qapi/error: Make QAPISourceError 'col' parameter optional

It's already treated as optional, with one direct caller and some
subclass callers passing 'None'. Make it officially optional, which
requires moving the position of the argument to come after all required
parameters.

QAPISemError becomes functionally identical to QAPISourceError. Keep the
name to preserve its semantic meaning and avoid code churn, but remove
the now-useless __init__ wrapper.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210421192233.3542904-4-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/error.py

@@ -18,11 +18,11 @@ class QAPIError(Exception):
 
 class QAPISourceError(QAPIError):
     """Error class for all exceptions identifying a source location."""
-    def __init__(self, info, col, msg):
+    def __init__(self, info, msg, col=None):
         super().__init__()
         self.info = info
-        self.col = col
         self.msg = msg
+        self.col = col
 
     def __str__(self):
         loc = str(self.info)
@@ -41,10 +41,8 @@ def __init__(self, parser, msg):
                 col = (col + 7) % 8 + 1
             else:
                 col += 1
-        super().__init__(parser.info, col, msg)
+        super().__init__(parser.info, msg, col)
 
 
 class QAPISemError(QAPISourceError):
     """Error class for semantic QAPI errors."""
-    def __init__(self, info, msg):
-        super().__init__(info, None, msg)

scripts/qapi/schema.py

@@ -875,8 +875,7 @@ def _def_entity(self, ent):
         other_ent = self._entity_dict.get(ent.name)
         if other_ent:
             if other_ent.info:
-                where = QAPISourceError(other_ent.info, None,
-                                        "previous definition")
+                where = QAPISourceError(other_ent.info, "previous definition")
                 raise QAPISemError(
                     ent.info,
                     "'%s' is already defined\n%s" % (ent.name, where))