Add argument name to error messages in conversion errors

pyo3-derive-backend/src/pymethod.rs

@@ -456,6 +456,19 @@ pub fn impl_arg_params(
     }}
 }
 
+macro_rules! transform_arg_error {
+    ($name:tt) => (quote!{
+        |e| {
+            let reason = e.instance(_py).str()
+                .unwrap_or_else(|_| pyo3::types::PyString::new(_py, ""));
+            pyo3::PyErr::from_type(
+                e.ptype(_py),
+                format!("argument '{}': {}", stringify!(#$name), reason)
+            )
+        }
+    });
+}
+
 /// Re option_pos: The option slice doesn't contain the py: Python argument, so the argument
 /// index and the index in option diverge when using py: Python
 fn impl_arg_param(
@@ -475,10 +488,12 @@ fn impl_arg_param(
 
     let ty = arg.ty;
     let name = arg.name;
+    let transform_error = transform_arg_error!(name);
 
     if spec.is_args(&name) {
         return quote! {
-            let #arg_name = <#ty as pyo3::FromPyObject>::extract(_args.as_ref())?;
+            let #arg_name = <#ty as pyo3::FromPyObject>::extract(_args.as_ref())
+                .map_err(#transform_error)?;
         };
     } else if spec.is_kwargs(&name) {
         return quote! {
@@ -518,15 +533,15 @@ fn impl_arg_param(
 
         quote! {
             let #mut_ _tmp: #target_ty = match #arg_value {
-                Some(_obj) => _obj.extract()?,
+                Some(_obj) => _obj.extract().map_err(#transform_error)?,
                 None => #default,
             };
             let #arg_name = #borrow_tmp;
         }
     } else {
         quote! {
             let #arg_name = match #arg_value {
-                Some(_obj) => _obj.extract()?,
+                Some(_obj) => _obj.extract().map_err(#transform_error)?,
                 None => #default,
             };
         }