Pass detailed error message to python when parsing an argument fails

src/impl_/extract_argument.rs

@@ -101,9 +101,21 @@ pub fn from_py_with_with_default<'py, T>(
 /// single string.)
 #[doc(hidden)]
 #[cold]
-pub fn argument_extraction_error(py: Python, arg_name: &str, error: PyErr) -> PyErr {
+pub fn argument_extraction_error(py: Python, arg_name: &str, mut error: PyErr) -> PyErr {
     if error.get_type(py) == PyTypeError::type_object(py) {
-        PyTypeError::new_err(format!("argument '{}': {}", arg_name, error.value(py)))
+        let mut reason = error
+            .value(py)
+            .str()
+            .unwrap_or_else(|_| PyString::new(py, ""))
+            .to_string();
+
+        while let Some(cause) = error.cause(py) {
+            reason.push_str(":\n\t");
+            reason.push_str(&cause.to_string());
+            error = cause
+        }
+
+        PyTypeError::new_err(format!("argument '{}': {}", arg_name, reason))
     } else {
         error
     }

tests/test_pyfunction.rs

@@ -165,11 +165,21 @@ fn test_function_with_custom_conversion_error() {
     );
 }
 
+#[derive(Debug, FromPyObject)]
+struct ValueClass {
+    value: usize,
+}
+
 #[pyfunction]
-fn conversion_error(str_arg: &str, int_arg: i64, tuple_arg: (&str, f64), option_arg: Option<i64>) {
+fn conversion_error(str_arg: &str,
+                    int_arg: i64,
+                    tuple_arg: (&str, f64),
+                    option_arg: Option<i64>,
+                    struct_arg: Option<ValueClass>
+) {
     println!(
-        "{:?} {:?} {:?} {:?}",
-        str_arg, int_arg, tuple_arg, option_arg
+        "{:?} {:?} {:?} {:?} {:?}",
+        str_arg, int_arg, tuple_arg, option_arg, struct_arg
     );
 }
 
@@ -182,38 +192,62 @@ fn test_conversion_error() {
     py_expect_exception!(
         py,
         conversion_error,
-        "conversion_error(None, None, None, None)",
+        "conversion_error(None, None, None, None, None)",
         PyTypeError,
         "argument 'str_arg': 'NoneType' object cannot be converted to 'PyString'"
     );
     py_expect_exception!(
         py,
         conversion_error,
-        "conversion_error(100, None, None, None)",
+        "conversion_error(100, None, None, None, None)",
         PyTypeError,
         "argument 'str_arg': 'int' object cannot be converted to 'PyString'"
     );
     py_expect_exception!(
         py,
         conversion_error,
-        "conversion_error('string1', 'string2', None, None)",
+        "conversion_error('string1', 'string2', None, None, None)",
         PyTypeError,
         "argument 'int_arg': 'str' object cannot be interpreted as an integer"
     );
     py_expect_exception!(
         py,
         conversion_error,
-        "conversion_error('string1', -100, 'string2', None)",
+        "conversion_error('string1', -100, 'string2', None, None)",
         PyTypeError,
         "argument 'tuple_arg': 'str' object cannot be converted to 'PyTuple'"
     );
     py_expect_exception!(
         py,
         conversion_error,
-        "conversion_error('string1', -100, ('string2', 10.), 'string3')",
+        "conversion_error('string1', -100, ('string2', 10.), 'string3', None)",
         PyTypeError,
         "argument 'option_arg': 'str' object cannot be interpreted as an integer"
     );
+    py_expect_exception!(
+        py,
+        conversion_error,
+        "
+class ValueClass:
+    def __init__(self, value):
+        self.value = value
+conversion_error('string1', -100, ('string2', 10.), None, ValueClass(\"no_expected_type\"))",
+        PyTypeError,
+        "argument 'struct_arg': failed to extract field ValueClass.value:\n\tTypeError: 'str' \
+        object cannot be interpreted as an integer"
+    );
+    py_expect_exception!(
+        py,
+        conversion_error,
+        "
+class ValueClass:
+    def __init__(self, value):
+        self.value = value
+conversion_error('string1', -100, ('string2', 10.), None, ValueClass(-5))",
+        PyTypeError,
+        "argument 'struct_arg': failed to extract field ValueClass.value:\n\tOverflowError: can't \
+        convert negative int to unsigned"
+    );
 }
 
 #[test]