146 lines
7.3 KiB
HTML
146 lines
7.3 KiB
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<title>Pretty Printing API - Debugging with GDB</title>
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<meta name="description" content="Debugging with GDB">
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<link rel="up" href="Python-API.html#Python-API" title="Python API">
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<link rel="prev" href="Types-In-Python.html#Types-In-Python" title="Types In Python">
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<link rel="next" href="Selecting-Pretty_002dPrinters.html#Selecting-Pretty_002dPrinters" title="Selecting Pretty-Printers">
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<body>
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<div class="node">
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<a name="Pretty-Printing-API"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Selecting-Pretty_002dPrinters.html#Selecting-Pretty_002dPrinters">Selecting Pretty-Printers</a>,
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Previous: <a rel="previous" accesskey="p" href="Types-In-Python.html#Types-In-Python">Types In Python</a>,
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Up: <a rel="up" accesskey="u" href="Python-API.html#Python-API">Python API</a>
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<hr>
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</div>
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<h5 class="subsubsection">23.2.2.5 Pretty Printing API</h5>
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<p><a name="index-python-pretty-printing-api-2004"></a>
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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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<div class="defun">
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— Function: <b>pretty_printer.children</b> (<var>self</var>)<var><a name="index-pretty_005fprinter_002echildren-2005"></a></var><br>
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<blockquote><p><span class="sc">gdb</span> 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>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 <span class="sc">gdb</span> value.
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<p>This method is optional. If it does not exist, <span class="sc">gdb</span> will act
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as though the value has no children.
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</p></blockquote></div>
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<div class="defun">
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— Function: <b>pretty_printer.display_hint</b> (<var>self</var>)<var><a name="index-pretty_005fprinter_002edisplay_005fhint-2006"></a></var><br>
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<blockquote><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><span class="samp">displayhint</span></samp>’ attribute of the variable being
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printed.
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<p>This method is optional. If it does exist, this method must return a
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string.
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<p>Some display hints are predefined by <span class="sc">gdb</span>:
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<dl>
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<dt>‘<samp><span class="samp">array</span></samp>’<dd>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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<br><dt>‘<samp><span class="samp">map</span></samp>’<dd>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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<br><dt>‘<samp><span class="samp">string</span></samp>’<dd>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 <span class="sc">gdb</span> 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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</dl>
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</p></blockquote></div>
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<div class="defun">
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— Function: <b>pretty_printer.to_string</b> (<var>self</var>)<var><a name="index-pretty_005fprinter_002eto_005fstring-2007"></a></var><br>
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<blockquote><p><span class="sc">gdb</span> 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>When printing from the CLI, if the <code>to_string</code> method exists,
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then <span class="sc">gdb</span> 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>If this method returns a string, it is printed verbatim.
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<p>Otherwise, if this method returns an instance of <code>gdb.Value</code>,
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then <span class="sc">gdb</span> prints this value. This may result in a call to
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another pretty-printer.
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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 <span class="sc">gdb</span> 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>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>If the result is not one of these types, an exception is raised.
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</p></blockquote></div>
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<p><span class="sc">gdb</span> 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><a name="index-gdb_002edefault_005fvisualizer-2008"></a>
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<div class="defun">
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— Function: <b>gdb.default_visualizer</b> (<var>value</var>)<var><a name="index-gdb_002edefault_005fvisualizer-2009"></a></var><br>
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<blockquote><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></blockquote></div>
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</body></html>
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