[PATCH v3 07/13] qapi/introspect.py: Introduce preliminary tree typing

[John Snow <jsnow@redhat.com>]

The types will be used in forthcoming patches to add typing. These types
describe the layout and structure of the objects passed to
_tree_to_qlit, but lack the power to describe annotations until the next
commit.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 scripts/qapi/introspect.py | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/scripts/qapi/introspect.py b/scripts/qapi/introspect.py
index 0aa3b77109f..b82efe16f6e 100644
--- a/scripts/qapi/introspect.py
+++ b/scripts/qapi/introspect.py
@@ -10,7 +10,13 @@
 See the COPYING file in the top-level directory.
 """
 
-from typing import Optional
+from typing import (
+    Any,
+    Dict,
+    List,
+    Optional,
+    Union,
+)
 
 from .common import (
     c_name,
@@ -26,6 +32,28 @@
 )
 
 
+# This module constructs a tree data structure that is used to
+# generate the introspection information for QEMU. It behaves similarly
+# to a JSON value.
+#
+# A complexity over JSON is that our values may or may not be annotated.
+#
+# Un-annotated values may be:
+#     Scalar: str, bool, None.
+#     Non-scalar: List, Dict
+# _value = Union[str, bool, None, Dict[str, TreeValue], List[TreeValue]]
+#
+# With optional annotations, the type of all values is:
+# TreeValue = Union[_value, Annotated[_value]]
+#
+# Sadly, mypy does not support recursive types, so we must approximate this.
+_stub = Any
+_scalar = Union[str, bool, None]
+_nonscalar = Union[Dict[str, _stub], List[_stub]]
+_value = Union[_scalar, _nonscalar]
+# TreeValue = Union[_value, 'Annotated[_value]']
+
+
 def _make_tree(obj, ifcond, comment=None):
     extra = {
         'if': ifcond,
-- 
2.26.2