gdb/python/py-prettyprint.c

   1 /* Python pretty-printing
   2
   3    Copyright (C) 2008-2019 Free Software Foundation, Inc.
   4
   5    This file is part of GDB.
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20 #include "defs.h"
  21 #include "objfiles.h"
  22 #include "symtab.h"
  23 #include "language.h"
  24 #include "valprint.h"
  25 #include "extension-priv.h"
  26 #include "python.h"
  27 #include "python-internal.h"
  28 #include "py-ref.h"
  29
  30 /* Return type of print_string_repr.  */
  31
  32 enum string_repr_result
  33   {
  34     /* The string method returned None.  */
  35     string_repr_none,
  36     /* The string method had an error.  */
  37     string_repr_error,
  38     /* Everything ok.  */
  39     string_repr_ok
  40   };
  41
  42 /* Helper function for find_pretty_printer which iterates over a list,
  43    calls each function and inspects output.  This will return a
  44    printer object if one recognizes VALUE.  If no printer is found, it
  45    will return None.  On error, it will set the Python error and
  46    return NULL.  */
  47
  48 static gdbpy_ref<>
  49 search_pp_list (PyObject *list, PyObject *value)
  50 {
  51   Py_ssize_t pp_list_size, list_index;
  52
  53   pp_list_size = PyList_Size (list);
  54   for (list_index = 0; list_index < pp_list_size; list_index++)
  55     {
  56       PyObject *function = PyList_GetItem (list, list_index);
  57       if (! function)
  58         return NULL;
  59
  60       /* Skip if disabled.  */
  61       if (PyObject_HasAttr (function, gdbpy_enabled_cst))
  62         {
  63           gdbpy_ref<> attr (PyObject_GetAttr (function, gdbpy_enabled_cst));
  64           int cmp;
  65
  66           if (attr == NULL)
  67             return NULL;
  68           cmp = PyObject_IsTrue (attr.get ());
  69           if (cmp == -1)
  70             return NULL;
  71
  72           if (!cmp)
  73             continue;
  74         }
  75
  76       gdbpy_ref<> printer (PyObject_CallFunctionObjArgs (function, value,
  77                                                          NULL));
  78       if (printer == NULL)
  79         return NULL;
  80       else if (printer != Py_None)
  81         return printer;
  82     }
  83
  84   return gdbpy_ref<>::new_reference (Py_None);
  85 }
  86
  87 /* Subroutine of find_pretty_printer to simplify it.
  88    Look for a pretty-printer to print VALUE in all objfiles.
  89    The result is NULL if there's an error and the search should be terminated.
  90    The result is Py_None, suitably inc-ref'd, if no pretty-printer was found.
  91    Otherwise the result is the pretty-printer function, suitably inc-ref'd.  */
  92
  93 static PyObject *
  94 find_pretty_printer_from_objfiles (PyObject *value)
  95 {
  96   for (objfile *obj : all_objfiles (current_program_space))
  97     {
  98       gdbpy_ref<> objf = objfile_to_objfile_object (obj);
  99       if (objf == NULL)
 100         {
 101           /* Ignore the error and continue.  */
 102           PyErr_Clear ();
 103           continue;
 104         }
 105
 106       gdbpy_ref<> pp_list (objfpy_get_printers (objf.get (), NULL));
 107       gdbpy_ref<> function (search_pp_list (pp_list.get (), value));
 108
 109       /* If there is an error in any objfile list, abort the search and exit.  */
 110       if (function == NULL)
 111         return NULL;
 112
 113       if (function != Py_None)
 114         return function.release ();
 115     }
 116
 117   Py_RETURN_NONE;
 118 }
 119
 120 /* Subroutine of find_pretty_printer to simplify it.
 121    Look for a pretty-printer to print VALUE in the current program space.
 122    The result is NULL if there's an error and the search should be terminated.
 123    The result is Py_None, suitably inc-ref'd, if no pretty-printer was found.
 124    Otherwise the result is the pretty-printer function, suitably inc-ref'd.  */
 125
 126 static gdbpy_ref<>
 127 find_pretty_printer_from_progspace (PyObject *value)
 128 {
 129   gdbpy_ref<> obj = pspace_to_pspace_object (current_program_space);
 130
 131   if (obj == NULL)
 132     return NULL;
 133   gdbpy_ref<> pp_list (pspy_get_printers (obj.get (), NULL));
 134   return search_pp_list (pp_list.get (), value);
 135 }
 136
 137 /* Subroutine of find_pretty_printer to simplify it.
 138    Look for a pretty-printer to print VALUE in the gdb module.
 139    The result is NULL if there's an error and the search should be terminated.
 140    The result is Py_None, suitably inc-ref'd, if no pretty-printer was found.
 141    Otherwise the result is the pretty-printer function, suitably inc-ref'd.  */
 142
 143 static gdbpy_ref<>
 144 find_pretty_printer_from_gdb (PyObject *value)
 145 {
 146   /* Fetch the global pretty printer list.  */
 147   if (gdb_python_module == NULL
 148       || ! PyObject_HasAttrString (gdb_python_module, "pretty_printers"))
 149     return gdbpy_ref<>::new_reference (Py_None);
 150   gdbpy_ref<> pp_list (PyObject_GetAttrString (gdb_python_module,
 151                                                "pretty_printers"));
 152   if (pp_list == NULL || ! PyList_Check (pp_list.get ()))
 153     return gdbpy_ref<>::new_reference (Py_None);
 154
 155   return search_pp_list (pp_list.get (), value);
 156 }
 157
 158 /* Find the pretty-printing constructor function for VALUE.  If no
 159    pretty-printer exists, return None.  If one exists, return a new
 160    reference.  On error, set the Python error and return NULL.  */
 161
 162 static gdbpy_ref<>
 163 find_pretty_printer (PyObject *value)
 164 {
 165   /* Look at the pretty-printer list for each objfile
 166      in the current program-space.  */
 167   gdbpy_ref<> function (find_pretty_printer_from_objfiles (value));
 168   if (function == NULL || function != Py_None)
 169     return function;
 170
 171   /* Look at the pretty-printer list for the current program-space.  */
 172   function = find_pretty_printer_from_progspace (value);
 173   if (function == NULL || function != Py_None)
 174     return function;
 175
 176   /* Look at the pretty-printer list in the gdb module.  */
 177   return find_pretty_printer_from_gdb (value);
 178 }
 179
 180 /* Pretty-print a single value, via the printer object PRINTER.
 181    If the function returns a string, a PyObject containing the string
 182    is returned.  If the function returns Py_NONE that means the pretty
 183    printer returned the Python None as a value.  Otherwise, if the
 184    function returns a value,  *OUT_VALUE is set to the value, and NULL
 185    is returned.  On error, *OUT_VALUE is set to NULL, NULL is
 186    returned, with a python exception set.  */
 187
 188 static gdbpy_ref<>
 189 pretty_print_one_value (PyObject *printer, struct value **out_value)
 190 {
 191   gdbpy_ref<> result;
 192
 193   *out_value = NULL;
 194   TRY
 195     {
 196       if (!PyObject_HasAttr (printer, gdbpy_to_string_cst))
 197         result = gdbpy_ref<>::new_reference (Py_None);
 198       else
 199         {
 200           result.reset (PyObject_CallMethodObjArgs (printer, gdbpy_to_string_cst,
 201                                                     NULL));
 202           if (result != NULL)
 203             {
 204               if (! gdbpy_is_string (result.get ())
 205                   && ! gdbpy_is_lazy_string (result.get ())
 206                   && result != Py_None)
 207                 {
 208                   *out_value = convert_value_from_python (result.get ());
 209                   if (PyErr_Occurred ())
 210                     *out_value = NULL;
 211                   result = NULL;
 212                 }
 213             }
 214         }
 215     }
 216   CATCH (except, RETURN_MASK_ALL)
 217     {
 218     }
 219   END_CATCH
 220
 221   return result;
 222 }
 223
 224 /* Return the display hint for the object printer, PRINTER.  Return
 225    NULL if there is no display_hint method, or if the method did not
 226    return a string.  On error, print stack trace and return NULL.  On
 227    success, return an xmalloc()d string.  */
 228 gdb::unique_xmalloc_ptr<char>
 229 gdbpy_get_display_hint (PyObject *printer)
 230 {
 231   gdb::unique_xmalloc_ptr<char> result;
 232
 233   if (! PyObject_HasAttr (printer, gdbpy_display_hint_cst))
 234     return NULL;
 235
 236   gdbpy_ref<> hint (PyObject_CallMethodObjArgs (printer, gdbpy_display_hint_cst,
 237                                                 NULL));
 238   if (hint != NULL)
 239     {
 240       if (gdbpy_is_string (hint.get ()))
 241         {
 242           result = python_string_to_host_string (hint.get ());
 243           if (result == NULL)
 244             gdbpy_print_stack ();
 245         }
 246     }
 247   else
 248     gdbpy_print_stack ();
 249
 250   return result;
 251 }
 252
 253 /* A wrapper for gdbpy_print_stack that ignores MemoryError.  */
 254
 255 static void
 256 print_stack_unless_memory_error (struct ui_file *stream)
 257 {
 258   if (PyErr_ExceptionMatches (gdbpy_gdb_memory_error))
 259     {
 260       gdbpy_err_fetch fetched_error;
 261       gdb::unique_xmalloc_ptr<char> msg = fetched_error.to_string ();
 262
 263       if (msg == NULL || *msg == '\0')
 264         fprintf_filtered (stream, _("<error reading variable>"));
 265       else
 266         fprintf_filtered (stream, _("<error reading variable: %s>"),
 267                           msg.get ());
 268     }
 269   else
 270     gdbpy_print_stack ();
 271 }
 272
 273 /* Helper for gdbpy_apply_val_pretty_printer which calls to_string and
 274    formats the result.  */
 275
 276 static enum string_repr_result
 277 print_string_repr (PyObject *printer, const char *hint,
 278                    struct ui_file *stream, int recurse,
 279                    const struct value_print_options *options,
 280                    const struct language_defn *language,
 281                    struct gdbarch *gdbarch)
 282 {
 283   struct value *replacement = NULL;
 284   enum string_repr_result result = string_repr_ok;
 285
 286   gdbpy_ref<> py_str = pretty_print_one_value (printer, &replacement);
 287   if (py_str != NULL)
 288     {
 289       if (py_str == Py_None)
 290         result = string_repr_none;
 291       else if (gdbpy_is_lazy_string (py_str.get ()))
 292         {
 293           CORE_ADDR addr;
 294           long length;
 295           struct type *type;
 296           gdb::unique_xmalloc_ptr<char> encoding;
 297           struct value_print_options local_opts = *options;
 298
 299           gdbpy_extract_lazy_string (py_str.get (), &addr, &type,
 300                                      &length, &encoding);
 301
 302           local_opts.addressprint = 0;
 303           val_print_string (type, encoding.get (), addr, (int) length,
 304                             stream, &local_opts);
 305         }
 306       else
 307         {
 308           gdbpy_ref<> string
 309             = python_string_to_target_python_string (py_str.get ());
 310           if (string != NULL)
 311             {
 312               char *output;
 313               long length;
 314               struct type *type;
 315
 316 #ifdef IS_PY3K
 317               output = PyBytes_AS_STRING (string.get ());
 318               length = PyBytes_GET_SIZE (string.get ());
 319 #else
 320               output = PyString_AsString (string.get ());
 321               length = PyString_Size (string.get ());
 322 #endif
 323               type = builtin_type (gdbarch)->builtin_char;
 324
 325               if (hint && !strcmp (hint, "string"))
 326                 LA_PRINT_STRING (stream, type, (gdb_byte *) output,
 327                                  length, NULL, 0, options);
 328               else
 329                 fputs_filtered (output, stream);
 330             }
 331           else
 332             {
 333               result = string_repr_error;
 334               print_stack_unless_memory_error (stream);
 335             }
 336         }
 337     }
 338   else if (replacement)
 339     {
 340       struct value_print_options opts = *options;
 341
 342       opts.addressprint = 0;
 343       common_val_print (replacement, stream, recurse, &opts, language);
 344     }
 345   else
 346     {
 347       result = string_repr_error;
 348       print_stack_unless_memory_error (stream);
 349     }
 350
 351   return result;
 352 }
 353
 354 #ifndef IS_PY3K
 355
 356 /* Create a dummy PyFrameObject, needed to work around
 357    a Python-2.4 bug with generators.  */
 358 class dummy_python_frame
 359 {
 360  public:
 361
 362   dummy_python_frame ();
 363
 364   ~dummy_python_frame ()
 365   {
 366     if (m_valid)
 367       m_tstate->frame = m_saved_frame;
 368   }
 369
 370   bool failed () const
 371   {
 372     return !m_valid;
 373   }
 374
 375  private:
 376
 377   bool m_valid;
 378   PyFrameObject *m_saved_frame;
 379   gdbpy_ref<> m_frame;
 380   PyThreadState *m_tstate;
 381 };
 382
 383 dummy_python_frame::dummy_python_frame ()
 384 : m_valid (false),
 385   m_saved_frame (NULL),
 386   m_tstate (NULL)
 387 {
 388   PyCodeObject *code;
 389   PyFrameObject *frame;
 390
 391   gdbpy_ref<> empty_string (PyString_FromString (""));
 392   if (empty_string == NULL)
 393     return;
 394
 395   gdbpy_ref<> null_tuple (PyTuple_New (0));
 396   if (null_tuple == NULL)
 397     return;
 398
 399   code = PyCode_New (0,                   /* argcount */
 400                      0,                   /* locals */
 401                      0,                   /* stacksize */
 402                      0,                   /* flags */
 403                      empty_string.get (), /* code */
 404                      null_tuple.get (),   /* consts */
 405                      null_tuple.get (),   /* names */
 406                      null_tuple.get (),   /* varnames */
 407 #if PYTHON_API_VERSION >= 1010
 408                      null_tuple.get (),   /* freevars */
 409                      null_tuple.get (),   /* cellvars */
 410 #endif
 411                      empty_string.get (), /* filename */
 412                      empty_string.get (), /* name */
 413                      1,                   /* firstlineno */
 414                      empty_string.get ()  /* lnotab */
 415                      );
 416   if (code == NULL)
 417     return;
 418   gdbpy_ref<> code_holder ((PyObject *) code);
 419
 420   gdbpy_ref<> globals (PyDict_New ());
 421   if (globals == NULL)
 422     return;
 423
 424   m_tstate = PyThreadState_GET ();
 425   frame = PyFrame_New (m_tstate, code, globals.get (), NULL);
 426   if (frame == NULL)
 427     return;
 428
 429   m_frame.reset ((PyObject *) frame);
 430   m_tstate->frame = frame;
 431   m_saved_frame = frame->f_back;
 432   m_valid = true;
 433 }
 434 #endif
 435
 436 /* Helper for gdbpy_apply_val_pretty_printer that formats children of the
 437    printer, if any exist.  If is_py_none is true, then nothing has
 438    been printed by to_string, and format output accordingly. */
 439 static void
 440 print_children (PyObject *printer, const char *hint,
 441                 struct ui_file *stream, int recurse,
 442                 const struct value_print_options *options,
 443                 const struct language_defn *language,
 444                 int is_py_none)
 445 {
 446   int is_map, is_array, done_flag, pretty;
 447   unsigned int i;
 448
 449   if (! PyObject_HasAttr (printer, gdbpy_children_cst))
 450     return;
 451
 452   /* If we are printing a map or an array, we want some special
 453      formatting.  */
 454   is_map = hint && ! strcmp (hint, "map");
 455   is_array = hint && ! strcmp (hint, "array");
 456
 457   gdbpy_ref<> children (PyObject_CallMethodObjArgs (printer, gdbpy_children_cst,
 458                                                     NULL));
 459   if (children == NULL)
 460     {
 461       print_stack_unless_memory_error (stream);
 462       return;
 463     }
 464
 465   gdbpy_ref<> iter (PyObject_GetIter (children.get ()));
 466   if (iter == NULL)
 467     {
 468       print_stack_unless_memory_error (stream);
 469       return;
 470     }
 471
 472   /* Use the prettyformat_arrays option if we are printing an array,
 473      and the pretty option otherwise.  */
 474   if (is_array)
 475     pretty = options->prettyformat_arrays;
 476   else
 477     {
 478       if (options->prettyformat == Val_prettyformat)
 479         pretty = 1;
 480       else
 481         pretty = options->prettyformat_structs;
 482     }
 483
 484   /* Manufacture a dummy Python frame to work around Python 2.4 bug,
 485      where it insists on having a non-NULL tstate->frame when
 486      a generator is called.  */
 487 #ifndef IS_PY3K
 488   dummy_python_frame frame;
 489   if (frame.failed ())
 490     {
 491       gdbpy_print_stack ();
 492       return;
 493     }
 494 #endif
 495
 496   done_flag = 0;
 497   for (i = 0; i < options->print_max; ++i)
 498     {
 499       PyObject *py_v;
 500       const char *name;
 501
 502       gdbpy_ref<> item (PyIter_Next (iter.get ()));
 503       if (item == NULL)
 504         {
 505           if (PyErr_Occurred ())
 506             print_stack_unless_memory_error (stream);
 507           /* Set a flag so we can know whether we printed all the
 508              available elements.  */
 509           else
 510             done_flag = 1;
 511           break;
 512         }
 513
 514       if (! PyTuple_Check (item.get ()) || PyTuple_Size (item.get ()) != 2)
 515         {
 516           PyErr_SetString (PyExc_TypeError,
 517                            _("Result of children iterator not a tuple"
 518                              " of two elements."));
 519           gdbpy_print_stack ();
 520           continue;
 521         }
 522       if (! PyArg_ParseTuple (item.get (), "sO", &name, &py_v))
 523         {
 524           /* The user won't necessarily get a stack trace here, so provide
 525              more context.  */
 526           if (gdbpy_print_python_errors_p ())
 527             fprintf_unfiltered (gdb_stderr,
 528                                 _("Bad result from children iterator.\n"));
 529           gdbpy_print_stack ();
 530           continue;
 531         }
 532
 533       /* Print initial "{".  For other elements, there are three
 534          cases:
 535          1. Maps.  Print a "," after each value element.
 536          2. Arrays.  Always print a ",".
 537          3. Other.  Always print a ",".  */
 538       if (i == 0)
 539         {
 540          if (is_py_none)
 541            fputs_filtered ("{", stream);
 542          else
 543            fputs_filtered (" = {", stream);
 544        }
 545
 546       else if (! is_map || i % 2 == 0)
 547         fputs_filtered (pretty ? "," : ", ", stream);
 548
 549       /* In summary mode, we just want to print "= {...}" if there is
 550          a value.  */
 551       if (options->summary)
 552         {
 553           /* This increment tricks the post-loop logic to print what
 554              we want.  */
 555           ++i;
 556           /* Likewise.  */
 557           pretty = 0;
 558           break;
 559         }
 560
 561       if (! is_map || i % 2 == 0)
 562         {
 563           if (pretty)
 564             {
 565               fputs_filtered ("\n", stream);
 566               print_spaces_filtered (2 + 2 * recurse, stream);
 567             }
 568           else
 569             wrap_here (n_spaces (2 + 2 *recurse));
 570         }
 571
 572       if (is_map && i % 2 == 0)
 573         fputs_filtered ("[", stream);
 574       else if (is_array)
 575         {
 576           /* We print the index, not whatever the child method
 577              returned as the name.  */
 578           if (options->print_array_indexes)
 579             fprintf_filtered (stream, "[%d] = ", i);
 580         }
 581       else if (! is_map)
 582         {
 583           fputs_filtered (name, stream);
 584           fputs_filtered (" = ", stream);
 585         }
 586
 587       if (gdbpy_is_lazy_string (py_v))
 588         {
 589           CORE_ADDR addr;
 590           struct type *type;
 591           long length;
 592           gdb::unique_xmalloc_ptr<char> encoding;
 593           struct value_print_options local_opts = *options;
 594
 595           gdbpy_extract_lazy_string (py_v, &addr, &type, &length, &encoding);
 596
 597           local_opts.addressprint = 0;
 598           val_print_string (type, encoding.get (), addr, (int) length, stream,
 599                             &local_opts);
 600         }
 601       else if (gdbpy_is_string (py_v))
 602         {
 603           gdb::unique_xmalloc_ptr<char> output;
 604
 605           output = python_string_to_host_string (py_v);
 606           if (!output)
 607             gdbpy_print_stack ();
 608           else
 609             fputs_filtered (output.get (), stream);
 610         }
 611       else
 612         {
 613           struct value *value = convert_value_from_python (py_v);
 614
 615           if (value == NULL)
 616             {
 617               gdbpy_print_stack ();
 618               error (_("Error while executing Python code."));
 619             }
 620           else
 621             common_val_print (value, stream, recurse + 1, options, language);
 622         }
 623
 624       if (is_map && i % 2 == 0)
 625         fputs_filtered ("] = ", stream);
 626     }
 627
 628   if (i)
 629     {
 630       if (!done_flag)
 631         {
 632           if (pretty)
 633             {
 634               fputs_filtered ("\n", stream);
 635               print_spaces_filtered (2 + 2 * recurse, stream);
 636             }
 637           fputs_filtered ("...", stream);
 638         }
 639       if (pretty)
 640         {
 641           fputs_filtered ("\n", stream);
 642           print_spaces_filtered (2 * recurse, stream);
 643         }
 644       fputs_filtered ("}", stream);
 645     }
 646 }
 647
 648 enum ext_lang_rc
 649 gdbpy_apply_val_pretty_printer (const struct extension_language_defn *extlang,
 650                                 struct type *type,
 651                                 LONGEST embedded_offset, CORE_ADDR address,
 652                                 struct ui_file *stream, int recurse,
 653                                 struct value *val,
 654                                 const struct value_print_options *options,
 655                                 const struct language_defn *language)
 656 {
 657   struct gdbarch *gdbarch = get_type_arch (type);
 658   struct value *value;
 659   enum string_repr_result print_result;
 660
 661   if (value_lazy (val))
 662     value_fetch_lazy (val);
 663
 664   /* No pretty-printer support for unavailable values.  */
 665   if (!value_bytes_available (val, embedded_offset, TYPE_LENGTH (type)))
 666     return EXT_LANG_RC_NOP;
 667
 668   if (!gdb_python_initialized)
 669     return EXT_LANG_RC_NOP;
 670
 671   gdbpy_enter enter_py (gdbarch, language);
 672
 673   /* Instantiate the printer.  */
 674   value = value_from_component (val, type, embedded_offset);
 675
 676   gdbpy_ref<> val_obj (value_to_value_object (value));
 677   if (val_obj == NULL)
 678     {
 679       print_stack_unless_memory_error (stream);
 680       return EXT_LANG_RC_ERROR;
 681     }
 682
 683   /* Find the constructor.  */
 684   gdbpy_ref<> printer (find_pretty_printer (val_obj.get ()));
 685   if (printer == NULL)
 686     {
 687       print_stack_unless_memory_error (stream);
 688       return EXT_LANG_RC_ERROR;
 689     }
 690
 691   if (printer == Py_None)
 692     return EXT_LANG_RC_NOP;
 693
 694   /* If we are printing a map, we want some special formatting.  */
 695   gdb::unique_xmalloc_ptr<char> hint (gdbpy_get_display_hint (printer.get ()));
 696
 697   /* Print the section */
 698   print_result = print_string_repr (printer.get (), hint.get (), stream,
 699                                     recurse, options, language, gdbarch);
 700   if (print_result != string_repr_error)
 701     print_children (printer.get (), hint.get (), stream, recurse, options,
 702                     language, print_result == string_repr_none);
 703
 704   if (PyErr_Occurred ())
 705     print_stack_unless_memory_error (stream);
 706   return EXT_LANG_RC_OK;
 707 }
 708
 709
 710 /* Apply a pretty-printer for the varobj code.  PRINTER_OBJ is the
 711    print object.  It must have a 'to_string' method (but this is
 712    checked by varobj, not here) which takes no arguments and
 713    returns a string.  The printer will return a value and in the case
 714    of a Python string being returned, this function will return a
 715    PyObject containing the string.  For any other type, *REPLACEMENT is
 716    set to the replacement value and this function returns NULL.  On
 717    error, *REPLACEMENT is set to NULL and this function also returns
 718    NULL.  */
 719 gdbpy_ref<>
 720 apply_varobj_pretty_printer (PyObject *printer_obj,
 721                              struct value **replacement,
 722                              struct ui_file *stream)
 723 {
 724   *replacement = NULL;
 725   gdbpy_ref<> py_str = pretty_print_one_value (printer_obj, replacement);
 726
 727   if (*replacement == NULL && py_str == NULL)
 728     print_stack_unless_memory_error (stream);
 729
 730   return py_str;
 731 }
 732
 733 /* Find a pretty-printer object for the varobj module.  Returns a new
 734    reference to the object if successful; returns NULL if not.  VALUE
 735    is the value for which a printer tests to determine if it
 736    can pretty-print the value.  */
 737 gdbpy_ref<>
 738 gdbpy_get_varobj_pretty_printer (struct value *value)
 739 {
 740   TRY
 741     {
 742       value = value_copy (value);
 743     }
 744   CATCH (except, RETURN_MASK_ALL)
 745     {
 746       GDB_PY_HANDLE_EXCEPTION (except);
 747     }
 748   END_CATCH
 749
 750   gdbpy_ref<> val_obj (value_to_value_object (value));
 751   if (val_obj == NULL)
 752     return NULL;
 753
 754   return find_pretty_printer (val_obj.get ());
 755 }
 756
 757 /* A Python function which wraps find_pretty_printer and instantiates
 758    the resulting class.  This accepts a Value argument and returns a
 759    pretty printer instance, or None.  This function is useful as an
 760    argument to the MI command -var-set-visualizer.  */
 761 PyObject *
 762 gdbpy_default_visualizer (PyObject *self, PyObject *args)
 763 {
 764   PyObject *val_obj;
 765   struct value *value;
 766
 767   if (! PyArg_ParseTuple (args, "O", &val_obj))
 768     return NULL;
 769   value = value_object_to_value (val_obj);
 770   if (! value)
 771     {
 772       PyErr_SetString (PyExc_TypeError,
 773                        _("Argument must be a gdb.Value."));
 774       return NULL;
 775     }
 776
 777   return find_pretty_printer (val_obj).release ();
 778 }