Support enum variant named fields and cover failures

guide/src/conversions/traits.md

@@ -492,7 +492,9 @@ If the input is neither a string nor an integer, the error message will be:
   - if the argument is set, uses the given default value.
   - in this case, the argument must be a Rust expression returning a value of the desired Rust type.
   - if the argument is not set, [`Default::default`](https://doc.rust-lang.org/std/default/trait.Default.html#tymethod.default) is used.
-  - this attribute is only supported on named struct fields.
+  - note that the default value is only used if the field is not set.
+    If the field is set and the conversion function from Python to Rust fails, an exception is raised and the default value is not used.
+  - this attribute is only supported on named fields.
 
 For example, the code below applies the given conversion function on the `"value"` dict item to compute its length or fall back to the type default value (0):
 
@@ -503,6 +505,8 @@ use pyo3::prelude::*;
 struct RustyStruct {
     #[pyo3(item("value"), default, from_py_with = "Bound::<'_, PyAny>::len")]
     len: usize,
+    #[pyo3(item)]
+    other: usize,
 }
 #
 # use pyo3::types::PyDict;
@@ -511,13 +515,17 @@ struct RustyStruct {
 #         // Filled case
 #         let dict = PyDict::new(py);
 #         dict.set_item("value", (1,)).unwrap();
+#         dict.set_item("other", 1).unwrap();
 #         let result = dict.extract::<RustyStruct>()?;
 #         assert_eq!(result.len, 1);
+#         assert_eq!(result.other, 1);
 #
 #         // Empty case
 #         let dict = PyDict::new(py);
+#         dict.set_item("other", 1).unwrap();
 #         let result = dict.extract::<RustyStruct>()?;
 #         assert_eq!(result.len, 0);
+#         assert_eq!(result.other, 1);
 #         Ok(())
 #     })
 # }

pyo3-macros-backend/src/frompyobject.rs

@@ -147,6 +147,10 @@ impl<'a> Container<'a> {
                             attrs.getter.is_none(),
                             field.span() => "`getter` is not permitted on tuple struct elements."
                         );
+                        ensure_spanned!(
+                            attrs.default.is_none(),
+                            field.span() => "`default` is not permitted on tuple struct elements."
+                        );
                         Ok(TupleStructField {
                             from_py_with: attrs.from_py_with,
                         })
@@ -200,7 +204,11 @@ impl<'a> Container<'a> {
                         })
                     })
                     .collect::<Result<Vec<_>>>()?;
-                if options.transparent {
+                if struct_fields.iter().all(|field| field.default.is_some()) {
+                    bail_spanned!(
+                        fields.span() => "cannot derive FromPyObject for structs and variants with only default values"
+                    )
+                } else if options.transparent {
                     ensure_spanned!(
                         struct_fields.len() == 1,
                         fields.span() => "transparent structs and variants can only have 1 field"

tests/test_frompyobject.rs

@@ -690,38 +690,49 @@ fn test_with_keyword_item() {
 #[derive(Debug, FromPyObject, PartialEq, Eq)]
 pub struct WithDefaultItem {
     #[pyo3(item, default)]
-    value: Option<usize>,
+    opt: Option<usize>,
+    #[pyo3(item)]
+    value: usize,
 }
 
 #[test]
 fn test_with_default_item() {
     Python::with_gil(|py| {
         let dict = PyDict::new(py);
+        dict.set_item("value", 3).unwrap();
         let result = dict.extract::<WithDefaultItem>().unwrap();
-        let expected = WithDefaultItem { value: None };
+        let expected = WithDefaultItem {
+            value: 3,
+            opt: None,
+        };
         assert_eq!(result, expected);
     });
 }
 
 #[derive(Debug, FromPyObject, PartialEq, Eq)]
 pub struct WithExplicitDefaultItem {
     #[pyo3(item, default = 1)]
+    opt: usize,
+    #[pyo3(item)]
     value: usize,
 }
 
 #[test]
 fn test_with_explicit_default_item() {
     Python::with_gil(|py| {
         let dict = PyDict::new(py);
+        dict.set_item("value", 3).unwrap();
         let result = dict.extract::<WithExplicitDefaultItem>().unwrap();
-        let expected = WithExplicitDefaultItem { value: 1 };
+        let expected = WithExplicitDefaultItem { value: 3, opt: 1 };
         assert_eq!(result, expected);
     });
 }
 
 #[derive(Debug, FromPyObject, PartialEq, Eq)]
 pub struct WithDefaultItemAndConversionFunction {
     #[pyo3(item, default, from_py_with = "Bound::<'_, PyAny>::len")]
+    opt: usize,
+    #[pyo3(item)]
     value: usize,
 }
 
@@ -730,26 +741,62 @@ fn test_with_default_item_and_conversion_function() {
     Python::with_gil(|py| {
         // Filled case
         let dict = PyDict::new(py);
-        dict.set_item("value", (1,)).unwrap();
+        dict.set_item("opt", (1,)).unwrap();
+        dict.set_item("value", 3).unwrap();
         let result = dict
             .extract::<WithDefaultItemAndConversionFunction>()
             .unwrap();
-        let expected = WithDefaultItemAndConversionFunction { value: 1 };
+        let expected = WithDefaultItemAndConversionFunction { opt: 1, value: 3 };
         assert_eq!(result, expected);
 
         // Empty case
         let dict = PyDict::new(py);
+        dict.set_item("value", 3).unwrap();
         let result = dict
             .extract::<WithDefaultItemAndConversionFunction>()
             .unwrap();
-        let expected = WithDefaultItemAndConversionFunction { value: 0 };
+        let expected = WithDefaultItemAndConversionFunction { opt: 0, value: 3 };
         assert_eq!(result, expected);
 
         // Error case
         let dict = PyDict::new(py);
-        dict.set_item("value", 1).unwrap();
+        dict.set_item("value", 3).unwrap();
+        dict.set_item("opt", 1).unwrap();
         assert!(dict
             .extract::<WithDefaultItemAndConversionFunction>()
             .is_err());
     });
 }
+
+#[derive(Debug, FromPyObject, PartialEq, Eq)]
+pub enum WithDefaultItemEnum {
+    #[pyo3(from_item_all)]
+    Foo {
+        a: usize,
+        #[pyo3(default)]
+        b: usize,
+    },
+    NeverUsedA {
+        a: usize,
+    },
+}
+
+#[test]
+fn test_with_default_item_enum() {
+    Python::with_gil(|py| {
+        // A and B filled
+        let dict = PyDict::new(py);
+        dict.set_item("a", 1).unwrap();
+        dict.set_item("b", 2).unwrap();
+        let result = dict.extract::<WithDefaultItemEnum>().unwrap();
+        let expected = WithDefaultItemEnum::Foo { a: 1, b: 2 };
+        assert_eq!(result, expected);
+
+        // A filled
+        let dict = PyDict::new(py);
+        dict.set_item("a", 1).unwrap();
+        let result = dict.extract::<WithDefaultItemEnum>().unwrap();
+        let expected = WithDefaultItemEnum::Foo { a: 1, b: 0 };
+        assert_eq!(result, expected);
+    });
+}

tests/ui/invalid_frompy_derive.rs

@@ -213,4 +213,21 @@ struct FromItemAllConflictAttrWithArgs {
     field: String,
 }
 
+#[derive(FromPyObject)]
+struct StructWithOnlyDefaultValues {
+    #[pyo3(default)]
+    field: String,
+}
+
+#[derive(FromPyObject)]
+enum EnumVariantWithOnlyDefaultValues {
+    Foo {
+        #[pyo3(default)]
+        field: String,
+    },
+}
+
+#[derive(FromPyObject)]
+struct NamedTuplesWithDefaultValues(#[pyo3(default)] String);
+
 fn main() {}

tests/ui/invalid_frompy_derive.stderr

@@ -223,3 +223,29 @@ error: The struct is already annotated with `from_item_all`, `attribute` is not
     |
 210 | #[pyo3(from_item_all)]
     |        ^^^^^^^^^^^^^
+
+error: cannot derive FromPyObject for structs and variants with only default values
+   --> tests/ui/invalid_frompy_derive.rs:217:36
+    |
+217 |   struct StructWithOnlyDefaultValues {
+    |  ____________________________________^
+218 | |     #[pyo3(default)]
+219 | |     field: String,
+220 | | }
+    | |_^
+
+error: cannot derive FromPyObject for structs and variants with only default values
+   --> tests/ui/invalid_frompy_derive.rs:224:9
+    |
+224 |       Foo {
+    |  _________^
+225 | |         #[pyo3(default)]
+226 | |         field: String,
+227 | |     },
+    | |_____^
+
+error: `default` is not permitted on tuple struct elements.
+   --> tests/ui/invalid_frompy_derive.rs:231:37
+    |
+231 | struct NamedTuplesWithDefaultValues(#[pyo3(default)] String);
+    |                                     ^