Also relax the PySequence check when extracting fixed-sized arrays.

newsfragments/2675.fixed.md

@@ -0,0 +1 @@
+`impl FromPyObject for [T; N]` will accept anything passing `PySequence_Check`, e.g. NumPy arrays, in the same way that `impl FromPyObject for Vec<T>` was modified after [#2477](https://github.com/PyO3/pyo3/pull/2477).

pytests/src/sequence.rs

@@ -6,6 +6,11 @@ fn vec_to_vec_i32(vec: Vec<i32>) -> Vec<i32> {
     vec
 }
 
+#[pyfunction]
+fn array_to_array_i32(arr: [i32; 3]) -> [i32; 3] {
+    arr
+}
+
 #[pyfunction]
 fn vec_to_vec_pystring(vec: Vec<&PyString>) -> Vec<&PyString> {
     vec
@@ -14,6 +19,7 @@ fn vec_to_vec_pystring(vec: Vec<&PyString>) -> Vec<&PyString> {
 #[pymodule]
 pub fn sequence(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
     m.add_function(wrap_pyfunction!(vec_to_vec_i32, m)?)?;
+    m.add_function(wrap_pyfunction!(array_to_array_i32, m)?)?;
     m.add_function(wrap_pyfunction!(vec_to_vec_pystring, m)?)?;
     Ok(())
 }

pytests/tests/test_sequence.py

@@ -29,3 +29,9 @@ def test_vec_from_array():
     import numpy
 
     assert sequence.vec_to_vec_i32(numpy.array([1, 2, 3])) == [1, 2, 3]
+
+
+def test_rust_array_from_array():
+    import numpy
+
+    assert sequence.array_to_array_i32(numpy.array([1, 2, 3])) == [1, 2, 3]

src/conversions/array.rs

@@ -3,9 +3,11 @@ use crate::{exceptions, PyErr};
 #[cfg(min_const_generics)]
 mod min_const_generics {
     use super::invalid_sequence_length;
-    use crate::conversion::IntoPyPointer;
+    use crate::conversion::{AsPyPointer, IntoPyPointer};
+    use crate::types::PySequence;
     use crate::{
-        ffi, FromPyObject, IntoPy, Py, PyAny, PyObject, PyResult, PyTryFrom, Python, ToPyObject,
+        ffi, FromPyObject, IntoPy, Py, PyAny, PyDowncastError, PyObject, PyResult, PyTryFrom,
+        Python, ToPyObject,
     };
 
     impl<T, const N: usize> IntoPy<PyObject> for [T; N]
@@ -61,8 +63,16 @@ mod min_const_generics {
     where
         T: FromPyObject<'s>,
     {
-        let seq = <crate::types::PySequence as PyTryFrom>::try_from(obj)?;
-        let seq_len = seq.len()? as usize;
+        // Types that pass `PySequence_Check` usually implement enough of the sequence protocol
+        // to support this function and if not, we will only fail extraction safely.
+        let seq = unsafe {
+            if ffi::PySequence_Check(obj.as_ptr()) != 0 {
+                <PySequence as PyTryFrom>::try_from_unchecked(obj)
+            } else {
+                return Err(PyDowncastError::new(obj, "Sequence").into());
+            }
+        };
+        let seq_len = seq.len()?;
         if seq_len != N {
             return Err(invalid_sequence_length(N, seq_len));
         }
@@ -174,9 +184,11 @@ mod min_const_generics {
 #[cfg(not(min_const_generics))]
 mod array_impls {
     use super::invalid_sequence_length;
-    use crate::conversion::IntoPyPointer;
+    use crate::conversion::{AsPyPointer, IntoPyPointer};
+    use crate::types::PySequence;
     use crate::{
-        ffi, FromPyObject, IntoPy, Py, PyAny, PyObject, PyResult, PyTryFrom, Python, ToPyObject,
+        ffi, FromPyObject, IntoPy, Py, PyAny, PyDowncastError, PyObject, PyResult, PyTryFrom,
+        Python, ToPyObject,
     };
     use std::mem::{transmute_copy, ManuallyDrop};
 
@@ -274,8 +286,16 @@ mod array_impls {
     where
         T: FromPyObject<'s>,
     {
-        let seq = <crate::types::PySequence as PyTryFrom>::try_from(obj)?;
-        let seq_len = seq.len()? as usize;
+        // Types that pass `PySequence_Check` usually implement enough of the sequence protocol
+        // to support this function and if not, we will only fail extraction safely.
+        let seq = unsafe {
+            if ffi::PySequence_Check(obj.as_ptr()) != 0 {
+                <PySequence as PyTryFrom>::try_from_unchecked(obj)
+            } else {
+                return Err(PyDowncastError::new(obj, "Sequence").into());
+            }
+        };
+        let seq_len = seq.len()?;
         if seq_len != slice.len() {
             return Err(invalid_sequence_length(slice.len(), seq_len));
         }

src/types/sequence.rs

@@ -309,7 +309,7 @@ where
         }
     };
 
-    let mut v = Vec::with_capacity(seq.len().unwrap_or(0) as usize);
+    let mut v = Vec::with_capacity(seq.len().unwrap_or(0));
     for item in seq.iter()? {
         v.push(item?.extract::<T>()?);
     }