qapi/parser: Rework _check_pragma_list_of_str as a TypeGuard

TypeGuards wont exist in Python proper until 3.10. Ah well. We can hack
up our own by declaring this function to return the type we claim it
checks for and using this to safely downcast object -> List[str].

In so doing, I bring this function under _pragma so it can use the
'info' object in its closure. Having done this, _pragma also now no
longer needs to take a 'self' parameter, so drop it.

To help with line-length, and with the context evident from its new
scope, rename the function to the shorter check_list_str().

Signed-off-by: John Snow <jsnow@redhat.com>

Message-Id: <20210519183951.3946870-12-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/parser.py

@@ -17,6 +17,7 @@
 from collections import OrderedDict
 import os
 import re
+from typing import List
 
 from .common import must_match
 from .error import QAPISemError, QAPISourceError
@@ -154,28 +155,29 @@ class QAPISchemaParser:
             ) from err
 
     @staticmethod
-    def _check_pragma_list_of_str(name, value, info):
-        if (not isinstance(value, list)
-                or any([not isinstance(elt, str) for elt in value])):
-            raise QAPISemError(
-                info,
-                "pragma %s must be a list of strings" % name)
+    def _pragma(name, value, info):
+
+        def check_list_str(name, value) -> List[str]:
+            if (not isinstance(value, list) or
+                    any([not isinstance(elt, str) for elt in value])):
+                raise QAPISemError(
+                    info,
+                    "pragma %s must be a list of strings" % name)
+            return value
+
+        pragma = info.pragma
 
-    def _pragma(self, name, value, info):
         if name == 'doc-required':
             if not isinstance(value, bool):
                 raise QAPISemError(info,
                                    "pragma 'doc-required' must be boolean")
-            info.pragma.doc_required = value
+            pragma.doc_required = value
         elif name == 'command-name-exceptions':
-            self._check_pragma_list_of_str(name, value, info)
-            info.pragma.command_name_exceptions = value
+            pragma.command_name_exceptions = check_list_str(name, value)
         elif name == 'command-returns-exceptions':
-            self._check_pragma_list_of_str(name, value, info)
-            info.pragma.command_returns_exceptions = value
+            pragma.command_returns_exceptions = check_list_str(name, value)
         elif name == 'member-name-exceptions':
-            self._check_pragma_list_of_str(name, value, info)
-            info.pragma.member_name_exceptions = value
+            pragma.member_name_exceptions = check_list_str(name, value)
         else:
             raise QAPISemError(info, "unknown pragma '%s'" % name)