qapi/expr.py: Consolidate check_if_str calls in check_if

This is a small rewrite to address some minor style nits.

Don't compare against the empty list to check for the empty condition, and
move the normalization forward to unify the check on the now-normalized
structure.

With the check unified, the local nested function isn't needed anymore
and can be brought down into the normal flow of the function. With the
nesting level changed, shuffle the error strings around a bit to get
them to fit in 79 columns.

Note: although ifcond is typed as Sequence[str] elsewhere, we *know* that
the parser will produce real, bona-fide lists. It's okay to check
isinstance(ifcond, list) here.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210421182032.3521476-12-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/expr.py

@@ -146,30 +146,29 @@ def check_flags(expr: _JSONObject, info: QAPISourceInfo) -> None:
 
 def check_if(expr: _JSONObject, info: QAPISourceInfo, source: str) -> None:
 
-    def check_if_str(ifcond: object) -> None:
-        if not isinstance(ifcond, str):
+    ifcond = expr.get('if')
+    if ifcond is None:
+        return
+
+    if isinstance(ifcond, list):
+        if not ifcond:
+            raise QAPISemError(
+                info, "'if' condition [] of %s is useless" % source)
+    else:
+        # Normalize to a list
+        ifcond = expr['if'] = [ifcond]
+
+    for elt in ifcond:
+        if not isinstance(elt, str):
             raise QAPISemError(
                 info,
                 "'if' condition of %s must be a string or a list of strings"
                 % source)
-        if ifcond.strip() == '':
+        if not elt.strip():
             raise QAPISemError(
                 info,
                 "'if' condition '%s' of %s makes no sense"
-                % (ifcond, source))
-
-    ifcond = expr.get('if')
-    if ifcond is None:
-        return
-    if isinstance(ifcond, list):
-        if ifcond == []:
-            raise QAPISemError(
-                info, "'if' condition [] of %s is useless" % source)
-        for elt in ifcond:
-            check_if_str(elt)
-    else:
-        check_if_str(ifcond)
-        expr['if'] = [ifcond]
+                % (elt, source))
 
 
 def normalize_members(members: object) -> None: