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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<char,.*>$") |
| 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=<" + str(self.val["a"]) + |
| "> b=<" + str(self.val["b"]) + ">") |
| |
| class barPrinter: |
| """Print a bar object.""" |
| |
| def __init__(self, val): |
| self.val = val |
| |
| def to_string(self): |
| return ("x=<" + str(self.val["x"]) + |
| "> y=<" + str(self.val["y"]) + ">") |
| </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 ‘<samp><span class="samp">info pretty-printer</span></samp>’: |
| |
| <pre class="smallexample"> (gdb) info pretty-printer |
| my_library.so: |
| my_library |
| foo |
| bar |
| </pre> |
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