Added an as_bytes method for Py<PyBytes>

This allows for obtaining a slice that's not lexically bound to the GIL which can be helpful to avoid copying.

CHANGELOG.md

@@ -14,6 +14,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Panic during compilation when `PYO3_CROSS_LIB_DIR` is set for some host/target combinations. [#2232](https://github.com/PyO3/pyo3/pull/2232)
 
+### Added
+
+- Added `as_bytes` on `Py<PyBytes>`. [#2235](https://github.com/PyO3/pyo3/pull/2235)
+
 ## [0.16.2] - 2022-03-15
 
 ### Packaging

src/types/bytes.rs

@@ -97,6 +97,24 @@ impl PyBytes {
     }
 }
 
+impl Py<PyBytes> {
+    /// Gets the Python bytes as a byte slice. Because Python bytes are
+    /// immutable, the result may be used for as long as the reference to
+    /// `self` is held.
+    pub fn as_bytes(&self, _py: Python<'_>) -> &[u8] {
+        // py is required here because `PyBytes_AsString` and `PyBytes_Size`
+        // can both technically raise exceptions which require the GIL to be
+        // held. The only circumstance in which they raise is if the value
+        // isn't really a `PyBytes`, but better safe than sorry.
+        unsafe {
+            let buffer = ffi::PyBytes_AsString(self.as_ptr()) as *const u8;
+            let length = ffi::PyBytes_Size(self.as_ptr()) as usize;
+            debug_assert!(!buffer.is_null());
+            std::slice::from_raw_parts(buffer, length)
+        }
+    }
+}
+
 /// This is the same way [Vec] is indexed.
 impl<I: SliceIndex<[u8]>> Index<I> for PyBytes {
     type Output = I::Output;

tests/test_bytes.rs

@@ -41,3 +41,18 @@ fn test_bytearray_vec_conversion() {
     let f = wrap_pyfunction!(bytes_vec_conversion)(py).unwrap();
     py_assert!(py, f, "f(bytearray(b'Hello World')) == b'Hello World'");
 }
+
+#[test]
+fn test_py_as_bytes() {
+    let pyobj: pyo3::Py<pyo3::types::PyBytes>;
+    let data: &[u8];
+
+    {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        pyobj = pyo3::types::PyBytes::new(py, b"abc").into_py(py);
+        data = pyobj.as_bytes(py);
+    }
+
+    assert_eq!(data, b"abc");
+}