give reg_name, insn_name, group_name a customizable default instead of returning something else for id 0

remove the check for self._raw.id on reg_name and group_name (in CsInsn, since the to_name functions don't operate on the current instruction)
Add reg_name, insn_name and group_name to Cs.
update test_group_name.py with the new api.

bindings/python/capstone/__init__.py

@@ -435,6 +435,8 @@ def copy_ctypes(src):
     ctypes.pointer(dst)[0] = src
     return dst
 
+def _ascii_name_or_default(name, default):
+    return default if name is None else name.decode('ascii')
 
 # Python-style class to disasm code
 class CsInsn(object):
@@ -573,43 +575,31 @@ class CsInsn(object):
         return _cs.cs_errno(self._cs.csh)
 
     # get the register name, given the register ID
-    def reg_name(self, reg_id):
-        if self._raw.id == 0:
-            raise CsError(CS_ERR_SKIPDATA)
-
+    def reg_name(self, reg_id, default=None):
         if self._cs._diet:
             # Diet engine cannot provide register name
             raise CsError(CS_ERR_DIET)
 
-        if reg_id == 0:
-            return "(invalid)"
-
-        return _cs.cs_reg_name(self._cs.csh, reg_id).decode('ascii')
+        return _ascii_name_or_default(_cs.cs_reg_name(self._cs.csh, reg_id), default)
 
     # get the instruction name
-    def insn_name(self):
+    def insn_name(self, default=None):
         if self._cs._diet:
             # Diet engine cannot provide instruction name
             raise CsError(CS_ERR_DIET)
 
         if self._raw.id == 0:
-            return "(invalid)"
+            return default
 
-        return _cs.cs_insn_name(self._cs.csh, self.id).decode('ascii')
+        return _ascii_name_or_default(_cs.cs_insn_name(self._cs.csh, self.id).decode('ascii'), default)
 
     # get the group name
-    def group_name(self, group_id):
-        if self._raw.id == 0:
-            raise CsError(CS_ERR_SKIPDATA)
-
+    def group_name(self, group_id, default=None):
         if self._cs._diet:
             # Diet engine cannot provide group name
             raise CsError(CS_ERR_DIET)
 
-        if group_id == 0:
-            return "(invalid)"
-
-        return _cs.cs_group_name(self._cs.csh, group_id).decode('ascii')
+        return _ascii_name_or_default(_cs.cs_group_name(self._cs.csh, group_id), default)
 
 
     # verify if this insn belong to group with id as @group_id
@@ -862,6 +852,33 @@ class Cs(object):
         # save mode
         self._mode = opt
 
+    # get the last error code
+    def errno(self):
+        return _cs.cs_errno(self.csh)
+
+    # get the register name, given the register ID
+    def reg_name(self, reg_id, default=None):
+        if self._diet:
+            # Diet engine cannot provide register name
+            raise CsError(CS_ERR_DIET)
+
+        return _ascii_name_or_default(_cs.cs_reg_name(self.csh, reg_id), default)
+
+    # get the instruction name, given the instruction ID
+    def insn_name(self, insn_id, default=None):
+        if self._diet:
+            # Diet engine cannot provide instruction name
+            raise CsError(CS_ERR_DIET)
+
+        return _ascii_name_or_default(_cs.cs_insn_name(self.csh, insn_id), default)
+
+    # get the group name
+    def group_name(self, group_id, default=None):
+        if self._diet:
+            # Diet engine cannot provide group name
+            raise CsError(CS_ERR_DIET)
+
+        return _ascii_name_or_default(_cs.cs_group_name(self.csh, group_id), default)
 
     # Disassemble binary & return disassembled instructions in CsInsn objects
     def disasm(self, code, offset, count=0):

suite/test_group_name.py

@@ -11,10 +11,6 @@ from capstone.x86 import *
 from capstone.xcore import *
 import sys
 
-# yes this is bad, importing ctypes like this,
-# but the Cs object did not have the group_name function
-from capstone import _cs
-
 class GroupTest:
     def __init__(self, name, arch, mode, data):
         self.name = name
@@ -27,7 +23,7 @@ class GroupTest:
         cap = Cs(self.arch, self.mode)
         for group_id in xrange(0,255):
             name = self.data.get(group_id)
-            res = _cs.cs_group_name(cap.csh, group_id)
+            res = cap.group_name(group_id)
             if res != name:
                 print("ERROR: id = %u expected '%s', but got '%s'" %(group_id, name, res))
         print("")