Add FiniteElement python wrapper

python/dolfinx/fem/__init__.py

@@ -32,7 +32,7 @@
     locate_dofs_topological,
 )
 from dolfinx.fem.dofmap import DofMap
-from dolfinx.fem.element import CoordinateElement, coordinate_element
+from dolfinx.fem.element import CoordinateElement, FiniteElement, coordinate_element, finite_element
 from dolfinx.fem.forms import (
     Form,
     compile_form,
@@ -91,7 +91,11 @@ def create_interpolation_data(
     """
     return _PointOwnershipData(
         _create_interpolation_data(
-            V_to.mesh._cpp_object.geometry, V_to.element, V_from.mesh._cpp_object, cells, padding
+            V_to.mesh._cpp_object.geometry,
+            V_to.element._cpp_object,
+            V_from.mesh._cpp_object,
+            cells,
+            padding,
         )
     )
 
@@ -169,6 +173,7 @@ def compute_integration_domains(
     "DofMap",
     "ElementMetaData",
     "Expression",
+    "FiniteElement",
     "Form",
     "Function",
     "FunctionSpace",
@@ -189,6 +194,7 @@ def compute_integration_domains(
     "dirichletbc",
     "discrete_gradient",
     "extract_function_spaces",
+    "finite_element",
     "form",
     "form_cpp_class",
     "functionspace",

python/dolfinx/fem/element.py

@@ -1,4 +1,4 @@
-# Copyright (C) 2024 Garth N. Wells
+# Copyright (C) 2024 Garth N. Wells and Paul T. Kühner
 #
 # This file is part of DOLFINx (https://www.fenicsproject.org)
 #
@@ -12,6 +12,8 @@
 import numpy.typing as npt
 
 import basix
+import ufl
+import ufl.finiteelement
 from dolfinx import cpp as _cpp
 
 
@@ -160,3 +162,162 @@ def _(e: basix.finite_element.FiniteElement):
         return CoordinateElement(_cpp.fem.CoordinateElement_float32(e._e))
     except TypeError:
         return CoordinateElement(_cpp.fem.CoordinateElement_float64(e._e))
+
+
+class FiniteElement:
+    _cpp_object: typing.Union[_cpp.fem.FiniteElement_float32, _cpp.fem.FiniteElement_float64]
+
+    def __init__(self, cpp_object):
+        """Creates a Python wrapper for the exported finite element class.
+
+        Note:
+            Do not use this constructor directly. Instead use :func:`finite_element`.
+
+        Args:
+            The underlying cpp instance that this object will wrap.
+        """
+        self._cpp_object = cpp_object
+
+    def __eq__(self, other):
+        return self._cpp_object == other._cpp_object
+
+    @property
+    def dtype(self):
+        """Geometry type of the Mesh that the FunctionSpace is defined on."""
+        return self._cpp_object.dtype
+
+    @property
+    def basix_element(self):
+        """Return underlying Basix element (if it exists).
+
+        Raises:
+            Runtime error if Basix element does not exist.
+        """
+        return self._cpp_object.basix_element
+
+    @property
+    def num_sub_elements(self):
+        """Number of sub elements (for a mixed or blocked element)."""
+        return self._cpp_object.num_sub_elements
+
+    @property
+    def value_shape(self):
+        """Value shape of the finite element field.
+
+        The value shape describes the shape of the finite element field, e.g. `{}` for a scalar,
+        `{2}` for a vector in 2D, `{3, 3}` for a rank-2 tensor in 3D, etc.
+
+        Returns:
+            The value shape.
+        """
+        return self._cpp_object.value_shape
+
+    @property
+    def interpolation_points(self):
+        """Points on the reference cell at which an expression needs to be evaluated in order to
+        interpolate the expression in the finite element space.
+
+        Note:
+            For Lagrange elements the points will just be the nodal positions. For other elements
+            the points will typically be the quadrature points used to evaluate moment degrees of
+            freedom.
+
+        Returns:
+            Interpolation point coordinates on the reference cell, returning the (0) coordinates
+            data (row-major) storage and (1) the shape `(num_points, tdim)`.
+        """
+        return self._cpp_object.interpolation_points
+
+    @property
+    def interpolation_ident(self):
+        """Check if interpolation into the finite element space is an identity operation given the
+        evaluation on an expression at specific points, i.e. the degree-of-freedom are equal to
+        point evaluations. The function will return `true` for Lagrange elements.
+
+        Returns:
+            True if interpolation is an identity operation"""
+        return self._cpp_object.interpolation_ident
+
+    @property
+    def space_dimension(self):
+        """Dimension of the finite element function space (the number of degrees-of-freedom for the
+        element).
+
+        For 'blocked' elements, this function returns the dimension of the full element rather than
+        the dimension of the base element.
+
+        Returns:
+            Dimension of the finite element space.
+        """
+        return self._cpp_object.space_dimension
+
+    @property
+    def needs_dof_transformations(self):
+        """Check if DOF transformations are needed for this element.
+
+        DOF transformations will be needed for elements which might not be continuous when two
+        neighbouring cells disagree on the orientation of a shared sub-entity, and when this cannot
+        be corrected for by permuting the DOF numbering in the dofmap.
+
+        For example, Raviart-Thomas elements will need DOF transformations, as the neighbouring
+        cells may disagree on the orientation of a basis function, and this orientation cannot be
+        corrected for by permuting the DOF numbers on each cell.
+
+        Returns:
+            True if DOF transformations are required.
+        """
+        return self._cpp_object.needs_dof_transformations
+
+    @property
+    def signature(self):
+        """String identifying the finite element.
+
+        Returns:
+            Element signature
+        """
+        return self._cpp_object.signature
+
+    def T_apply(self, x, cell_permutations, dim):
+        """Transform basis functions from the reference element ordering and orientation to the
+        globally consistent physical element ordering and orientation.
+        """
+        self._cpp_object.T_apply(x, cell_permutations, dim)
+
+    def Tt_apply(self, x, cell_permutations, dim):
+        """Apply the transpose of the operator applied by T_apply()."""
+        self._cpp_object.Tt_apply(x, cell_permutations, dim)
+
+    def Tt_inv_apply(self, x, cell_permutations, dim):
+        """Apply the inverse transpose of the operator applied by T_apply()."""
+        self._cpp_object.Tt_apply(x, cell_permutations, dim)
+
+
+def finite_element(
+    cell_type: _cpp.mesh.CellType,
+    ufl_e: ufl.finiteelement,
+    dtype: np.dtype,
+) -> FiniteElement:
+    """Create a DOLFINx element from a basix.ufl element."""
+    if np.issubdtype(dtype, np.float32):
+        CppElement = _cpp.fem.FiniteElement_float32
+    elif np.issubdtype(dtype, np.float64):
+        CppElement = _cpp.fem.FiniteElement_float64
+    else:
+        raise ValueError(f"Unsupported dtype: {dtype}")
+
+    if ufl_e.is_mixed:
+        elements = [finite_element(cell_type, e, dtype)._cpp_object for e in ufl_e.sub_elements]
+        return FiniteElement(CppElement(elements))
+    elif ufl_e.is_quadrature:
+        return FiniteElement(
+            CppElement(
+                cell_type,
+                ufl_e.custom_quadrature()[0],
+                ufl_e.reference_value_shape,
+                ufl_e.is_symmetric,
+            )
+        )
+    else:
+        basix_e = ufl_e.basix_element._e
+        value_shape = ufl_e.reference_value_shape if ufl_e.block_size > 1 else None
+        return FiniteElement(CppElement(basix_e, value_shape, ufl_e.is_symmetric))

python/dolfinx/fem/function.py

@@ -18,6 +18,7 @@
 from dolfinx import cpp as _cpp
 from dolfinx import default_scalar_type, jit, la
 from dolfinx.fem import dofmap
+from dolfinx.fem.element import FiniteElement, finite_element
 from dolfinx.geometry import PointOwnershipData
 
 if typing.TYPE_CHECKING:
@@ -461,7 +462,7 @@ def _(e0: Expression):
             # u0 is callable
             assert callable(u0)
             x = _cpp.fem.interpolation_coords(
-                self._V.element, self._V.mesh.geometry._cpp_object, cells0
+                self._V.element._cpp_object, self._V.mesh.geometry._cpp_object, cells0
             )
             self._cpp_object.interpolate(np.asarray(u0(x), dtype=self.dtype), cells0)  # type: ignore
 
@@ -560,32 +561,6 @@ class ElementMetaData(typing.NamedTuple):
     symmetry: typing.Optional[bool] = None
 
 
-def _create_dolfinx_element(
-    cell_type: _cpp.mesh.CellType,
-    ufl_e: ufl.FiniteElementBase,
-    dtype: np.dtype,
-) -> typing.Union[_cpp.fem.FiniteElement_float32, _cpp.fem.FiniteElement_float64]:
-    """Create a DOLFINx element from a basix.ufl element."""
-    if np.issubdtype(dtype, np.float32):
-        CppElement = _cpp.fem.FiniteElement_float32
-    elif np.issubdtype(dtype, np.float64):
-        CppElement = _cpp.fem.FiniteElement_float64
-    else:
-        raise ValueError(f"Unsupported dtype: {dtype}")
-
-    if ufl_e.is_mixed:
-        elements = [_create_dolfinx_element(cell_type, e, dtype) for e in ufl_e.sub_elements]
-        return CppElement(elements)
-    elif ufl_e.is_quadrature:
-        return CppElement(
-            cell_type, ufl_e.custom_quadrature()[0], ufl_e.reference_value_shape, ufl_e.is_symmetric
-        )
-    else:
-        basix_e = ufl_e.basix_element._e
-        value_shape = ufl_e.reference_value_shape if ufl_e.block_size > 1 else None
-        return CppElement(basix_e, value_shape, ufl_e.is_symmetric)
-
-
 def functionspace(
     mesh: Mesh,
     element: typing.Union[ufl.FiniteElementBase, ElementMetaData, tuple[str, int, tuple, bool]],
@@ -614,18 +589,18 @@ def functionspace(
         raise ValueError("Non-matching UFL cell and mesh cell shapes.")
 
     # Create DOLFINx objects
-    cpp_element = _create_dolfinx_element(mesh.topology.cell_type, ufl_e, dtype)
-    cpp_dofmap = _cpp.fem.create_dofmap(mesh.comm, mesh.topology._cpp_object, cpp_element)
+    element = finite_element(mesh.topology.cell_type, ufl_e, dtype)
+    cpp_dofmap = _cpp.fem.create_dofmap(mesh.comm, mesh.topology._cpp_object, element._cpp_object)
 
     assert np.issubdtype(
-        mesh.geometry.x.dtype, cpp_element.dtype
+        mesh.geometry.x.dtype, element.dtype
     ), "Mesh and element dtype are not compatible."
 
     # Initialize the cpp.FunctionSpace
     try:
-        cppV = _cpp.fem.FunctionSpace_float64(mesh._cpp_object, cpp_element, cpp_dofmap)
+        cppV = _cpp.fem.FunctionSpace_float64(mesh._cpp_object, element._cpp_object, cpp_dofmap)
     except TypeError:
-        cppV = _cpp.fem.FunctionSpace_float32(mesh._cpp_object, cpp_element, cpp_dofmap)
+        cppV = _cpp.fem.FunctionSpace_float32(mesh._cpp_object, element._cpp_object, cpp_dofmap)
 
     return FunctionSpace(mesh, ufl_e, cppV)
 
@@ -746,11 +721,9 @@ def ufl_function_space(self) -> ufl.FunctionSpace:
         return self
 
     @property
-    def element(
-        self,
-    ) -> typing.Union[_cpp.fem.FiniteElement_float32, _cpp.fem.FiniteElement_float64]:
+    def element(self) -> FiniteElement:
         """Function space finite element."""
-        return self._cpp_object.element  # type: ignore
+        return FiniteElement(self._cpp_object.element)
 
     @property
     def dofmap(self) -> dofmap.DofMap: