Improve typing for queries.

ExpectType grew its Widget bound and then it was moved out of the body of the class so that it could be referenced inside methods, because it was needed inside the body of 'only_one'.

src/textual/css/query.py

@@ -48,7 +48,10 @@ class WrongType(QueryError):
     """Query result was not of the correct type."""
 
 
-QueryType = TypeVar("QueryType", bound="Widget")
+QueryType = TypeVar("QueryType", bound=Widget)
+"""Type variable used to type generic queries."""
+ExpectType = TypeVar("ExpectType", bound=Widget)
+"""Type variable used to further restrict queries."""
 
 
 @rich.repr.auto(angular=True)
@@ -187,10 +190,8 @@ def exclude(self, selector: str) -> DOMQuery[QueryType]:
         """
         return DOMQuery(self.node, exclude=selector, parent=self)
 
-    ExpectType = TypeVar("ExpectType")
-
     @overload
-    def first(self) -> Widget:
+    def first(self) -> QueryType:
         ...
 
     @overload
@@ -226,7 +227,7 @@ def first(
             raise NoMatches(f"No nodes match {self!r}")
 
     @overload
-    def only_one(self) -> Widget:
+    def only_one(self) -> QueryType:
         ...
 
     @overload
@@ -235,7 +236,7 @@ def only_one(self, expect_type: type[ExpectType]) -> ExpectType:
 
     def only_one(
         self, expect_type: type[ExpectType] | None = None
-    ) -> Widget | ExpectType:
+    ) -> QueryType | ExpectType:
         """Get the *only* matching node.
 
         Args:
@@ -253,7 +254,9 @@ def only_one(
         _rich_traceback_omit = True
         # Call on first to get the first item. Here we'll use all of the
         # testing and checking it provides.
-        the_one = self.first(expect_type) if expect_type is not None else self.first()
+        the_one: ExpectType | QueryType = (
+            self.first(expect_type) if expect_type is not None else self.first()
+        )
         try:
             # Now see if we can access a subsequent item in the nodes. There
             # should *not* be anything there, so we *should* get an
@@ -268,10 +271,10 @@ def only_one(
             # The IndexError was got, that's a good thing in this case. So
             # we return what we found.
             pass
-        return cast("Widget", the_one)
+        return the_one
 
     @overload
-    def last(self) -> Widget:
+    def last(self) -> QueryType:
         ...
 
     @overload
@@ -304,7 +307,7 @@ def last(
         return last
 
     @overload
-    def results(self) -> Iterator[Widget]:
+    def results(self) -> Iterator[QueryType]:
         ...
 
     @overload
@@ -313,7 +316,7 @@ def results(self, filter_type: type[ExpectType]) -> Iterator[ExpectType]:
 
     def results(
         self, filter_type: type[ExpectType] | None = None
-    ) -> Iterator[Widget | ExpectType]:
+    ) -> Iterator[QueryType | ExpectType]:
         """Get query results, optionally filtered by a given type.
 
         Args: