validate value of 'const' properties (helps with discriminated unions) (#1024)

For 'const' properties, the parsed value will now be checked against the
value in the schema at runtime and a ValueError will be raised if they
do not match. Until now, the values would just be `cast` to a `Literal`
without checking.

This helps with with parsing `oneOf` when matching a const property is
the only way to resolve which of the types should be returned. i.e. if a
`oneOf` contains two schemas, but the properties in one are a subset of
the other, then the only way to know which one to return is to validate
the `const` value.

I know that for the most part this library parses but doesn't validate,
but I don't see a way around it in this case. The spec itself mentions
validation when talking about `oneOf`:

```
In OAS 3.0, a response payload MAY be described to be exactly one of any number of types:
[...]
which means the payload MUST, by validation, match exactly one of the schemas described by Cat, Dog, or Lizard.
```

Note that this also addresses some of the use cases for discriminated
unions. Per the spec, the `a discriminator MAY act as a "hint" to
shortcut validation and selection of the matching schema which may be a
costly operation, depending on the complexity of the schema.` so
implementing discriminators would be a speedup but with this change
we'll at least construct the correct type.

I'm happy to take any feedback, or abandon this if you don't like fact
that it is validation. I originally went to try to implement
discriminated unions fully, but this seemed like a much easier step
towards that that gets the same result.

---------

Co-authored-by: Dylan Anthony <dbanty@users.noreply.github.com>

Author: peter-greenatlas

.changeset/const_values_are_now_validated_at_runtime.md

@@ -0,0 +1,10 @@
+---
+default: major
+---
+
+# `const` values in responses are now validated at runtime
+
+Prior to this version, `const` values returned from servers were assumed to always be correct. Now, if a server returns 
+an unexpected value, the client will raise a `ValueError`. This should enable better usage with `oneOf`.
+
+PR #1024. Thanks @peter-greenatlas!

end_to_end_tests/test-3-1-golden-record/test_3_1_features_client/api/const/post_const_path.py

@@ -46,6 +46,10 @@ def _parse_response(
 ) -> Optional[Literal["Why have a fixed response? I dunno"]]:
     if response.status_code == HTTPStatus.OK:
         response_200 = cast(Literal["Why have a fixed response? I dunno"], response.json())
+        if response_200 != "Why have a fixed response? I dunno":
+            raise ValueError(
+                f"response_200 must match const 'Why have a fixed response? I dunno', got '{response_200}'"
+            )
         return response_200
     if client.raise_on_unexpected_status:
         raise errors.UnexpectedStatus(response.status_code, response.content)

end_to_end_tests/test-3-1-golden-record/test_3_1_features_client/models/post_const_path_body.py

@@ -46,16 +46,26 @@ def to_dict(self) -> Dict[str, Any]:
     @classmethod
     def from_dict(cls: Type[T], src_dict: Dict[str, Any]) -> T:
         d = src_dict.copy()
-        required = d.pop("required")
+        required = cast(Literal["this always goes in the body"], d.pop("required"))
+        if required != "this always goes in the body":
+            raise ValueError(f"required must match const 'this always goes in the body', got '{required}'")
 
         def _parse_nullable(data: object) -> Union[Literal["this or null goes in the body"], None]:
             if data is None:
                 return data
+            nullable_type_1 = cast(Literal["this or null goes in the body"], data)
+            if nullable_type_1 != "this or null goes in the body":
+                raise ValueError(
+                    f"nullable_type_1 must match const 'this or null goes in the body', got '{nullable_type_1}'"
+                )
+            return nullable_type_1
             return cast(Union[Literal["this or null goes in the body"], None], data)
 
         nullable = _parse_nullable(d.pop("nullable"))
 
-        optional = d.pop("optional", UNSET)
+        optional = cast(Union[Literal["this sometimes goes in the body"], Unset], d.pop("optional", UNSET))
+        if optional != "this sometimes goes in the body" and not isinstance(optional, Unset):
+            raise ValueError(f"optional must match const 'this sometimes goes in the body', got '{optional}'")
 
         post_const_path_body = cls(
             required=required,

openapi_python_client/parser/properties/const.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import Any, overload
+from typing import Any, ClassVar, overload
 
 from attr import define
 
@@ -12,7 +12,7 @@
 
 @define
 class ConstProperty(PropertyProtocol):
-    """A property representing a Union (anyOf) of other properties"""
+    """A property representing a const value"""
 
     name: str
     required: bool
@@ -21,6 +21,7 @@ class ConstProperty(PropertyProtocol):
     python_name: PythonIdentifier
     description: str | None
     example: None
+    template: ClassVar[str] = "const_property.py.jinja"
 
     @classmethod
     def build(

openapi_python_client/templates/property_templates/const_property.py.jinja

@@ -0,0 +1,5 @@
+{% macro construct(property, source) %}
+{{ property.python_name }} = cast({{ property.get_type_string() }} , {{ source }})
+if {{ property.python_name }} != {{ property.value }}{% if not property.required %}and not isinstance({{ property.python_name }}, Unset){% endif %}:
+    raise ValueError(f"{{ property.name }} must match const {{ property.value }}, got '{{'{' + property.python_name + '}' }}'")
+{%- endmacro %}