pyclass: mapping flag

CHANGELOG.md

@@ -30,6 +30,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - Added `as_bytes` on `Py<PyBytes>`. [#2235](https://github.com/PyO3/pyo3/pull/2235)
+- Add `#[pyclass(mapping)]` option to leave sequence slots empty in container implementations. [#2265](https://github.com/PyO3/pyo3/pull/2265)
 
 ### Packaging
 

guide/src/class.md

@@ -203,6 +203,7 @@ Python::with_gil(|py|{
 [params-4]: https://doc.rust-lang.org/stable/std/rc/struct.Rc.html
 [params-5]: https://doc.rust-lang.org/stable/std/sync/struct.Rc.html
 [params-6]: https://docs.python.org/3/library/weakref.html
+[params-mapping]: ./class/protocols.md#mapping--sequence-types
 
 These parameters are covered in various sections of this guide.
 

guide/src/class/protocols.md

@@ -192,6 +192,18 @@ and `Return` a final value - see its docs for further details and an example.
 
 ### Mapping & Sequence types
 
+The magic methods in this section can be used to implement Python container types. They are two main categories of container in Python: "mappings" such as `dict`, with arbitrary keys, and "sequences" such as `list` and `tuple`, with integer keys.
+
+The Python C-API which PyO3 is built upon has separate "slots" for sequences and mappings. When writing a `class` in pure Python, there is no such distinction in the implementation - a `__getitem__` implementation will fill the slots for both the mapping and sequence forms, for example.
+
+By default PyO3 reproduces the Python behaviour of filling both mapping and sequence slots. This makes sense for the "simple" case which matches Python, and also for sequences, where the mapping slot is used anyway to implement slice indexing.
+
+Mapping types usually will not want the sequence slots filled. Having them filled will lead to outcomes which may be unwanted, such as:
+- The mapping type will successfully cast to [`PySequence`]. This may lead to consumers of the type handling it incorrectly.
+- Python provides a default implementation of `__iter__` for sequences, which calls `__getitem__` with consecutive positive integers starting from 0 until an `IndexError` is returned. Unless the mapping only contains consecutive positive integer keys, this `__iter__` implementation will likely not be the intended behavior.
+
+Use the `#[pyclass(mapping)]` annotation to instruct PyO3 to only fill the mapping slots, leaving the sequence ones empty. This will apply to `__getitem__`, `__setitem__`, and `__delitem__`.
+
   - `__len__(<self>) -> usize`
 
     Implements the built-in function `len()` for the sequence.
@@ -265,7 +277,6 @@ and `Return` a final value - see its docs for further details and an example.
     Used by the `*=` operator, after trying the numeric multiplication via
     the `__imul__` method.
 
-
 ### Descriptors
 
   - `__get__(<self>, object, object) -> object`
@@ -599,3 +610,4 @@ For details, look at the `#[pymethods]` regarding GC methods.
 [`PyObjectProtocol`]: {{#PYO3_DOCS_URL}}/pyo3/class/basic/trait.PyObjectProtocol.html
 [`PySequenceProtocol`]: {{#PYO3_DOCS_URL}}/pyo3/class/sequence/trait.PySequenceProtocol.html
 [`PyIterProtocol`]: {{#PYO3_DOCS_URL}}/pyo3/class/iter/trait.PyIterProtocol.html
+[`PySequence`]: {{#PYO3_DOCS_URL}}/pyo3/types/struct.PySequence.html

pyo3-ffi/src/object.rs

@@ -337,6 +337,12 @@ extern "C" {
 // Flag bits for printing:
 pub const Py_PRINT_RAW: c_int = 1; // No string quotes etc.
 
+#[cfg(all(Py_3_10, not(Py_LIMITED_API)))]
+pub const Py_TPFLAGS_SEQUENCE: c_ulong = 1 << 5;
+
+#[cfg(all(Py_3_10, not(Py_LIMITED_API)))]
+pub const Py_TPFLAGS_MAPPING: c_ulong = 1 << 6;
+
 #[cfg(Py_3_10)]
 pub const Py_TPFLAGS_DISALLOW_INSTANTIATION: c_ulong = 1 << 7;
 

pyo3-macros-backend/src/attributes.rs

@@ -18,6 +18,7 @@ pub mod kw {
     syn::custom_keyword!(gc);
     syn::custom_keyword!(get);
     syn::custom_keyword!(item);
+    syn::custom_keyword!(mapping);
     syn::custom_keyword!(module);
     syn::custom_keyword!(name);
     syn::custom_keyword!(pass_module);

pyo3-macros-backend/src/pyclass.rs

@@ -54,6 +54,7 @@ pub struct PyClassPyO3Options {
     pub dict: Option<kw::dict>,
     pub extends: Option<ExtendsAttribute>,
     pub freelist: Option<FreelistAttribute>,
+    pub mapping: Option<kw::mapping>,
     pub module: Option<ModuleAttribute>,
     pub name: Option<NameAttribute>,
     pub subclass: Option<kw::subclass>,
@@ -69,6 +70,7 @@ enum PyClassPyO3Option {
     Dict(kw::dict),
     Extends(ExtendsAttribute),
     Freelist(FreelistAttribute),
+    Mapping(kw::mapping),
     Module(ModuleAttribute),
     Name(NameAttribute),
     Subclass(kw::subclass),
@@ -90,6 +92,8 @@ impl Parse for PyClassPyO3Option {
             input.parse().map(PyClassPyO3Option::Extends)
         } else if lookahead.peek(attributes::kw::freelist) {
             input.parse().map(PyClassPyO3Option::Freelist)
+        } else if lookahead.peek(attributes::kw::mapping) {
+            input.parse().map(PyClassPyO3Option::Mapping)
         } else if lookahead.peek(attributes::kw::module) {
             input.parse().map(PyClassPyO3Option::Module)
         } else if lookahead.peek(kw::name) {
@@ -145,6 +149,7 @@ impl PyClassPyO3Options {
             PyClassPyO3Option::Dict(dict) => set_option!(dict),
             PyClassPyO3Option::Extends(extends) => set_option!(extends),
             PyClassPyO3Option::Freelist(freelist) => set_option!(freelist),
+            PyClassPyO3Option::Mapping(mapping) => set_option!(mapping),
             PyClassPyO3Option::Module(module) => set_option!(module),
             PyClassPyO3Option::Name(name) => set_option!(name),
             PyClassPyO3Option::Subclass(subclass) => set_option!(subclass),
@@ -749,6 +754,7 @@ impl<'a> PyClassImplsBuilder<'a> {
             .map(|extends_attr| extends_attr.value.clone())
             .unwrap_or_else(|| parse_quote! { _pyo3::PyAny });
         let is_subclass = self.attr.options.extends.is_some();
+        let is_mapping: bool = self.attr.options.mapping.is_some();
 
         let dict_offset = if self.attr.options.dict.is_some() {
             quote! {
@@ -830,6 +836,7 @@ impl<'a> PyClassImplsBuilder<'a> {
                 const DOC: &'static str = #doc;
                 const IS_BASETYPE: bool = #is_basetype;
                 const IS_SUBCLASS: bool = #is_subclass;
+                const IS_MAPPING: bool = #is_mapping;
 
                 type Layout = _pyo3::PyCell<Self>;
                 type BaseType = #base;

pyo3-macros/docs/pyclass_parameters.md

@@ -6,6 +6,7 @@
 | `dict` | Gives instances of this class an empty `__dict__` to store custom attributes. |
 | <span style="white-space: pre">`extends = BaseType`</span>  | Use a custom baseclass. Defaults to [`PyAny`][params-1] |
 | <span style="white-space: pre">`freelist = N`</span> |  Implements a [free list][params-2] of size N. This can improve performance for types that are often created and deleted in quick succession. Profile your code to see whether `freelist` is right for you.  |
+| `mapping` |  Inform PyO3 that this class is a [`Mapping`][params-mapping], and so leave its implementation of sequence C-API slots empty. |
 | <span style="white-space: pre">`module = "module_name"`</span> |  Python code will see the class as being defined in this module. Defaults to `builtins`. |
 | <span style="white-space: pre">`name = "python_name"`</span> | Sets the name that Python sees this class as. Defaults to the name of the Rust struct. |
 | <span style="white-space: pre">`text_signature = "(arg1, arg2, ...)"`</span> |  Sets the text signature for the Python class' `__new__` method. |

pyo3-macros/src/lib.rs

@@ -93,6 +93,7 @@ pub fn pyproto(_: TokenStream, input: TokenStream) -> TokenStream {
 /// [params-4]: std::rc::Rc
 /// [params-5]: std::sync::Arc
 /// [params-6]: https://docs.python.org/3/library/weakref.html
+/// [params-mapping]: https://pyo3.rs/latest/class/protocols.html#mapping--sequence-types
 #[proc_macro_attribute]
 pub fn pyclass(attr: TokenStream, input: TokenStream) -> TokenStream {
     use syn::Item;

src/impl_/pyclass.rs

@@ -155,6 +155,9 @@ pub trait PyClassImpl: Sized {
     /// #[pyclass(extends=...)]
     const IS_SUBCLASS: bool = false;
 
+    /// #[pyclass(mapping)]
+    const IS_MAPPING: bool = false;
+
     /// Layout
     type Layout: PyLayout<Self>;
 

src/pyclass.rs

@@ -55,6 +55,7 @@ where
             T::weaklist_offset(),
             &T::for_all_items,
             T::IS_BASETYPE,
+            T::IS_MAPPING,
         )
     } {
         Ok(type_object) => type_object,
@@ -75,6 +76,7 @@ unsafe fn create_type_object_impl(
     weaklist_offset: Option<ffi::Py_ssize_t>,
     for_all_items: &dyn Fn(&mut dyn FnMut(&PyClassItems)),
     is_basetype: bool,
+    is_mapping: bool,
 ) -> PyResult<*mut ffi::PyTypeObject> {
     let mut slots = Vec::new();
 
@@ -147,26 +149,30 @@ unsafe fn create_type_object_impl(
         slots.extend_from_slice(items.slots);
     });
 
-    // If mapping methods implemented, define sequence methods get implemented too.
-    // CPython does the same for Python `class` statements.
+    if !is_mapping {
+        // If mapping methods implemented, define sequence methods get implemented too.
+        // CPython does the same for Python `class` statements.
 
-    // NB we don't implement sq_length to avoid annoying CPython behaviour of automatically adding
-    // the length to negative indices.
+        // NB we don't implement sq_length to avoid annoying CPython behaviour of automatically adding
+        // the length to negative indices.
 
-    if has_getitem {
-        push_slot(
-            &mut slots,
-            ffi::Py_sq_item,
-            get_sequence_item_from_mapping as _,
-        );
-    }
+        // Don't add these methods for "pure" mappings.
 
-    if has_setitem {
-        push_slot(
-            &mut slots,
-            ffi::Py_sq_ass_item,
-            assign_sequence_item_from_mapping as _,
-        );
+        if has_getitem {
+            push_slot(
+                &mut slots,
+                ffi::Py_sq_item,
+                get_sequence_item_from_mapping as _,
+            );
+        }
+
+        if has_setitem {
+            push_slot(
+                &mut slots,
+                ffi::Py_sq_ass_item,
+                assign_sequence_item_from_mapping as _,
+            );
+        }
     }
 
     if !has_new {

tests/test_mapping.rs

@@ -7,10 +7,12 @@ use pyo3::prelude::*;
 use pyo3::py_run;
 use pyo3::types::IntoPyDict;
 use pyo3::types::PyList;
+use pyo3::types::PyMapping;
+use pyo3::types::PySequence;
 
 mod common;
 
-#[pyclass]
+#[pyclass(mapping)]
 struct Mapping {
     index: HashMap<String, usize>,
 }
@@ -55,6 +57,13 @@ impl Mapping {
             Ok(())
         }
     }
+
+    fn get(&self, py: Python<'_>, key: &str, default: Option<PyObject>) -> Option<PyObject> {
+        self.index
+            .get(key)
+            .map(|value| value.into_py(py))
+            .or(default)
+    }
 }
 
 /// Return a dict with `m = Mapping(['1', '2', '3'])`.
@@ -103,3 +112,15 @@ fn test_delitem() {
     py_expect_exception!(py, *d, "del m[-1]", PyTypeError);
     py_expect_exception!(py, *d, "del m['4']", PyKeyError);
 }
+
+#[test]
+fn mapping_is_not_sequence() {
+    Python::with_gil(|py| {
+        let mut index = HashMap::new();
+        index.insert("Foo".into(), 1);
+        index.insert("Bar".into(), 2);
+        let m = Py::new(py, Mapping { index }).unwrap();
+        assert!(m.as_ref(py).downcast::<PyMapping>().is_ok());
+        assert!(m.as_ref(py).downcast::<PySequence>().is_err());
+    });
+}

tests/ui/invalid_pyclass_args.stderr

@@ -1,4 +1,4 @@
-error: expected one of: `crate`, `dict`, `extends`, `freelist`, `module`, `name`, `subclass`, `text_signature`, `unsendable`, `weakref`, `gc`
+error: expected one of: `crate`, `dict`, `extends`, `freelist`, `mapping`, `module`, `name`, `subclass`, `text_signature`, `unsendable`, `weakref`, `gc`
  --> tests/ui/invalid_pyclass_args.rs:3:11
   |
 3 | #[pyclass(extend=pyo3::types::PyDict)]
@@ -34,7 +34,7 @@ error: expected string literal
 18 | #[pyclass(module = my_module)]
    |                    ^^^^^^^^^
 
-error: expected one of: `crate`, `dict`, `extends`, `freelist`, `module`, `name`, `subclass`, `text_signature`, `unsendable`, `weakref`, `gc`
+error: expected one of: `crate`, `dict`, `extends`, `freelist`, `mapping`, `module`, `name`, `subclass`, `text_signature`, `unsendable`, `weakref`, `gc`
   --> tests/ui/invalid_pyclass_args.rs:21:11
    |
 21 | #[pyclass(weakrev)]