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<title>Debugging with GDB: Pretty Printing API</title>
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<a name="Pretty-Printing-API"></a>
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<div class="header">
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<p>
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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="previous">Types In Python</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> [<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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<hr>
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<a name="Pretty-Printing-API-1"></a>
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<h4 class="subsubsection">23.2.2.5 Pretty Printing API</h4>
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<a name="index-python-pretty-printing-api"></a>
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<p>A pretty-printer is just an object that holds a value and implements a
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specific interface, defined here. An example output is provided
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(see <a href="Pretty-Printing.html#Pretty-Printing">Pretty Printing</a>).
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</p>
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<dl>
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<dt><a name="index-pretty_005fprinter_002echildren"></a>Function: <strong>pretty_printer.children</strong> <em>(self)</em></dt>
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<dd><p><small>GDB</small> will call this method on a pretty-printer to compute the
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children of the pretty-printer’s value.
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</p>
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<p>This method must return an object conforming to the Python iterator
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protocol. Each item returned by the iterator must be a tuple holding
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two elements. The first element is the “name” of the child; the
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second element is the child’s value. The value can be any Python
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object which is convertible to a <small>GDB</small> value.
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</p>
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<p>This method is optional. If it does not exist, <small>GDB</small> will act
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as though the value has no children.
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</p>
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<p>For efficiency, the <code>children</code> method should lazily compute its
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results. This will let <small>GDB</small> read as few elements as
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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.
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</p>
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<p>Children may be hidden from display based on the value of ‘<samp>set
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print max-depth</samp>’ (see <a href="Print-Settings.html#Print-Settings">Print Settings</a>).
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</p></dd></dl>
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<dl>
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<dt><a name="index-pretty_005fprinter_002edisplay_005fhint"></a>Function: <strong>pretty_printer.display_hint</strong> <em>(self)</em></dt>
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<dd><p>The CLI may call this method and use its result to change the
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formatting of a value. The result will also be supplied to an MI
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consumer as a ‘<samp>displayhint</samp>’ attribute of the variable being
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printed.
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</p>
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<p>This method is optional. If it does exist, this method must return a
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string or the special value <code>None</code>.
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</p>
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<p>Some display hints are predefined by <small>GDB</small>:
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</p>
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<dl compact="compact">
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<dt>‘<samp>array</samp>’</dt>
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<dd><p>Indicate that the object being printed is “array-like”. The CLI
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uses this to respect parameters such as <code>set print elements</code> and
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<code>set print array</code>.
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</p>
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</dd>
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<dt>‘<samp>map</samp>’</dt>
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<dd><p>Indicate that the object being printed is “map-like”, and that the
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children of this value can be assumed to alternate between keys and
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values.
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</p>
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</dd>
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<dt>‘<samp>string</samp>’</dt>
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<dd><p>Indicate that the object being printed is “string-like”. If the
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printer’s <code>to_string</code> method returns a Python string of some
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kind, then <small>GDB</small> will call its internal language-specific
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string-printing function to format the string. For the CLI this means
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adding quotation marks, possibly escaping some characters, respecting
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<code>set print elements</code>, and the like.
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</p></dd>
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</dl>
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<p>The special value <code>None</code> causes <small>GDB</small> to apply the default
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display rules.
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</p></dd></dl>
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<dl>
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<dt><a name="index-pretty_005fprinter_002eto_005fstring"></a>Function: <strong>pretty_printer.to_string</strong> <em>(self)</em></dt>
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<dd><p><small>GDB</small> will call this method to display the string
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representation of the value passed to the object’s constructor.
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</p>
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<p>When printing from the CLI, if the <code>to_string</code> method exists,
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then <small>GDB</small> will prepend its result to the values returned by
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<code>children</code>. Exactly how this formatting is done is dependent on
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the display hint, and may change as more hints are added. Also,
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depending on the print settings (see <a href="Print-Settings.html#Print-Settings">Print Settings</a>), the CLI may
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print just the result of <code>to_string</code> in a stack trace, omitting
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the result of <code>children</code>.
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</p>
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<p>If this method returns a string, it is printed verbatim.
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</p>
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<p>Otherwise, if this method returns an instance of <code>gdb.Value</code>,
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then <small>GDB</small> prints this value. This may result in a call to
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another pretty-printer.
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</p>
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<p>If instead the method returns a Python value which is convertible to a
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<code>gdb.Value</code>, then <small>GDB</small> performs the conversion and prints
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the resulting value. Again, this may result in a call to another
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pretty-printer. Python scalars (integers, floats, and booleans) and
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strings are convertible to <code>gdb.Value</code>; other types are not.
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</p>
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<p>Finally, if this method returns <code>None</code> then no further operations
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are peformed in this method and nothing is printed.
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</p>
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<p>If the result is not one of these types, an exception is raised.
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</p></dd></dl>
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<p><small>GDB</small> provides a function which can be used to look up the
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default pretty-printer for a <code>gdb.Value</code>:
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</p>
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<a name="index-gdb_002edefault_005fvisualizer"></a>
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<dl>
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<dt><a name="index-gdb_002edefault_005fvisualizer-1"></a>Function: <strong>gdb.default_visualizer</strong> <em>(value)</em></dt>
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<dd><p>This function takes a <code>gdb.Value</code> object as an argument. If a
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pretty-printer for this value exists, then it is returned. If no such
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printer exists, then this returns <code>None</code>.
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</p></dd></dl>
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<div class="header">
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<p>
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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="previous">Types In Python</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> [<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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