[PATCH 13/22] qapi/parser: [RFC] overload the return type of get_expr

[John Snow]

Teach mypy that there are two possible return types here: either an
Expression, or ... something else.

Not a SLOC reduction, but it does remove an assertion. It also isn't
much safer than a cast, mypy has no insight into if overloads are true
or not. It's on the honor system.

I thought I'd demonstrate its use, though.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 scripts/qapi/parser.py | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/scripts/qapi/parser.py b/scripts/qapi/parser.py
index f2b57d5642a..cbdddc344e7 100644
--- a/scripts/qapi/parser.py
+++ b/scripts/qapi/parser.py
@@ -23,6 +23,7 @@
     Optional,
     Set,
     Union,
+    overload,
 )
 
 from .common import match_nofail
@@ -95,8 +96,7 @@ def _parse(self) -> None:
                     self.docs.append(cur_doc)
                 continue
 
-            expr = self.get_expr(False)
-            assert isinstance(expr, dict)  # Guaranteed when nested=False
+            expr = self.get_expr()
 
             if 'include' in expr:
                 self.reject_expr_doc(cur_doc)
@@ -304,8 +304,15 @@ def get_values(self) -> List[object]:
                 raise QAPIParseError(self, "expected ',' or ']'")
             self.accept()
 
+    @overload
+    # No nesting, must be an Expression.
+    def get_expr(self) -> Expression: ...
+
+    @overload
+    # Possibly nested, might be anything.
+    def get_expr(self, nested: bool) -> _ExprValue: ...
+
     def get_expr(self, nested: bool = False) -> _ExprValue:
-        # TODO: Teach mypy that nested=False means the retval is a Dict.
         expr: _ExprValue
         if self.tok != '{' and not nested:
             raise QAPIParseError(self, "expected '{'")
-- 
2.30.2