derive(FromPyObject): adds default option

Takes an optional expression to set a custom value that is not the one from the Default trait

Author: Tpt

newsfragments/4829.added.md

@@ -0,0 +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()`.

pyo3-macros-backend/src/attributes.rs

@@ -45,6 +45,7 @@ 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 {
@@ -351,6 +352,8 @@ 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>;
+
 /// For specifying the path to the pyo3 crate.
 pub type CrateAttribute = KeywordAttribute<Token![crate], LitStrValue<Path>>;
 

pyo3-macros-backend/src/frompyobject.rs

@@ -1,7 +1,9 @@
-use crate::attributes::{self, get_pyo3_options, CrateAttribute, FromPyWithAttribute};
+use crate::attributes::{
+    self, get_pyo3_options, CrateAttribute, DefaultAttribute, FromPyWithAttribute,
+};
 use crate::utils::Ctx;
 use proc_macro2::TokenStream;
-use quote::{format_ident, quote};
+use quote::{format_ident, quote, ToTokens};
 use syn::{
     ext::IdentExt,
     parenthesized,
@@ -90,6 +92,7 @@ struct NamedStructField<'a> {
     ident: &'a syn::Ident,
     getter: Option<FieldGetter>,
     from_py_with: Option<FromPyWithAttribute>,
+    default: Option<DefaultAttribute>,
 }
 
 struct TupleStructField {
@@ -193,6 +196,7 @@ impl<'a> Container<'a> {
                             ident,
                             getter: attrs.getter,
                             from_py_with: attrs.from_py_with,
+                            default: attrs.default,
                         })
                     })
                     .collect::<Result<Vec<_>>>()?;
@@ -346,18 +350,33 @@ impl<'a> Container<'a> {
                     quote!(#pyo3_path::types::PyAnyMethods::get_item(obj, #pyo3_path::intern!(obj.py(), #field_name)))
                 }
             };
-            let extractor = match &field.from_py_with {
-                None => {
-                    quote!(#pyo3_path::impl_::frompyobject::extract_struct_field(&#getter?, #struct_name, #field_name)?)
-                }
-                Some(FromPyWithAttribute {
-                    value: expr_path, ..
-                }) => {
-                    quote! (#pyo3_path::impl_::frompyobject::extract_struct_field_with(#expr_path as fn(_) -> _, &#getter?, #struct_name, #field_name)?)
-                }
+            let extractor = if let Some(FromPyWithAttribute {
+                value: expr_path, ..
+            }) = &field.from_py_with
+            {
+                quote!(#pyo3_path::impl_::frompyobject::extract_struct_field_with(#expr_path as fn(_) -> _, &value, #struct_name, #field_name)?)
+            } else {
+                quote!(#pyo3_path::impl_::frompyobject::extract_struct_field(&value, #struct_name, #field_name)?)
+            };
+            let extracted = if let Some(default) = &field.default {
+                let default_expr = if let Some(default_expr) = &default.value {
+                    default_expr.to_token_stream()
+                } else {
+                    quote!(Default::default())
+                };
+                quote!(if let Ok(value) = #getter {
+                    #extractor
+                } else {
+                    #default_expr
+                })
+            } else {
+                quote!({
+                    let value = #getter?;
+                    #extractor
+                })
             };
 
-            fields.push(quote!(#ident: #extractor));
+            fields.push(quote!(#ident: #extracted));
         }
 
         quote!(::std::result::Result::Ok(#self_ty{#fields}))
@@ -458,6 +477,7 @@ impl ContainerOptions {
 struct FieldPyO3Attributes {
     getter: Option<FieldGetter>,
     from_py_with: Option<FromPyWithAttribute>,
+    default: Option<DefaultAttribute>,
 }
 
 #[derive(Clone, Debug)]
@@ -469,6 +489,7 @@ enum FieldGetter {
 enum FieldPyO3Attribute {
     Getter(FieldGetter),
     FromPyWith(FromPyWithAttribute),
+    Default(DefaultAttribute),
 }
 
 impl Parse for FieldPyO3Attribute {
@@ -512,6 +533,8 @@ 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) {
+            input.parse().map(FieldPyO3Attribute::Default)
         } else {
             Err(lookahead.error())
         }
@@ -523,6 +546,7 @@ impl FieldPyO3Attributes {
     fn from_attrs(attrs: &[Attribute]) -> Result<Self> {
         let mut getter = None;
         let mut from_py_with = None;
+        let mut default = None;
 
         for attr in attrs {
             if let Some(pyo3_attrs) = get_pyo3_options(attr)? {
@@ -542,6 +566,13 @@ impl FieldPyO3Attributes {
                             );
                             from_py_with = Some(from_py_with_attr);
                         }
+                        FieldPyO3Attribute::Default(default_attr) => {
+                            ensure_spanned!(
+                                default.is_none(),
+                                attr.span() => "`default` may only be provided once"
+                            );
+                            default = Some(default_attr);
+                        }
                     }
                 }
             }
@@ -550,6 +581,7 @@ impl FieldPyO3Attributes {
         Ok(FieldPyO3Attributes {
             getter,
             from_py_with,
+            default,
         })
     }
 }

tests/test_frompyobject.rs

@@ -686,3 +686,35 @@ fn test_with_keyword_item() {
         assert_eq!(result, expected);
     });
 }
+
+#[derive(Debug, FromPyObject, PartialEq, Eq)]
+pub struct WithDefaultItem {
+    #[pyo3(item, default)]
+    value: Option<usize>,
+}
+
+#[test]
+fn test_with_default_item() {
+    Python::with_gil(|py| {
+        let dict = PyDict::new(py);
+        let result = dict.extract::<WithDefaultItem>().unwrap();
+        let expected = WithDefaultItem { value: None };
+        assert_eq!(result, expected);
+    });
+}
+
+#[derive(Debug, FromPyObject, PartialEq, Eq)]
+pub struct WithExplicitDefaultItem {
+    #[pyo3(item, default = 1)]
+    value: usize,
+}
+
+#[test]
+fn test_with_explicit_default_item() {
+    Python::with_gil(|py| {
+        let dict = PyDict::new(py);
+        let result = dict.extract::<WithExplicitDefaultItem>().unwrap();
+        let expected = WithExplicitDefaultItem { value: 1 };
+        assert_eq!(result, expected);
+    });
+}

tests/ui/invalid_frompy_derive.stderr

@@ -84,7 +84,7 @@ error: transparent structs and variants can only have 1 field
 70 | |     },
    | |_____^
 
-error: expected one of: `attribute`, `item`, `from_py_with`
+error: expected one of: `attribute`, `item`, `from_py_with`, `default`
   --> tests/ui/invalid_frompy_derive.rs:76:12
    |
 76 |     #[pyo3(attr)]