qapi/expr.py: Add casts in a few select cases

Casts are instructions to the type checker only, they aren't "safe" and
should probably be avoided in general. In this case, when we perform
type checking on a nested structure, the type of each field does not
"stick".

(See PEP 647 for an example of "type narrowing" that does "stick".
 It is available in Python 3.10, so we can't use it yet.)

We don't need to assert that something is a str if we've already checked
or asserted that it is -- use a cast instead for these cases.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210421182032.3521476-9-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/expr.py

@@ -15,7 +15,7 @@
 # See the COPYING file in the top-level directory.
 
 import re
-from typing import Dict, Optional
+from typing import Dict, Optional, cast
 
 from .common import c_name
 from .error import QAPISemError
@@ -261,7 +261,7 @@ def check_enum(expr, info):
 
 
 def check_struct(expr, info):
-    name = expr['struct']
+    name = cast(str, expr['struct'])  # Checked in check_exprs
     members = expr['data']
 
     check_type(members, info, "'data'", allow_dict=name)
@@ -269,7 +269,7 @@ def check_struct(expr, info):
 
 
 def check_union(expr, info):
-    name = expr['union']
+    name = cast(str, expr['union'])  # Checked in check_exprs
     base = expr.get('base')
     discriminator = expr.get('discriminator')
     members = expr['data']
@@ -368,8 +368,8 @@ def check_exprs(exprs):
         else:
             raise QAPISemError(info, "expression is missing metatype")
 
-        name = expr[meta]
-        check_name_is_str(name, info, "'%s'" % meta)
+        check_name_is_str(expr[meta], info, "'%s'" % meta)
+        name = cast(str, expr[meta])
         info.set_defn(meta, name)
         check_defn_name_str(name, info, meta)