macros: raise AttributeError on property deletion requests

The setter function will receive a NULL value on deletion requests.
This wasn't properly handled before, leading to a panic.

The new code raises AttributeError in this scenario instead.

A test for the behavior has been added. Documentation has also
been updated to reflect the behavior.

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Fixed
 
 - Restrict FFI definitions `PyGILState_Check` and `Py_tracefunc` to the unlimited API. [#1787](https://github.com/PyO3/pyo3/pull/1787)
+- Raise `AttributeError` to avoid panic when calling `del` on a `[setter]` defined class property. [#1775](https://github.com/PyO3/pyo3/issues/1775)
 
 ## [0.14.2] - 2021-08-09
 

guide/src/class.md

@@ -454,6 +454,10 @@ impl MyClass {
 
 In this case, the property `number` is defined and available from Python code as `self.number`.
 
+Attributes defined by `#[setter]` or `#[pyo3(set)]` will always raise `AttributeError` on `del`
+operations. Support for defining custom `del` behavior is tracked in
+[#1778](https://github.com/PyO3/pyo3/issues/1778).
+
 ## Instance methods
 
 To define a Python compatible method, an `impl` block for your struct has to be annotated with the

pyo3-macros-backend/src/pymethod.rs

@@ -218,7 +218,11 @@ pub fn impl_py_setter_def(cls: &syn::Type, property_type: PropertyType) -> Resul
                     ) -> std::os::raw::c_int {
                         pyo3::callback::handle_panic(|_py| {
                             #slf
-                            let _value = _py.from_borrowed_ptr::<pyo3::types::PyAny>(_value);
+                            let _value = _py
+                                .from_borrowed_ptr_or_opt(_value)
+                                .ok_or_else(|| {
+                                    pyo3::exceptions::PyAttributeError::new_err("can't delete attribute")
+                                })?;
                             let _val = pyo3::FromPyObject::extract(_value)?;
 
                             pyo3::callback::convert(_py, #setter_impl)

tests/test_getter_setter.rs

@@ -54,6 +54,8 @@ fn class_with_properties() {
     py_run!(py, inst, "inst.DATA = 20");
     py_run!(py, inst, "assert inst.get_num() == 20 == inst.DATA");
 
+    py_expect_exception!(py, inst, "del inst.DATA", PyAttributeError);
+
     py_run!(py, inst, "assert inst.get_num() == inst.unwrapped == 20");
     py_run!(py, inst, "inst.unwrapped = 42");
     py_run!(py, inst, "assert inst.get_num() == inst.unwrapped == 42");