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<h5 class="subsubsection">23.2.2.21 Parameters In Python</h5>

<p><a name="index-parameters-in-python-2183"></a><a name="index-python-parameters-2184"></a><a name="index-gdb_002eParameter-2185"></a><a name="index-Parameter-2186"></a>You can implement new <span class="sc">gdb</span> parameters using Python.  A new
parameter is implemented as an instance of the <code>gdb.Parameter</code>
class.

   <p>Parameters are exposed to the user via the <code>set</code> and
<code>show</code> commands.  See <a href="Help.html#Help">Help</a>.

   <p>There are many parameters that already exist and can be set in
<span class="sc">gdb</span>.  Two examples are: <code>set follow fork</code> and
<code>set charset</code>.  Setting these parameters influences certain
behavior in <span class="sc">gdb</span>.  Similarly, you can define parameters that
can be used to influence behavior in custom Python scripts and commands.

<div class="defun">
&mdash; Function: <b>Parameter.__init__</b> (<var>name, command-class, parameter-class </var><span class="roman">[</span><var>, enum-sequence</var><span class="roman">]</span>)<var><a name="index-Parameter_002e_005f_005finit_005f_005f-2187"></a></var><br>
<blockquote><p>The object initializer for <code>Parameter</code> registers the new
parameter with <span class="sc">gdb</span>.  This initializer is normally invoked
from the subclass' own <code>__init__</code> method.

        <p><var>name</var> is the name of the new parameter.  If <var>name</var> consists
of multiple words, then the initial words are looked for as prefix
parameters.  An example of this can be illustrated with the
<code>set print</code> set of parameters.  If <var>name</var> is
<code>print foo</code>, then <code>print</code> will be searched as the prefix
parameter.  In this case the parameter can subsequently be accessed in
<span class="sc">gdb</span> as <code>set print foo</code>.

        <p>If <var>name</var> consists of multiple words, and no prefix parameter group
can be found, an exception is raised.

        <p><var>command-class</var> should be one of the &lsquo;<samp><span class="samp">COMMAND_</span></samp>&rsquo; constants
(see <a href="Commands-In-Python.html#Commands-In-Python">Commands In Python</a>).  This argument tells <span class="sc">gdb</span> how to
categorize the new parameter in the help system.

        <p><var>parameter-class</var> should be one of the &lsquo;<samp><span class="samp">PARAM_</span></samp>&rsquo; constants
defined below.  This argument tells <span class="sc">gdb</span> the type of the new
parameter; this information is used for input validation and
completion.

        <p>If <var>parameter-class</var> is <code>PARAM_ENUM</code>, then
<var>enum-sequence</var> must be a sequence of strings.  These strings
represent the possible values for the parameter.

        <p>If <var>parameter-class</var> is not <code>PARAM_ENUM</code>, then the presence
of a fourth argument will cause an exception to be thrown.

        <p>The help text for the new parameter is taken from the Python
documentation string for the parameter's class, if there is one.  If
there is no documentation string, a default value is used. 
</p></blockquote></div>

<div class="defun">
&mdash; Variable: <b>Parameter.set_doc</b><var><a name="index-Parameter_002eset_005fdoc-2188"></a></var><br>
<blockquote><p>If this attribute exists, and is a string, then its value is used as
the help text for this parameter's <code>set</code> command.  The value is
examined when <code>Parameter.__init__</code> is invoked; subsequent changes
have no effect. 
</p></blockquote></div>

<div class="defun">
&mdash; Variable: <b>Parameter.show_doc</b><var><a name="index-Parameter_002eshow_005fdoc-2189"></a></var><br>
<blockquote><p>If this attribute exists, and is a string, then its value is used as
the help text for this parameter's <code>show</code> command.  The value is
examined when <code>Parameter.__init__</code> is invoked; subsequent changes
have no effect. 
</p></blockquote></div>

<div class="defun">
&mdash; Variable: <b>Parameter.value</b><var><a name="index-Parameter_002evalue-2190"></a></var><br>
<blockquote><p>The <code>value</code> attribute holds the underlying value of the
parameter.  It can be read and assigned to just as any other
attribute.  <span class="sc">gdb</span> does validation when assignments are made. 
</p></blockquote></div>

   <p>There are two methods that may be implemented in any <code>Parameter</code>
class.  These are:

<div class="defun">
&mdash; Function: <b>Parameter.get_set_string</b> (<var>self</var>)<var><a name="index-Parameter_002eget_005fset_005fstring-2191"></a></var><br>
<blockquote><p>If this method exists, <span class="sc">gdb</span> will call it when a
<var>parameter</var>'s value has been changed via the <code>set</code> API (for
example, <kbd>set foo off</kbd>).  The <code>value</code> attribute has already
been populated with the new value and may be used in output.  This
method must return a string.  If the returned string is not empty,
<span class="sc">gdb</span> will present it to the user.

        <p>If this method raises the <code>gdb.GdbError</code> exception
(see <a href="Exception-Handling.html#Exception-Handling">Exception Handling</a>), then <span class="sc">gdb</span> will print the
exception's string and the <code>set</code> command will fail.  Note,
however, that the <code>value</code> attribute will not be reset in this
case.  So, if your parameter must validate values, it should store the
old value internally and reset the exposed value, like so:

     <pre class="smallexample">          class ExampleParam (gdb.Parameter):
             def __init__ (self, name):
                super (ExampleParam, self).__init__ (name,
                             gdb.COMMAND_DATA,
                             gdb.PARAM_BOOLEAN)
                self.value = True
                self.saved_value = True
             def validate(self):
                return False
             def get_set_string (self):
                if not self.validate():
                  self.value = self.saved_value
                  raise gdb.GdbError('Failed to validate')
                self.saved_value = self.value
</pre>
        </blockquote></div>

<div class="defun">
&mdash; Function: <b>Parameter.get_show_string</b> (<var>self, svalue</var>)<var><a name="index-Parameter_002eget_005fshow_005fstring-2192"></a></var><br>
<blockquote><p><span class="sc">gdb</span> will call this method when a <var>parameter</var>'s
<code>show</code> API has been invoked (for example, <kbd>show foo</kbd>).  The
argument <code>svalue</code> receives the string representation of the
current value.  This method must return a string. 
</p></blockquote></div>

   <p>When a new parameter is defined, its type must be specified.  The
available types are represented by constants defined in the <code>gdb</code>
module:

     
<a name="index-PARAM_005fBOOLEAN-2193"></a>
<a name="index-gdb_002ePARAM_005fBOOLEAN-2194"></a>
<dl><dt><code>gdb.PARAM_BOOLEAN</code><dd>The value is a plain boolean.  The Python boolean values, <code>True</code>
and <code>False</code> are the only valid values.

     <p><a name="index-PARAM_005fAUTO_005fBOOLEAN-2195"></a><a name="index-gdb_002ePARAM_005fAUTO_005fBOOLEAN-2196"></a><br><dt><code>gdb.PARAM_AUTO_BOOLEAN</code><dd>The value has three possible states: true, false, and &lsquo;<samp><span class="samp">auto</span></samp>&rsquo;.  In
Python, true and false are represented using boolean constants, and
&lsquo;<samp><span class="samp">auto</span></samp>&rsquo; is represented using <code>None</code>.

     <p><a name="index-PARAM_005fUINTEGER-2197"></a><a name="index-gdb_002ePARAM_005fUINTEGER-2198"></a><br><dt><code>gdb.PARAM_UINTEGER</code><dd>The value is an unsigned integer.  The value of 0 should be
interpreted to mean &ldquo;unlimited&rdquo;.

     <p><a name="index-PARAM_005fINTEGER-2199"></a><a name="index-gdb_002ePARAM_005fINTEGER-2200"></a><br><dt><code>gdb.PARAM_INTEGER</code><dd>The value is a signed integer.  The value of 0 should be interpreted
to mean &ldquo;unlimited&rdquo;.

     <p><a name="index-PARAM_005fSTRING-2201"></a><a name="index-gdb_002ePARAM_005fSTRING-2202"></a><br><dt><code>gdb.PARAM_STRING</code><dd>The value is a string.  When the user modifies the string, any escape
sequences, such as &lsquo;<samp><span class="samp">\t</span></samp>&rsquo;, &lsquo;<samp><span class="samp">\f</span></samp>&rsquo;, and octal escapes, are
translated into corresponding characters and encoded into the current
host charset.

     <p><a name="index-PARAM_005fSTRING_005fNOESCAPE-2203"></a><a name="index-gdb_002ePARAM_005fSTRING_005fNOESCAPE-2204"></a><br><dt><code>gdb.PARAM_STRING_NOESCAPE</code><dd>The value is a string.  When the user modifies the string, escapes are
passed through untranslated.

     <p><a name="index-PARAM_005fOPTIONAL_005fFILENAME-2205"></a><a name="index-gdb_002ePARAM_005fOPTIONAL_005fFILENAME-2206"></a><br><dt><code>gdb.PARAM_OPTIONAL_FILENAME</code><dd>The value is a either a filename (a string), or <code>None</code>.

     <p><a name="index-PARAM_005fFILENAME-2207"></a><a name="index-gdb_002ePARAM_005fFILENAME-2208"></a><br><dt><code>gdb.PARAM_FILENAME</code><dd>The value is a filename.  This is just like
<code>PARAM_STRING_NOESCAPE</code>, but uses file names for completion.

     <p><a name="index-PARAM_005fZINTEGER-2209"></a><a name="index-gdb_002ePARAM_005fZINTEGER-2210"></a><br><dt><code>gdb.PARAM_ZINTEGER</code><dd>The value is an integer.  This is like <code>PARAM_INTEGER</code>, except 0
is interpreted as itself.

     <p><a name="index-PARAM_005fZUINTEGER-2211"></a><a name="index-gdb_002ePARAM_005fZUINTEGER-2212"></a><br><dt><code>gdb.PARAM_ZUINTEGER</code><dd>The value is an unsigned integer.  This is like <code>PARAM_INTEGER</code>,
except 0 is interpreted as itself, and the value cannot be negative.

     <p><a name="index-PARAM_005fZUINTEGER_005fUNLIMITED-2213"></a><a name="index-gdb_002ePARAM_005fZUINTEGER_005fUNLIMITED-2214"></a><br><dt><code>gdb.PARAM_ZUINTEGER_UNLIMITED</code><dd>The value is a signed integer.  This is like <code>PARAM_ZUINTEGER</code>,
except the special value -1 should be interpreted to mean
&ldquo;unlimited&rdquo;.  Other negative values are not allowed.

     <p><a name="index-PARAM_005fENUM-2215"></a><a name="index-gdb_002ePARAM_005fENUM-2216"></a><br><dt><code>gdb.PARAM_ENUM</code><dd>The value is a string, which must be one of a collection string
constants provided when the parameter is created. 
</dl>

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