Added customizable conversion error message for type errors encounted in FromPyObject::extract

pyo3-macros-backend/src/attributes.rs

@@ -12,6 +12,7 @@ pub mod kw {
     syn::custom_keyword!(annotation);
     syn::custom_keyword!(attribute);
     syn::custom_keyword!(from_py_with);
+    syn::custom_keyword!(conversion_error);
     syn::custom_keyword!(item);
     syn::custom_keyword!(pass_module);
     syn::custom_keyword!(name);

pyo3-macros-backend/src/from_pyobject.rs

@@ -241,21 +241,24 @@ impl<'a> Container<'a> {
                 FieldGetter::GetItem(Some(key)) => quote!(get_item(#key)),
                 FieldGetter::GetItem(None) => quote!(get_item(stringify!(#ident))),
             };
-
+            let conversion_error_msg = attrs.conversion_error
+                .as_ref()
+                .map_or(format!("failed to extract field {}.{}",
+                                quote!(#self_ty),
+                                ident),
+                |msg| msg.value());
             let get_field = quote!(obj.#getter?);
             let extractor = match &attrs.from_py_with {
                 None => quote!(#get_field.extract().map_err(|inner| {
-                    let err_msg = format!("failed to extract field {}.{}\n\nCaused by:\n    {}\n",
-                        stringify!(#self_ty),
-                        stringify!(#ident),
+                    let err_msg = format!("{}\n\nCaused by:\n    {}\n",
+                        #conversion_error_msg,
                         inner);
                     pyo3::exceptions::PyTypeError::new_err(err_msg)
                 })?),
                 Some(FromPyWithAttribute(expr_path)) => quote! (#expr_path(#get_field).
                     map_err(|inner| {
-                        let err_msg = format!("failed to extract field {}.{}\n\nCaused by:\n    {}\n",
-                            stringify!(#self_ty),
-                            stringify!(#ident),
+                        let err_msg = format!("{}\n\nCaused by:\n    {}\n",
+                            #conversion_error_msg,
                             inner);
                         pyo3::exceptions::PyTypeError::new_err(err_msg)
                     })?),
@@ -336,6 +339,7 @@ impl ContainerOptions {
 struct FieldPyO3Attributes {
     getter: FieldGetter,
     from_py_with: Option<FromPyWithAttribute>,
+    conversion_error: Option<LitStr>,
 }
 
 #[derive(Clone, Debug)]
@@ -347,6 +351,7 @@ enum FieldGetter {
 enum FieldPyO3Attribute {
     Getter(FieldGetter),
     FromPyWith(FromPyWithAttribute),
+    ConversionError(LitStr),
 }
 
 impl Parse for FieldPyO3Attribute {
@@ -390,6 +395,10 @@ impl Parse for FieldPyO3Attribute {
             }
         } else if lookahead.peek(attributes::kw::from_py_with) {
             input.parse().map(FieldPyO3Attribute::FromPyWith)
+        } else if lookahead.peek(attributes::kw::conversion_error) {
+            let _: attributes::kw::conversion_error = input.parse()?;
+            let _: Token![=] = input.parse()?;
+            input.parse().map(FieldPyO3Attribute::ConversionError)
         } else {
             Err(lookahead.error())
         }
@@ -402,6 +411,7 @@ impl FieldPyO3Attributes {
     fn from_attrs(attrs: &[Attribute]) -> Result<Self> {
         let mut getter = None;
         let mut from_py_with = None;
+        let mut conversion_error = None;
 
         for attr in attrs {
             if let Some(pyo3_attrs) = get_pyo3_attributes(attr)? {
@@ -421,6 +431,13 @@ impl FieldPyO3Attributes {
                             );
                             from_py_with = Some(from_py_with_attr);
                         }
+                        FieldPyO3Attribute::ConversionError(conversion_error_msg) => {
+                            ensure_spanned!(
+                                conversion_error.is_none(),
+                                attr.span() => "`conversion_error` may only be provided once"
+                            );
+                            conversion_error = Some(conversion_error_msg)
+                        }
                     }
                 }
             }
@@ -429,6 +446,7 @@ impl FieldPyO3Attributes {
         Ok(FieldPyO3Attributes {
             getter: getter.unwrap_or(FieldGetter::GetAttr(None)),
             from_py_with,
+            conversion_error,
         })
     }
 }