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<h5 class="subsubsection">23.2.2.7 Writing a Pretty-Printer</h5>

<p><a name="index-writing-a-pretty_002dprinter-2011"></a>
A pretty-printer consists of two parts: a lookup function to detect
if the type is supported, and the printer itself.

   <p>Here is an example showing how a <code>std::string</code> printer might be
written.  See <a href="Pretty-Printing-API.html#Pretty-Printing-API">Pretty Printing API</a>, for details on the API this class
must provide.

<pre class="smallexample">     class StdStringPrinter(object):
         "Print a std::string"
     
         def __init__(self, val):
             self.val = val
     
         def to_string(self):
             return self.val['_M_dataplus']['_M_p']
     
         def display_hint(self):
             return 'string'
</pre>
   <p>And here is an example showing how a lookup function for the printer
example above might be written.

<pre class="smallexample">     def str_lookup_function(val):
         lookup_tag = val.type.tag
         if lookup_tag == None:
             return None
         regex = re.compile("^std::basic_string&lt;char,.*&gt;$")
         if regex.match(lookup_tag):
             return StdStringPrinter(val)
         return None
</pre>
   <p>The example lookup function extracts the value's type, and attempts to
match it to a type that it can pretty-print.  If it is a type the
printer can pretty-print, it will return a printer object.  If not, it
returns <code>None</code>.

   <p>We recommend that you put your core pretty-printers into a Python
package.  If your pretty-printers are for use with a library, we
further recommend embedding a version number into the package name. 
This practice will enable <span class="sc">gdb</span> to load multiple versions of
your pretty-printers at the same time, because they will have
different names.

   <p>You should write auto-loaded code (see <a href="Python-Auto_002dloading.html#Python-Auto_002dloading">Python Auto-loading</a>) such that it
can be evaluated multiple times without changing its meaning.  An
ideal auto-load file will consist solely of <code>import</code>s of your
printer modules, followed by a call to a register pretty-printers with
the current objfile.

   <p>Taken as a whole, this approach will scale nicely to multiple
inferiors, each potentially using a different library version. 
Embedding a version number in the Python package name will ensure that
<span class="sc">gdb</span> is able to load both sets of printers simultaneously. 
Then, because the search for pretty-printers is done by objfile, and
because your auto-loaded code took care to register your library's
printers with a specific objfile, <span class="sc">gdb</span> will find the correct
printers for the specific version of the library used by each
inferior.

   <p>To continue the <code>std::string</code> example (see <a href="Pretty-Printing-API.html#Pretty-Printing-API">Pretty Printing API</a>),
this code might appear in <code>gdb.libstdcxx.v6</code>:

<pre class="smallexample">     def register_printers(objfile):
         objfile.pretty_printers.append(str_lookup_function)
</pre>
   <p class="noindent">And then the corresponding contents of the auto-load file would be:

<pre class="smallexample">     import gdb.libstdcxx.v6
     gdb.libstdcxx.v6.register_printers(gdb.current_objfile())
</pre>
   <p>The previous example illustrates a basic pretty-printer. 
There are a few things that can be improved on. 
The printer doesn't have a name, making it hard to identify in a
list of installed printers.  The lookup function has a name, but
lookup functions can have arbitrary, even identical, names.

   <p>Second, the printer only handles one type, whereas a library typically has
several types.  One could install a lookup function for each desired type
in the library, but one could also have a single lookup function recognize
several types.  The latter is the conventional way this is handled. 
If a pretty-printer can handle multiple data types, then its
<dfn>subprinters</dfn> are the printers for the individual data types.

   <p>The <code>gdb.printing</code> module provides a formal way of solving these
problems (see <a href="gdb_002eprinting.html#gdb_002eprinting">gdb.printing</a>). 
Here is another example that handles multiple types.

   <p>These are the types we are going to pretty-print:

<pre class="smallexample">     struct foo { int a, b; };
     struct bar { struct foo x, y; };
</pre>
   <p>Here are the printers:

<pre class="smallexample">     class fooPrinter:
         """Print a foo object."""
     
         def __init__(self, val):
             self.val = val
     
         def to_string(self):
             return ("a=&lt;" + str(self.val["a"]) +
                     "&gt; b=&lt;" + str(self.val["b"]) + "&gt;")
     
     class barPrinter:
         """Print a bar object."""
     
         def __init__(self, val):
             self.val = val
     
         def to_string(self):
             return ("x=&lt;" + str(self.val["x"]) +
                     "&gt; y=&lt;" + str(self.val["y"]) + "&gt;")
</pre>
   <p>This example doesn't need a lookup function, that is handled by the
<code>gdb.printing</code> module.  Instead a function is provided to build up
the object that handles the lookup.

<pre class="smallexample">     import gdb.printing
     
     def build_pretty_printer():
         pp = gdb.printing.RegexpCollectionPrettyPrinter(
             "my_library")
         pp.add_printer('foo', '^foo$', fooPrinter)
         pp.add_printer('bar', '^bar$', barPrinter)
         return pp
</pre>
   <p>And here is the autoload support:

<pre class="smallexample">     import gdb.printing
     import my_library
     gdb.printing.register_pretty_printer(
         gdb.current_objfile(),
         my_library.build_pretty_printer())
</pre>
   <p>Finally, when this printer is loaded into <span class="sc">gdb</span>, here is the
corresponding output of &lsquo;<samp><span class="samp">info pretty-printer</span></samp>&rsquo;:

<pre class="smallexample">     (gdb) info pretty-printer
     my_library.so:
       my_library
         foo
         bar
</pre>
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