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| <h5 class="subsubsection">23.2.2.5 Pretty Printing API</h5> |
| |
| <p><a name="index-python-pretty-printing-api-2004"></a> |
| 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>). |
| |
| <div class="defun"> |
| — Function: <b>pretty_printer.children</b> (<var>self</var>)<var><a name="index-pretty_005fprinter_002echildren-2005"></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 “name” 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"> |
| — Function: <b>pretty_printer.display_hint</b> (<var>self</var>)<var><a name="index-pretty_005fprinter_002edisplay_005fhint-2006"></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 ‘<samp><span class="samp">displayhint</span></samp>’ 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>‘<samp><span class="samp">array</span></samp>’<dd>Indicate that the object being printed is “array-like”. The CLI |
| uses this to respect parameters such as <code>set print elements</code> and |
| <code>set print array</code>. |
| |
| <br><dt>‘<samp><span class="samp">map</span></samp>’<dd>Indicate that the object being printed is “map-like”, and that the |
| children of this value can be assumed to alternate between keys and |
| values. |
| |
| <br><dt>‘<samp><span class="samp">string</span></samp>’<dd>Indicate that the object being printed is “string-like”. 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"> |
| — Function: <b>pretty_printer.to_string</b> (<var>self</var>)<var><a name="index-pretty_005fprinter_002eto_005fstring-2007"></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-2008"></a> |
| |
| <div class="defun"> |
| — Function: <b>gdb.default_visualizer</b> (<var>value</var>)<var><a name="index-gdb_002edefault_005fvisualizer-2009"></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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