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<p>
Next:&nbsp;<a rel="next" accesskey="n" href="Writing-a-Frame-Filter.html#Writing-a-Frame-Filter">Writing a Frame Filter</a>,
Previous:&nbsp;<a rel="previous" accesskey="p" href="Frame-Filter-API.html#Frame-Filter-API">Frame Filter API</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="Python-API.html#Python-API">Python API</a>
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<h5 class="subsubsection">23.2.2.10 Decorating Frames</h5>
<p><a name="index-frame-decorator-api-2023"></a>
Frame decorators are sister objects to frame filters (see <a href="Frame-Filter-API.html#Frame-Filter-API">Frame Filter API</a>). Frame decorators are applied by a frame filter and can
only be used in conjunction with frame filters.
<p>The purpose of a frame decorator is to customize the printed content
of each <code>gdb.Frame</code> in commands where frame filters are executed.
This concept is called decorating a frame. Frame decorators decorate
a <code>gdb.Frame</code> with Python code contained within each API call.
This separates the actual data contained in a <code>gdb.Frame</code> from
the decorated data produced by a frame decorator. This abstraction is
necessary to maintain integrity of the data contained in each
<code>gdb.Frame</code>.
<p>Frame decorators have a mandatory interface, defined below.
<p><span class="sc">gdb</span> already contains a frame decorator called
<code>FrameDecorator</code>. This contains substantial amounts of
boilerplate code to decorate the content of a <code>gdb.Frame</code>. It is
recommended that other frame decorators inherit and extend this
object, and only to override the methods needed.
<p><a name="index-gdb_002eFrameDecorator-2024"></a><code>FrameDecorator</code> is defined in the Python module
<code>gdb.FrameDecorator</code>, so your code can import it like:
<pre class="smallexample"> from gdb.FrameDecorator import FrameDecorator
</pre>
<div class="defun">
&mdash; Function: <b>FrameDecorator.elided</b> (<var>self</var>)<var><a name="index-FrameDecorator_002eelided-2025"></a></var><br>
<blockquote>
<p>The <code>elided</code> method groups frames together in a hierarchical
system. An example would be an interpreter, where multiple low-level
frames make up a single call in the interpreted language. In this
example, the frame filter would elide the low-level frames and present
a single high-level frame, representing the call in the interpreted
language, to the user.
<p>The <code>elided</code> function must return an iterable and this iterable
must contain the frames that are being elided wrapped in a suitable
frame decorator. If no frames are being elided this function may
return an empty iterable, or <code>None</code>. Elided frames are indented
from normal frames in a <code>CLI</code> backtrace, or in the case of
<code>GDB/MI</code>, are placed in the <code>children</code> field of the eliding
frame.
<p>It is the frame filter's task to also filter out the elided frames from
the source iterator. This will avoid printing the frame twice.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>FrameDecorator.function</b> (<var>self</var>)<var><a name="index-FrameDecorator_002efunction-2026"></a></var><br>
<blockquote>
<p>This method returns the name of the function in the frame that is to
be printed.
<p>This method must return a Python string describing the function, or
<code>None</code>.
<p>If this function returns <code>None</code>, <span class="sc">gdb</span> will not print any
data for this field.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>FrameDecorator.address</b> (<var>self</var>)<var><a name="index-FrameDecorator_002eaddress-2027"></a></var><br>
<blockquote>
<p>This method returns the address of the frame that is to be printed.
<p>This method must return a Python numeric integer type of sufficient
size to describe the address of the frame, or <code>None</code>.
<p>If this function returns a <code>None</code>, <span class="sc">gdb</span> will not print
any data for this field.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>FrameDecorator.filename</b> (<var>self</var>)<var><a name="index-FrameDecorator_002efilename-2028"></a></var><br>
<blockquote>
<p>This method returns the filename and path associated with this frame.
<p>This method must return a Python string containing the filename and
the path to the object file backing the frame, or <code>None</code>.
<p>If this function returns a <code>None</code>, <span class="sc">gdb</span> will not print
any data for this field.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>FrameDecorator.line</b> (<var>self</var>)<var>:<a name="index-FrameDecorator_002eline-2029"></a></var><br>
<blockquote>
<p>This method returns the line number associated with the current
position within the function addressed by this frame.
<p>This method must return a Python integer type, or <code>None</code>.
<p>If this function returns a <code>None</code>, <span class="sc">gdb</span> will not print
any data for this field.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>FrameDecorator.frame_args</b> (<var>self</var>)<var><a name="index-FrameDecorator_002eframe_005fargs-2030"></a></var><br>
<blockquote><p><a name="frame_005fargs"></a>This method must return an iterable, or <code>None</code>. Returning an
empty iterable, or <code>None</code> means frame arguments will not be
printed for this frame. This iterable must contain objects that
implement two methods, described here.
<p>This object must implement a <code>argument</code> method which takes a
single <code>self</code> parameter and must return a <code>gdb.Symbol</code>
(see <a href="Symbols-In-Python.html#Symbols-In-Python">Symbols In Python</a>), or a Python string. The object must also
implement a <code>value</code> method which takes a single <code>self</code>
parameter and must return a <code>gdb.Value</code> (see <a href="Values-From-Inferior.html#Values-From-Inferior">Values From Inferior</a>), a Python value, or <code>None</code>. If the <code>value</code>
method returns <code>None</code>, and the <code>argument</code> method returns a
<code>gdb.Symbol</code>, <span class="sc">gdb</span> will look-up and print the value of
the <code>gdb.Symbol</code> automatically.
<p>A brief example:
<pre class="smallexample"> class SymValueWrapper():
def __init__(self, symbol, value):
self.sym = symbol
self.val = value
def value(self):
return self.val
def symbol(self):
return self.sym
class SomeFrameDecorator()
...
...
def frame_args(self):
args = []
try:
block = self.inferior_frame.block()
except:
return None
# Iterate over all symbols in a block. Only add
# symbols that are arguments.
for sym in block:
if not sym.is_argument:
continue
args.append(SymValueWrapper(sym,None))
# Add example synthetic argument.
args.append(SymValueWrapper(``foo'', 42))
return args
</pre>
</blockquote></div>
<div class="defun">
&mdash; Function: <b>FrameDecorator.frame_locals</b> (<var>self</var>)<var><a name="index-FrameDecorator_002eframe_005flocals-2031"></a></var><br>
<blockquote>
<p>This method must return an iterable or <code>None</code>. Returning an
empty iterable, or <code>None</code> means frame local arguments will not be
printed for this frame.
<p>The object interface, the description of the various strategies for
reading frame locals, and the example are largely similar to those
described in the <code>frame_args</code> function, (see <a href="frame_005fargs.html#frame_005fargs">The frame filter frame_args function</a>). Below is a modified example:
<pre class="smallexample"> class SomeFrameDecorator()
...
...
def frame_locals(self):
vars = []
try:
block = self.inferior_frame.block()
except:
return None
# Iterate over all symbols in a block. Add all
# symbols, except arguments.
for sym in block:
if sym.is_argument:
continue
vars.append(SymValueWrapper(sym,None))
# Add an example of a synthetic local variable.
vars.append(SymValueWrapper(``bar'', 99))
return vars
</pre>
</blockquote></div>
<div class="defun">
&mdash; Function: <b>FrameDecorator.inferior_frame</b> (<var>self</var>)<var>:<a name="index-FrameDecorator_002einferior_005fframe-2032"></a></var><br>
<blockquote>
<p>This method must return the underlying <code>gdb.Frame</code> that this
frame decorator is decorating. <span class="sc">gdb</span> requires the underlying
frame for internal frame information to determine how to print certain
values when printing a frame.
</p></blockquote></div>
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