#[pyclass(dict)] makes objects without __dict__

[temoto]

I tried to mitigate problem of storing &PyObject in pyo3 class struct by going back to Python, using __dict__ instance storage. Documentation mentions single-token attribute to py::class macros. I have studied code that uses this attribute and found it is sophisticated machinery, using special Python C-API construct in PyType header. Respect for special handling of __dict__, AFAIK you could get away with heap allocated PyDict and provide access via getter.

I guess it's some minor scholar mistake again, please point in right direction.

#[py::class(dict)]
struct Hub {
    token: PyToken,
}

#[py::methods]
impl Hub {
    #[new]
    fn __new__(obj: &PyRawObject, py: Python) -> PyResult<()> {
        obj.init(|token| Hub { token: token })
    }
}

% ../venv-27/bin/python -c 'import ext ; print(vars(ext.Hub()))
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: vars() argument must have __dict__ attribute

Darwin Kernel Version 16.7.0: Thu Jun 15 17:36:27 PDT 2017; root:xnu-3789.70.16~2/RELEASE_X86_64
rustc 1.25.0-nightly (27a046e93 2018-02-18)
Python 2.7.10
setuptools-rust 0.8.3

[Alexander-N]

I'd like to work on this, but I would need some hints on what needs to be done.

[konstin]

As far as I know, tp_dictoffsetin ffi::PyTypeObject needs to be set: https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_dictoffset

Code snippets that should already add __dict__ (but don't work):

pyo3/src/type_object.rs

Lines 323 to 327 in 6c58766

	// __dict__ support
	if T::FLAGS & PY_TYPE_FLAG_DICT != 0 {
	offset -= std::mem::size_of::<*const ffi::PyObject>();
	type_object.tp_dictoffset = offset as isize;
	}

pyo3/pyo3-derive-backend/src/pyclass.rs

Lines 278 to 299 in 6c58766

	// insert space for weak ref
	let mut has_weakref = false;
	let mut has_dict = false;
	for f in attr.flags.iter() {
	if let syn::Expr::Path(ref epath) = f {
	if epath.path == parse_quote! {::pyo3::type_object::PY_TYPE_FLAG_WEAKREF} {
	has_weakref = true;
	} else if epath.path == parse_quote! {::pyo3::type_object::PY_TYPE_FLAG_DICT} {
	has_dict = true;
	}
	}
	}
	let weakref = if has_weakref {
	quote! {std::mem::size_of::<*const ::pyo3::ffi::PyObject>()}
	} else {
	quote! {0}
	};
	let dict = if has_dict {
	quote! {std::mem::size_of::<*const ::pyo3::ffi::PyObject>()}
	} else {
	quote! {0}
	};