Pass detailed error message to python when parsing an argument fails

CHANGELOG.md

@@ -72,6 +72,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - `PyDate_FromTimestamp`
 - Deprecate the `gc` option for `pyclass` (e.g. `#[pyclass(gc)]`). Just implement a `__traverse__` `#[pymethod]`. [#2159](https://github.com/PyO3/pyo3/pull/2159)
 - The `ml_meth` field of `PyMethodDef` is now represented by the `PyMethodDefPointer` union [2166](https://github.com/PyO3/pyo3/pull/2166)
+- The `PyTypeError` thrown when argument parsing failed now contains the nested causes [2177](https://github.com/PyO3/pyo3/pull/2178)
 
 ### Removed
 

src/impl_/extract_argument.rs

@@ -103,7 +103,10 @@ pub fn from_py_with_with_default<'py, T>(
 #[cold]
 pub fn argument_extraction_error(py: Python, arg_name: &str, error: PyErr) -> PyErr {
     if error.get_type(py) == PyTypeError::type_object(py) {
-        PyTypeError::new_err(format!("argument '{}': {}", arg_name, error.value(py)))
+        let remapped_error =
+            PyTypeError::new_err(format!("argument '{}': {}", arg_name, error.value(py)));
+        remapped_error.set_cause(py, error.cause(py));
+        remapped_error
     } else {
         error
     }

tests/test_pyfunction.rs

@@ -165,11 +165,24 @@ fn test_function_with_custom_conversion_error() {
     );
 }
 
+#[pyclass]
+#[derive(Debug, FromPyObject)]
+struct ValueClass {
+    #[pyo3(get)]
+    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 +195,82 @@ 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"
     );
+    let exception = 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
+    );
+    assert_eq!(
+        extract_traceback(py, exception),
+        "TypeError: argument 'struct_arg': failed to \
+    extract field ValueClass.value: TypeError: 'str' object cannot be interpreted as an integer"
+    );
+
+    let exception = 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
+    );
+    assert_eq!(
+        extract_traceback(py, exception),
+        "TypeError: argument 'struct_arg': failed to \
+    extract field ValueClass.value: OverflowError: can't convert negative int to unsigned"
+    );
+}
+
+/// Helper function that concatenates the error message from
+/// each error in the traceback into a single string that can
+/// be tested.
+fn extract_traceback(py: Python, mut error: PyErr) -> String {
+    let mut error_msg = error.to_string();
+    while let Some(cause) = error.cause(py) {
+        error_msg.push_str(": ");
+        error_msg.push_str(&cause.to_string());
+        error = cause
+    }
+    error_msg
 }
 
 #[test]