Add PyFixedString and PyFixedUnicode implementors of Element to support Unicode arrays whose element length is know at compile time.

Author: adamreichold

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Changelog
 
 - Unreleased
+  - Add support for ASCII (`PyFixedString<N>`) and Unicode (`PyFixedUnicode<N>`) string arrays, i.e. dtypes `SN` and `UN` where `N` is the number of characters. ([#378](https://github.com/PyO3/rust-numpy/pull/378))
 
 - v0.19.0
   - Add `PyUntypedArray` as an untyped base type for `PyArray` which can be used to inspect arguments before more targeted downcasts. This is accompanied by some methods like `dtype` and `shape` moving from `PyArray` to `PyUntypedArray`. They are still accessible though, as `PyArray` dereferences to `PyUntypedArray` via the `Deref` trait. ([#369](https://github.com/PyO3/rust-numpy/pull/369))

src/datetime.rs

@@ -223,8 +223,8 @@ impl TypeDescriptors {
     fn from_unit<'py>(&'py self, py: Python<'py>, unit: NPY_DATETIMEUNIT) -> &'py PyArrayDescr {
         let mut dtypes = self.dtypes.get(py).borrow_mut();
 
-        match dtypes.get_or_insert_with(Default::default).entry(unit) {
-            Entry::Occupied(entry) => entry.into_mut().clone().into_ref(py),
+        let dtype = match dtypes.get_or_insert_with(Default::default).entry(unit) {
+            Entry::Occupied(entry) => entry.into_mut(),
             Entry::Vacant(entry) => {
                 let dtype = PyArrayDescr::new_from_npy_type(py, self.npy_type);
 
@@ -237,9 +237,11 @@ impl TypeDescriptors {
                     metadata.meta.num = 1;
                 }
 
-                entry.insert(dtype.into()).clone().into_ref(py)
+                entry.insert(dtype.into())
             }
-        }
+        };
+
+        dtype.clone().into_ref(py)
     }
 }
 

src/lib.rs

@@ -82,6 +82,7 @@ mod dtype;
 mod error;
 pub mod npyffi;
 mod slice_container;
+mod strings;
 mod sum_products;
 mod untyped_array;
 
@@ -105,6 +106,7 @@ pub use crate::convert::{IntoPyArray, NpyIndex, ToNpyDims, ToPyArray};
 pub use crate::dtype::{dtype, Complex32, Complex64, Element, PyArrayDescr};
 pub use crate::error::{BorrowError, FromVecError, NotContiguousError};
 pub use crate::npyffi::{PY_ARRAY_API, PY_UFUNC_API};
+pub use crate::strings::{PyFixedString, PyFixedUnicode};
 pub use crate::sum_products::{dot, einsum, inner};
 pub use crate::untyped_array::PyUntypedArray;
 

src/strings.rs

@@ -0,0 +1,230 @@
+//! Types to support arrays of [ASCII][ascii] and [UCS4][ucs4] strings
+//!
+//! [ascii]: https://numpy.org/doc/stable/reference/c-api/dtype.html#c.NPY_STRING
+//! [ucs4]: https://numpy.org/doc/stable/reference/c-api/dtype.html#c.NPY_UNICODE
+
+use std::cell::RefCell;
+use std::collections::hash_map::Entry;
+use std::convert::TryInto;
+use std::fmt;
+use std::mem::size_of;
+use std::os::raw::c_char;
+use std::str;
+
+use pyo3::{
+    ffi::{Py_UCS1, Py_UCS4},
+    sync::GILProtected,
+    Py, Python,
+};
+use rustc_hash::FxHashMap;
+
+use crate::dtype::{Element, PyArrayDescr};
+use crate::npyffi::NPY_TYPES;
+
+/// A newtype wrapper around [`[u8; N]`][Py_UCS1] to handle [`byte` scalars][numpy-bytes] while satisfying coherence.
+///
+/// Note that when creating arrays of ASCII strings without an explicit `dtype`,
+/// NumPy will automatically determine the smallest possible array length at runtime.
+///
+/// For example,
+///
+/// ```python
+/// array = numpy.array([b"foo", b"bar", b"foobar"])
+/// ```
+///
+/// yields `S6` for `array.dtype`.
+///
+/// On the Rust side however, the length `N` of `PyFixedString<N>` must always be given
+/// explicitly and as a compile-time constant. For this work reliably, the Python code
+/// should set the `dtype` explicitly, e.g.
+///
+/// ```python
+/// numpy.array([b"foo", b"bar", b"foobar"], dtype='S12')
+/// ```
+///
+/// always matching `PyArray1<PyFixedString<12>>`.
+///
+/// # Example
+///
+/// ```rust
+/// # use pyo3::Python;
+/// use numpy::{PyArray1, PyFixedString};
+///
+/// # Python::with_gil(|py| {
+/// let array = PyArray1::<PyFixedString<3>>::from_vec(py, vec![[b'f', b'o', b'o'].into()]);
+///
+/// assert!(array.dtype().to_string().contains("S3"));
+/// # });
+/// ```
+///
+/// [numpy-bytes]: https://numpy.org/doc/stable/reference/arrays.scalars.html#numpy.bytes_
+#[repr(transparent)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct PyFixedString<const N: usize>(pub [Py_UCS1; N]);
+
+impl<const N: usize> fmt::Display for PyFixedString<N> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.write_str(str::from_utf8(&self.0).unwrap().trim_end_matches('\0'))
+    }
+}
+
+impl<const N: usize> From<[Py_UCS1; N]> for PyFixedString<N> {
+    fn from(val: [Py_UCS1; N]) -> Self {
+        Self(val)
+    }
+}
+
+unsafe impl<const N: usize> Element for PyFixedString<N> {
+    const IS_COPY: bool = true;
+
+    fn get_dtype(py: Python) -> &PyArrayDescr {
+        static DTYPES: TypeDescriptors = TypeDescriptors::new();
+
+        unsafe { DTYPES.from_size(py, NPY_TYPES::NPY_STRING, b'|' as _, size_of::<Self>()) }
+    }
+}
+
+/// A newtype wrapper around [`[PyUCS4; N]`][Py_UCS4] to handle [`str_` scalars][numpy-str] while satisfying coherence.
+///
+/// Note that when creating arrays of Unicode strings without an explicit `dtype`,
+/// NumPy will automatically determine the smallest possible array length at runtime.
+///
+/// For example,
+///
+/// ```python
+/// numpy.array(["foo🐍", "bar🦀", "foobar"])
+/// ```
+///
+/// yields `U6` for `array.dtype`.
+///
+/// On the Rust side however, the length `N` of `PyFixedUnicode<N>` must always be given
+/// explicitly and as a compile-time constant. For this work reliably, the Python code
+/// should set the `dtype` explicitly, e.g.
+///
+/// ```python
+/// numpy.array(["foo🐍", "bar🦀", "foobar"], dtype='U12')
+/// ```
+///
+/// always matching `PyArray1<PyFixedUnicode<12>>`.
+///
+/// # Example
+///
+/// ```rust
+/// # use pyo3::Python;
+/// use numpy::{PyArray1, PyFixedUnicode};
+///
+/// # Python::with_gil(|py| {
+/// let array = PyArray1::<PyFixedUnicode<3>>::from_vec(py, vec![[b'b' as _, b'a' as _, b'r' as _].into()]);
+///
+/// assert!(array.dtype().to_string().contains("U3"));
+/// # });
+/// ```
+///
+/// [numpy-str]: https://numpy.org/doc/stable/reference/arrays.scalars.html#numpy.str_
+#[repr(transparent)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct PyFixedUnicode<const N: usize>(pub [Py_UCS4; N]);
+
+impl<const N: usize> fmt::Display for PyFixedUnicode<N> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        for character in self.0 {
+            if character == 0 {
+                break;
+            }
+
+            write!(fmt, "{}", char::from_u32(character).unwrap())?;
+        }
+
+        Ok(())
+    }
+}
+
+impl<const N: usize> From<[Py_UCS4; N]> for PyFixedUnicode<N> {
+    fn from(val: [Py_UCS4; N]) -> Self {
+        Self(val)
+    }
+}
+
+unsafe impl<const N: usize> Element for PyFixedUnicode<N> {
+    const IS_COPY: bool = true;
+
+    fn get_dtype(py: Python) -> &PyArrayDescr {
+        static DTYPES: TypeDescriptors = TypeDescriptors::new();
+
+        unsafe { DTYPES.from_size(py, NPY_TYPES::NPY_UNICODE, b'=' as _, size_of::<Self>()) }
+    }
+}
+
+struct TypeDescriptors {
+    #[allow(clippy::type_complexity)]
+    dtypes: GILProtected<RefCell<Option<FxHashMap<usize, Py<PyArrayDescr>>>>>,
+}
+
+impl TypeDescriptors {
+    const fn new() -> Self {
+        Self {
+            dtypes: GILProtected::new(RefCell::new(None)),
+        }
+    }
+
+    /// `npy_type` must be either `NPY_STRING` or `NPY_UNICODE` with matching `byteorder` and `size`
+    #[allow(clippy::wrong_self_convention)]
+    unsafe fn from_size<'py>(
+        &'py self,
+        py: Python<'py>,
+        npy_type: NPY_TYPES,
+        byteorder: c_char,
+        size: usize,
+    ) -> &'py PyArrayDescr {
+        let mut dtypes = self.dtypes.get(py).borrow_mut();
+
+        let dtype = match dtypes.get_or_insert_with(Default::default).entry(size) {
+            Entry::Occupied(entry) => entry.into_mut(),
+            Entry::Vacant(entry) => {
+                let dtype = PyArrayDescr::new_from_npy_type(py, npy_type);
+
+                let descr = &mut *dtype.as_dtype_ptr();
+                descr.elsize = size.try_into().unwrap();
+                descr.byteorder = byteorder;
+
+                entry.insert(dtype.into())
+            }
+        };
+
+        dtype.clone().into_ref(py)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn format_fixed_string() {
+        assert_eq!(
+            PyFixedString([b'f', b'o', b'o', 0, 0, 0]).to_string(),
+            "foo"
+        );
+        assert_eq!(
+            PyFixedString([b'f', b'o', b'o', b'b', b'a', b'r']).to_string(),
+            "foobar"
+        );
+    }
+
+    #[test]
+    fn format_fixed_unicode() {
+        assert_eq!(
+            PyFixedUnicode([b'f' as _, b'o' as _, b'o' as _, 0, 0, 0]).to_string(),
+            "foo"
+        );
+        assert_eq!(
+            PyFixedUnicode([0x1F980, 0x1F40D, 0, 0, 0, 0]).to_string(),
+            "🦀🐍"
+        );
+        assert_eq!(
+            PyFixedUnicode([b'f' as _, b'o' as _, b'o' as _, b'b' as _, b'a' as _, b'r' as _])
+                .to_string(),
+            "foobar"
+        );
+    }
+}