Documentation, testing and hygiene

guide/src/conversions/traits.md

@@ -488,6 +488,11 @@ If the input is neither a string nor an integer, the error message will be:
     - apply a custom function to convert the field from Python the desired Rust type.
     - the argument must be the name of the function as a string.
     - the function signature must be `fn(&Bound<PyAny>) -> PyResult<T>` where `T` is the Rust type of the argument.
+- `pyo3(default)`, `pyo3(default = ...)`
+  - 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.
 
 ### `IntoPyObject`
 The ['IntoPyObject'] trait defines the to-python conversion for a Rust type. All types in PyO3 implement this trait,

newsfragments/4829.added.md

@@ -1 +1 @@
-`derive(FromPyObject)` allow a `default` attribute to set a default value for extracted fields. The default value is either provided explicitly or fetched via `Default::default()`.
+`derive(FromPyObject)` allow a `default` attribute to set a default value for extracted fields of named structs. The default value is either provided explicitly or fetched via `Default::default()`.

pyo3-macros-backend/src/attributes.rs

@@ -45,7 +45,6 @@ pub mod kw {
     syn::custom_keyword!(unsendable);
     syn::custom_keyword!(weakref);
     syn::custom_keyword!(gil_used);
-    syn::custom_keyword!(default);
 }
 
 fn take_int(read: &mut &str, tracker: &mut usize) -> String {
@@ -352,7 +351,7 @@ impl<K: ToTokens, V: ToTokens> ToTokens for OptionalKeywordAttribute<K, V> {
 
 pub type FromPyWithAttribute = KeywordAttribute<kw::from_py_with, LitStrValue<ExprPath>>;
 
-pub type DefaultAttribute = OptionalKeywordAttribute<kw::default, Expr>;
+pub type DefaultAttribute = OptionalKeywordAttribute<Token![default], Expr>;
 
 /// For specifying the path to the pyo3 crate.
 pub type CrateAttribute = KeywordAttribute<Token![crate], LitStrValue<Path>>;

pyo3-macros-backend/src/frompyobject.rs

@@ -362,9 +362,9 @@ impl<'a> Container<'a> {
                 let default_expr = if let Some(default_expr) = &default.value {
                     default_expr.to_token_stream()
                 } else {
-                    quote!(Default::default())
+                    quote!(::std::default::Default::default())
                 };
-                quote!(if let Ok(value) = #getter {
+                quote!(if let ::std::result::Result::Ok(value) = #getter {
                     #extractor
                 } else {
                     #default_expr
@@ -533,7 +533,7 @@ impl Parse for FieldPyO3Attribute {
             }
         } else if lookahead.peek(attributes::kw::from_py_with) {
             input.parse().map(FieldPyO3Attribute::FromPyWith)
-        } else if lookahead.peek(attributes::kw::default) {
+        } else if lookahead.peek(Token![default]) {
             input.parse().map(FieldPyO3Attribute::Default)
         } else {
             Err(lookahead.error())

src/tests/hygiene/misc.rs

@@ -12,6 +12,8 @@ struct Derive3 {
     f: i32,
     #[pyo3(item(42))]
     g: i32,
+    #[pyo3(default)]
+    h: i32,
 } // struct case
 
 #[derive(crate::FromPyObject)]

tests/test_frompyobject.rs

@@ -718,3 +718,38 @@ fn test_with_explicit_default_item() {
         assert_eq!(result, expected);
     });
 }
+
+#[derive(Debug, FromPyObject, PartialEq, Eq)]
+pub struct WithDefaultItemAndConversionFunction {
+    #[pyo3(item, default, from_py_with = "Bound::<'_, PyAny>::len")]
+    value: usize,
+}
+
+#[test]
+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();
+        let result = dict
+            .extract::<WithDefaultItemAndConversionFunction>()
+            .unwrap();
+        let expected = WithDefaultItemAndConversionFunction { value: 1 };
+        assert_eq!(result, expected);
+
+        // Empty case
+        let dict = PyDict::new(py);
+        let result = dict
+            .extract::<WithDefaultItemAndConversionFunction>()
+            .unwrap();
+        let expected = WithDefaultItemAndConversionFunction { value: 0 };
+        assert_eq!(result, expected);
+
+        // Error case
+        let dict = PyDict::new(py);
+        dict.set_item("value", 1).unwrap();
+        assert!(dict
+            .extract::<WithDefaultItemAndConversionFunction>()
+            .is_err());
+    });
+}