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<a name="Parameters-In-Python"></a>
<div class="header">
<p>
Next: <a href="Functions-In-Python.html#Functions-In-Python" accesskey="n" rel="next">Functions In Python</a>, Previous: <a href="Commands-In-Python.html#Commands-In-Python" accesskey="p" rel="prev">Commands 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="Parameters-In-Python-1"></a>
<h4 class="subsubsection">23.2.2.21 Parameters In Python</h4>

<a name="index-parameters-in-python"></a>
<a name="index-python-parameters"></a>
<a name="index-gdb_002eParameter"></a>
<a name="index-Parameter"></a>
<p>You can implement new <small>GDB</small> parameters using Python.  A new
parameter is implemented as an instance of the <code>gdb.Parameter</code>
class.
</p>
<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>
<p>There are many parameters that already exist and can be set in
<small>GDB</small>.  Two examples are: <code>set follow fork</code> and
<code>set charset</code>.  Setting these parameters influences certain
behavior in <small>GDB</small>.  Similarly, you can define parameters that
can be used to influence behavior in custom Python scripts and commands.
</p>
<dl>
<dt><a name="index-Parameter_002e_005f_005finit_005f_005f"></a>Function: <strong>Parameter.__init__</strong> <em>(name, <var>command-class</var>, <var>parameter-class</var> <span class="roman">[</span>, <var>enum-sequence</var><span class="roman">]</span>)</em></dt>
<dd><p>The object initializer for <code>Parameter</code> registers the new
parameter with <small>GDB</small>.  This initializer is normally invoked
from the subclass&rsquo; own <code>__init__</code> method.
</p>
<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
<small>GDB</small> as <code>set print foo</code>.
</p>
<p>If <var>name</var> consists of multiple words, and no prefix parameter group
can be found, an exception is raised.
</p>
<p><var>command-class</var> should be one of the &lsquo;<samp>COMMAND_</samp>&rsquo; constants
(see <a href="Commands-In-Python.html#Commands-In-Python">Commands In Python</a>).  This argument tells <small>GDB</small> how to
categorize the new parameter in the help system.
</p>
<p><var>parameter-class</var> should be one of the &lsquo;<samp>PARAM_</samp>&rsquo; constants
defined below.  This argument tells <small>GDB</small> the type of the new
parameter; this information is used for input validation and
completion.
</p>
<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>
<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>
<p>The help text for the new parameter is taken from the Python
documentation string for the parameter&rsquo;s class, if there is one.  If
there is no documentation string, a default value is used.
</p></dd></dl>

<dl>
<dt><a name="index-Parameter_002eset_005fdoc"></a>Variable: <strong>Parameter.set_doc</strong></dt>
<dd><p>If this attribute exists, and is a string, then its value is used as
the help text for this parameter&rsquo;s <code>set</code> command.  The value is
examined when <code>Parameter.__init__</code> is invoked; subsequent changes
have no effect.
</p></dd></dl>

<dl>
<dt><a name="index-Parameter_002eshow_005fdoc"></a>Variable: <strong>Parameter.show_doc</strong></dt>
<dd><p>If this attribute exists, and is a string, then its value is used as
the help text for this parameter&rsquo;s <code>show</code> command.  The value is
examined when <code>Parameter.__init__</code> is invoked; subsequent changes
have no effect.
</p></dd></dl>

<dl>
<dt><a name="index-Parameter_002evalue"></a>Variable: <strong>Parameter.value</strong></dt>
<dd><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.  <small>GDB</small> does validation when assignments are made.
</p></dd></dl>

<p>There are two methods that may be implemented in any <code>Parameter</code>
class.  These are:
</p>
<dl>
<dt><a name="index-Parameter_002eget_005fset_005fstring"></a>Function: <strong>Parameter.get_set_string</strong> <em>(self)</em></dt>
<dd><p>If this method exists, <small>GDB</small> will call it when a
<var>parameter</var>&rsquo;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,
<small>GDB</small> will present it to the user.
</p>
<p>If this method raises the <code>gdb.GdbError</code> exception
(see <a href="Exception-Handling.html#Exception-Handling">Exception Handling</a>), then <small>GDB</small> will print the
exception&rsquo;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:
</p>
<div class="smallexample">
<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></div>
</dd></dl>

<dl>
<dt><a name="index-Parameter_002eget_005fshow_005fstring"></a>Function: <strong>Parameter.get_show_string</strong> <em>(self, svalue)</em></dt>
<dd><p><small>GDB</small> will call this method when a <var>parameter</var>&rsquo;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></dd></dl>

<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:
</p>
<dl compact="compact">
<dd><a name="index-PARAM_005fBOOLEAN"></a>
<a name="index-gdb_002ePARAM_005fBOOLEAN"></a>
</dd>
<dt><code>gdb.PARAM_BOOLEAN</code></dt>
<dd><p>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"></a>
<a name="index-gdb_002ePARAM_005fAUTO_005fBOOLEAN"></a>
</dd>
<dt><code>gdb.PARAM_AUTO_BOOLEAN</code></dt>
<dd><p>The value has three possible states: true, false, and &lsquo;<samp>auto</samp>&rsquo;.  In
Python, true and false are represented using boolean constants, and
&lsquo;<samp>auto</samp>&rsquo; is represented using <code>None</code>.
</p>
<a name="index-PARAM_005fUINTEGER"></a>
<a name="index-gdb_002ePARAM_005fUINTEGER"></a>
</dd>
<dt><code>gdb.PARAM_UINTEGER</code></dt>
<dd><p>The value is an unsigned integer.  The value of 0 should be
interpreted to mean &ldquo;unlimited&rdquo;.
</p>
<a name="index-PARAM_005fINTEGER"></a>
<a name="index-gdb_002ePARAM_005fINTEGER"></a>
</dd>
<dt><code>gdb.PARAM_INTEGER</code></dt>
<dd><p>The value is a signed integer.  The value of 0 should be interpreted
to mean &ldquo;unlimited&rdquo;.
</p>
<a name="index-PARAM_005fSTRING"></a>
<a name="index-gdb_002ePARAM_005fSTRING"></a>
</dd>
<dt><code>gdb.PARAM_STRING</code></dt>
<dd><p>The value is a string.  When the user modifies the string, any escape
sequences, such as &lsquo;<samp>\t</samp>&rsquo;, &lsquo;<samp>\f</samp>&rsquo;, and octal escapes, are
translated into corresponding characters and encoded into the current
host charset.
</p>
<a name="index-PARAM_005fSTRING_005fNOESCAPE"></a>
<a name="index-gdb_002ePARAM_005fSTRING_005fNOESCAPE"></a>
</dd>
<dt><code>gdb.PARAM_STRING_NOESCAPE</code></dt>
<dd><p>The value is a string.  When the user modifies the string, escapes are
passed through untranslated.
</p>
<a name="index-PARAM_005fOPTIONAL_005fFILENAME"></a>
<a name="index-gdb_002ePARAM_005fOPTIONAL_005fFILENAME"></a>
</dd>
<dt><code>gdb.PARAM_OPTIONAL_FILENAME</code></dt>
<dd><p>The value is a either a filename (a string), or <code>None</code>.
</p>
<a name="index-PARAM_005fFILENAME"></a>
<a name="index-gdb_002ePARAM_005fFILENAME"></a>
</dd>
<dt><code>gdb.PARAM_FILENAME</code></dt>
<dd><p>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"></a>
<a name="index-gdb_002ePARAM_005fZINTEGER"></a>
</dd>
<dt><code>gdb.PARAM_ZINTEGER</code></dt>
<dd><p>The value is an integer.  This is like <code>PARAM_INTEGER</code>, except 0
is interpreted as itself.
</p>
<a name="index-PARAM_005fZUINTEGER"></a>
<a name="index-gdb_002ePARAM_005fZUINTEGER"></a>
</dd>
<dt><code>gdb.PARAM_ZUINTEGER</code></dt>
<dd><p>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"></a>
<a name="index-gdb_002ePARAM_005fZUINTEGER_005fUNLIMITED"></a>
</dd>
<dt><code>gdb.PARAM_ZUINTEGER_UNLIMITED</code></dt>
<dd><p>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"></a>
<a name="index-gdb_002ePARAM_005fENUM"></a>
</dd>
<dt><code>gdb.PARAM_ENUM</code></dt>
<dd><p>The value is a string, which must be one of a collection string
constants provided when the parameter is created.
</p></dd>
</dl>

<hr>
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