Add PyArrayDescr::new() constructor

src/dtype.rs

@@ -2,6 +2,7 @@ use std::mem::size_of;
 use std::os::raw::{
     c_char, c_int, c_long, c_longlong, c_short, c_uint, c_ulong, c_ulonglong, c_ushort,
 };
+use std::ptr;
 
 use num_traits::{Bounded, Zero};
 use pyo3::{
@@ -57,6 +58,23 @@ pub fn dtype<T: Element>(py: Python) -> &PyArrayDescr {
 }
 
 impl PyArrayDescr {
+    /// Creates a new dtype object from an arbitrary object.
+    ///
+    /// Equivalent to invoking the constructor of `np.dtype`.
+    pub fn new<'py, T: ToPyObject + ?Sized>(py: Python<'py>, obj: &T) -> PyResult<&'py Self> {
+        Self::new_impl(py, obj.to_object(py))
+    }
+
+    #[inline]
+    fn new_impl<'py>(py: Python<'py>, obj: PyObject) -> PyResult<&'py Self> {
+        let mut descr: *mut PyArray_Descr = ptr::null_mut();
+        unsafe {
+            // None is an invalid input here and is not converted to NPY_DEFAULT_TYPE
+            PY_ARRAY_API.PyArray_DescrConverter2(obj.as_ptr(), &mut descr as *mut _);
+            py.from_owned_ptr_or_err(descr as _)
+        }
+    }
+
     /// Returns `self` as `*mut PyArray_Descr`.
     pub fn as_dtype_ptr(&self) -> *mut PyArray_Descr {
         self.as_ptr() as _
@@ -423,6 +441,19 @@ mod tests {
     use super::{dtype, Complex32, Complex64, Element, PyArrayDescr};
     use crate::npyffi::{NPY_ALIGNED_STRUCT, NPY_ITEM_HASOBJECT, NPY_NEEDS_PYAPI, NPY_TYPES};
 
+    #[test]
+    fn test_dtype_new() {
+        pyo3::Python::with_gil(|py| {
+            assert_eq!(PyArrayDescr::new(py, "float64").unwrap(), dtype::<f64>(py));
+            let d = PyArrayDescr::new(py, [("a", "O"), ("b", "?")].as_ref()).unwrap();
+            assert_eq!(d.names(), Some(vec!["a", "b"]));
+            assert!(d.has_object());
+            assert_eq!(d.get_field("a").unwrap().0, dtype::<PyObject>(py));
+            assert_eq!(d.get_field("b").unwrap().0, dtype::<bool>(py));
+            assert!(PyArrayDescr::new(py, &123_usize).is_err());
+        });
+    }
+
     #[test]
     fn test_dtype_names() {
         fn type_name<T: Element>(py: pyo3::Python) -> &str {