pyclass #[new]: allow using custom error type

CHANGELOG.md

@@ -48,6 +48,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fix FFI definitions for `PyObject_Vectorcall` and `PyVectorcall_Call`. [#1287](https://github.com/PyO3/pyo3/pull/1285)
 - Fix building with Anaconda python inside a virtualenv. [#1290](https://github.com/PyO3/pyo3/pull/1290)
 - Fix definition of opaque FFI types. [#1312](https://github.com/PyO3/pyo3/pull/1312)
+- Fix using custom error type in pyclass `#[new]` methods. [#1319](https://github.com/PyO3/pyo3/pull/1319)
 
 ## [0.12.4] - 2020-11-28
 ### Fixed

pyo3-derive-backend/src/pymethod.rs

@@ -176,6 +176,7 @@ pub fn impl_wrap_new(cls: &syn::Type, spec: &FnSpec<'_>) -> TokenStream {
             _kwargs: *mut pyo3::ffi::PyObject) -> *mut pyo3::ffi::PyObject
         {
             use pyo3::type_object::PyTypeInfo;
+            use pyo3::callback::IntoPyCallbackOutput;
             use std::convert::TryFrom;
 
             const _LOCATION: &'static str = concat!(stringify!(#cls),".",stringify!(#python_name),"()");
@@ -183,7 +184,7 @@ pub fn impl_wrap_new(cls: &syn::Type, spec: &FnSpec<'_>) -> TokenStream {
                 let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
                 let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
 
-                let initializer = pyo3::PyClassInitializer::try_from(#body)?;
+                let initializer: pyo3::PyClassInitializer::<#cls> = #body.convert(_py)?;
                 let cell = initializer.create_cell_from_subtype(_py, subtype)?;
                 Ok(cell as *mut pyo3::ffi::PyObject)
             })

src/pyclass_init.rs

@@ -1,7 +1,7 @@
 //! Initialization utilities for `#[pyclass]`.
+use crate::callback::IntoPyCallbackOutput;
 use crate::type_object::{PyBorrowFlagLayout, PyLayout, PySizedLayout, PyTypeInfo};
-use crate::{PyCell, PyClass, PyErr, PyResult, Python};
-use std::convert::TryFrom;
+use crate::{PyCell, PyClass, PyResult, Python};
 use std::marker::PhantomData;
 
 /// Initializer for Python types.
@@ -182,16 +182,14 @@ where
     }
 }
 
-// Implementation which propagates the error from input PyResult. Useful in proc macro
-// code where `#[new]` may or may not return PyResult.
-impl<T, U> TryFrom<PyResult<U>> for PyClassInitializer<T>
+// Implementation used by proc macros to allow anything convertible to PyClassInitializer<T> to be
+// the return value of pyclass #[new] method (optionally wrapped in `Result<U, E>`).
+impl<T, U> IntoPyCallbackOutput<PyClassInitializer<T>> for U
 where
     T: PyClass,
     U: Into<PyClassInitializer<T>>,
 {
-    type Error = PyErr;
-
-    fn try_from(result: PyResult<U>) -> PyResult<Self> {
-        result.map(Into::into)
+    fn convert(self, _py: Python) -> PyResult<PyClassInitializer<T>> {
+        Ok(self.into())
     }
 }

tests/test_class_new.rs

@@ -1,3 +1,4 @@
+use pyo3::exceptions::PyValueError;
 use pyo3::prelude::*;
 
 #[pyclass]
@@ -120,3 +121,32 @@ assert c.from_rust is False
         .map_err(|e| e.print(py))
         .unwrap();
 }
+
+#[pyclass]
+#[derive(Debug)]
+struct NewWithCustomError {}
+
+struct CustomError;
+
+impl From<CustomError> for PyErr {
+    fn from(_error: CustomError) -> PyErr {
+        PyValueError::new_err("custom error")
+    }
+}
+
+#[pymethods]
+impl NewWithCustomError {
+    #[new]
+    fn new() -> Result<NewWithCustomError, CustomError> {
+        Err(CustomError)
+    }
+}
+
+#[test]
+fn new_with_custom_error() {
+    let gil = Python::acquire_gil();
+    let py = gil.python();
+    let typeobj = py.get_type::<NewWithCustomError>();
+    let err = typeobj.call0().unwrap_err();
+    assert_eq!(err.to_string(), "ValueError: custom error");
+}