add CompareOp::convert

to easily implement `__richcmp__` as the result of a Rust `std::cmp::Ordering` comparison.

CHANGELOG.md

@@ -19,6 +19,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add FFI definitions `Py_fstring_input`, `sendfunc`, and `_PyErr_StackItem`. [#2423](https://github.com/PyO3/pyo3/pull/2423)
 - Add `PyDateTime::new_with_fold`, `PyTime::new_with_fold`, `PyTime::get_fold`, `PyDateTime::get_fold` for PyPy. [#2428](https://github.com/PyO3/pyo3/pull/2428)
 - Allow `#[classattr]` take `Python` argument. [#2383](https://github.com/PyO3/pyo3/issues/2383)
+- Add `CompareOp::convert` to easily implement `__richcmp__` as the result of a
+  Rust `std::cmp::Ordering` comparison. [#2460](https://github.com/PyO3/pyo3/pull/2460)
 
 ### Changed
 

guide/src/class/object.md

@@ -128,15 +128,15 @@ impl Number {
 Unlike in Python, PyO3 does not provide the magic comparison methods you might expect like `__eq__`,
  `__lt__` and so on. Instead you have to implement all six operations at once with `__richcmp__`.
 This method will be called with a value of `CompareOp` depending on the operation.
- 
+
 ```rust
 use pyo3::class::basic::CompareOp;
 
 # use pyo3::prelude::*;
-# 
+#
 # #[pyclass]
 # struct Number(i32);
-# 
+#
 #[pymethods]
 impl Number {
     fn __richcmp__(&self, other: &Self, op: CompareOp) -> PyResult<bool> {
@@ -152,6 +152,28 @@ impl Number {
 }
 ```
 
+If you obtain the result by comparing two Rust values, as in this example, you
+can take a shortcut using `CompareOp::convert`:
+
+```rust
+use pyo3::class::basic::CompareOp;
+
+# use pyo3::prelude::*;
+#
+# #[pyclass]
+# struct Number(i32);
+#
+#[pymethods]
+impl Number {
+    fn __richcmp__(&self, other: &Self, op: CompareOp) -> bool {
+        op.convert(self.0.cmp(&other.0))
+    }
+}
+```
+
+It converts the `std::cmp::Ordering` obtained from Rust's `Ord` class to the
+required result for the given `CompareOp`.
+
 ### Truthyness
 
 We'll consider `Number` to be `True` if it is nonzero:
@@ -229,4 +251,4 @@ fn my_module(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
 [`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
 [`Hasher`]: https://doc.rust-lang.org/std/hash/trait.Hasher.html
 [`DefaultHasher`]: https://doc.rust-lang.org/std/collections/hash_map/struct.DefaultHasher.html
-[SipHash]: https://en.wikipedia.org/wiki/SipHash
+[SipHash]: https://en.wikipedia.org/wiki/SipHash

guide/src/class/protocols.md

@@ -70,8 +70,11 @@ given signatures should be interpreted as follows:
     <details>
     <summary>Return type</summary>
     The return type will normally be `PyResult<bool>`, but any Python object can be returned.
-    If the `object` is not of the type specified in the signature, the generated code will
-    automatically `return NotImplemented`.
+    If the second argument `object` is not of the type specified in the
+    signature, the generated code will automatically `return NotImplemented`.
+
+    You can use [`CompareOp::convert`] to adapt a Rust `std::cmp::Ordering` result
+    to the requested comparison.
     </details>
 
   - `__getattr__(<self>, object) -> object`
@@ -611,3 +614,4 @@ For details, look at the `#[pymethods]` regarding GC methods.
 [`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
+[`CompareOp::convert`]: {{#PYO3_DOCS_URL}}/pyo3/pyclass/enum.CompareOp.html#method.convert

src/pyclass.rs

@@ -11,6 +11,7 @@ use crate::{
     IntoPy, IntoPyPointer, PyCell, PyErr, PyMethodDefType, PyObject, PyResult, PyTypeInfo, Python,
 };
 use std::{
+    cmp::Ordering,
     convert::TryInto,
     ffi::{CStr, CString},
     os::raw::{c_char, c_int, c_uint, c_void},
@@ -457,6 +458,7 @@ pub enum CompareOp {
 }
 
 impl CompareOp {
+    /// Conversion from the C enum.
     pub fn from_raw(op: c_int) -> Option<Self> {
         match op {
             ffi::Py_LT => Some(CompareOp::Lt),
@@ -468,6 +470,38 @@ impl CompareOp {
             _ => None,
         }
     }
+
+    /// Converts a Rust [`std::cmp::Ordering`] into the correct Boolean result for
+    /// this ordering query.
+    ///
+    /// Usage example:
+    ///
+    /// ```rust
+    /// # use pyo3::prelude::*;
+    /// # use pyo3::class::basic::CompareOp;
+    ///
+    /// #[pyclass]
+    /// struct Size {
+    ///     size: usize
+    /// }
+    ///
+    /// #[pymethods]
+    /// impl Size {
+    ///     fn __richcmp__(&self, other: &Size, op: CompareOp) -> bool {
+    ///         op.convert(self.size.cmp(&other.size))
+    ///     }
+    /// }
+    /// ```
+    pub fn convert(&self, result: Ordering) -> bool {
+        match self {
+            CompareOp::Eq => result == Ordering::Equal,
+            CompareOp::Ne => result != Ordering::Equal,
+            CompareOp::Lt => result == Ordering::Less,
+            CompareOp::Le => result != Ordering::Greater,
+            CompareOp::Gt => result == Ordering::Greater,
+            CompareOp::Ge => result != Ordering::Less,
+        }
+    }
 }
 
 /// Output of `__next__` which can either `yield` the next value in the iteration, or
@@ -578,3 +612,15 @@ pub(crate) unsafe extern "C" fn no_constructor_defined(
         ))
     })
 }
+
+mod tests {
+    #[test]
+    fn test_compare_op_convert() {
+        use super::CompareOp;
+        use std::cmp::Ordering;
+
+        assert!(CompareOp::Eq.convert(Ordering::Equal));
+        assert!(!CompareOp::Eq.convert(Ordering::Less));
+        assert!(CompareOp::Ge.convert(Ordering::Greater));
+    }
+}