Introspection: fix return type hint for Result<()> (#5674)

* Introspection: fix return type hint for Result<()>

Uses a hack to get const specialization working

Closes #5645

* Reuse pyclass::boolean_struct

* Update newsfragments/5674.fixed.md

Co-authored-by: David Hewitt <[email protected]>

* Use Probe

* Update src/pyclass.rs

Co-authored-by: David Hewitt <[email protected]>

---------

Co-authored-by: David Hewitt <[email protected]>

Author: Tpt

newsfragments/5674.fixed.md

@@ -0,0 +1 @@
+Introspection: fix the return type annotation of `PyResult<()>` (must be `None` and not `tuple`)

pyo3-macros-backend/src/introspection.rs

@@ -112,13 +112,7 @@ pub fn function_introspection_code(
             } else {
                 match returns {
                     ReturnType::Default => PythonTypeHint::builtin("None"),
-                    ReturnType::Type(_, ty) => match *ty {
-                        Type::Tuple(t) if t.elems.is_empty() => {
-                            // () is converted to None in return types
-                            PythonTypeHint::builtin("None")
-                        }
-                        ty => PythonTypeHint::from_return_type(ty, parent),
-                    },
+                    ReturnType::Type(_, ty) => PythonTypeHint::from_return_type(*ty, parent),
                 }
                 .into()
             },

pyo3-macros-backend/src/type_hint.rs

@@ -148,7 +148,16 @@ impl PythonTypeHint {
                 }
             }
             PythonTypeHintVariant::ReturnType(t) => {
-                quote! { <#t as #pyo3_crate_path::impl_::introspection::PyReturnType>::OUTPUT_TYPE }
+                quote! {{
+                    #[allow(unused_imports)]
+                    use #pyo3_crate_path::impl_::pyclass::Probe as _;
+                    const TYPE: #pyo3_crate_path::inspect::TypeHint = if #pyo3_crate_path::impl_::pyclass::IsReturningEmptyTuple::<#t>::VALUE {
+                        #pyo3_crate_path::inspect::TypeHint::builtin("None")
+                    } else {
+                        <#t as #pyo3_crate_path::impl_::introspection::PyReturnType>::OUTPUT_TYPE
+                    };
+                    TYPE
+                }}
             }
             PythonTypeHintVariant::Type(t) => {
                 quote! { <#t as #pyo3_crate_path::type_object::PyTypeCheck>::TYPE_HINT }

pytests/stubs/pyclasses.pyi

@@ -1,5 +1,5 @@
 from _typeshed import Incomplete
-from typing import Any, final
+from typing import final
 
 class AssertingBaseClass:
     def __new__(cls, /, expected_type: type) -> AssertingBaseClass: ...
@@ -55,7 +55,7 @@ class PyClassThreadIter:
 
 @final
 class SubClassWithInit(dict):
-    def __init__(self, /, *args, **kwargs) -> Any: ...
+    def __init__(self, /, *args, **kwargs) -> None: ...
     def __new__(cls, /, *args, **kwargs) -> SubClassWithInit: ...
 
 def map_a_class(

src/conversion.rs

@@ -595,6 +595,9 @@ impl<'py> IntoPyObject<'py> for () {
     type Output = Bound<'py, Self::Target>;
     type Error = Infallible;
 
+    #[cfg(feature = "experimental-inspect")]
+    const OUTPUT_TYPE: TypeHint = PyTuple::TYPE_HINT; // TODO(Tpt): should be tuple[()]
+
     fn into_pyobject(self, py: Python<'py>) -> Result<Self::Output, Self::Error> {
         Ok(PyTuple::empty(py))
     }

src/impl_/pyclass/probes.rs

@@ -79,6 +79,16 @@ impl<T: Clone> IsClone<T> {
     pub const VALUE: bool = true;
 }
 
+probe!(IsReturningEmptyTuple);
+
+impl IsReturningEmptyTuple<()> {
+    pub const VALUE: bool = true;
+}
+
+impl<E> IsReturningEmptyTuple<Result<(), E>> {
+    pub const VALUE: bool = true;
+}
+
 #[cfg(test)]
 macro_rules! value_of {
     ($probe:ident, $ty:ty) => {{