  This may result in a call to
another pretty-printer.
</p>
<p>If instead the method returns a Python value which is convertible to a
<code>gdb.Value</code>, then <small>GDB</small> 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>
<p>Finally, if this method returns <code>None</code> then no further operations
are peformed in this method and nothing is printed.
</p>
<p>If the result is not one of these types, an exception is raised.
</p></dd></dl>

<p><small>GDB</small> 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"></a>
<dl>
<dt><a name="index-gdb_002edefault_005fvisualizer-1"></a>Function: <strong>gdb.default_visualizer</strong> <em>(value)</em></dt>
<dd><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></dd></dl>

<hr>
<div class="header">
<p>
Next: <a href="Selecting-Pretty_002dPrinters.html#Selecting-Pretty_002dPrinters" accesskey="n" rel="next">Selecting Pretty-Printers</a>, Previous: <a href="Types-In-Python.html#Types-In-Python" accesskey="p" rel="prev">Types In Python</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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