Make VTK optional again (#4713)

VTK releases are sporadic so when macOS eagerly upgrades to a new Python
version it breaks our install for some time.

Author: connorjward

docs/source/install.rst

@@ -426,6 +426,20 @@ To install Firedrake with SLEPc support you should:
 
    $ pip install --no-binary h5py 'firedrake[check,slepc]'
 
+VTK
+~~~
+
+To install Firedrake with VTK, it should be installed using the ``vtk`` optional
+dependency. For example::
+
+   $ pip install --no-binary h5py 'firedrake[check,vtk]'
+
+.. warning::
+
+   VTK make releases sporadically so will not always support the latest version
+   of Python. This is commonly an issue on macOS where homebrew will use the
+   latest Python very soon after it is released. To fix this you should use
+   an older version of Python (e.g. 3.13 instead of 3.14).
 
 PyTorch
 ~~~~~~~

firedrake/output/vtk_output.py

@@ -347,7 +347,7 @@ def get_array(function):
     return array
 
 
-class VTKFile(object):
+class VTKFile:
     _header = (b'<?xml version="1.0" ?>\n'
                b'<VTKFile type="Collection" version="0.1" '
                b'byte_order="LittleEndian">\n'

pyproject.toml

@@ -42,7 +42,6 @@ dependencies = [
   "scipy",
   "sympy",
   "islpy>=2025.1.5; sys_platform == 'darwin'",
-  "vtk",
 ]
 classifiers = [
   "Development Status :: 5 - Production/Stable",
@@ -85,6 +84,7 @@ docs = [
   "sphinxcontrib-svg2pdfconverter",
   "sphinxcontrib-youtube",
   "sphinx-copybutton",
+  "vtk",  # needed to resolve API
 ]
 jax = [
   "jax",
@@ -98,6 +98,9 @@ slepc = [
 torch = [  # requires passing '--extra-index-url' to work
   "torch",
 ]
+vtk = [
+  "vtk",
+]
 
 # Dependencies needed to run the full test suite
 ci = [
@@ -117,6 +120,7 @@ ci = [
   "pytest-xdist",
   "slepc4py==3.24.0",
   "torch",  # requires passing '--extra-index-url' to work
+  "vtk",
 ]
 docker = [  # Used in firedrake-vanilla container
   "ipympl",  # needed for notebook testing
@@ -132,6 +136,7 @@ docker = [  # Used in firedrake-vanilla container
   "pytest-timeout",
   "pytest-xdist",
   "slepc4py==3.24.0",
+  "vtk",
 ]
 
 [build-system]

tests/firedrake/conftest.py

@@ -68,6 +68,14 @@ def _skip_test_dependency(dependency):
         except ImportError:
             return skip
 
+    elif dependency == "vtk":
+        try:
+            import vtk  # noqa: F401
+            del vtk
+            return not skip
+        except ImportError:
+            return skip
+
     elif dependency in ("mumps", "hypre"):
         return dependency not in get_external_packages()
 
@@ -83,6 +91,8 @@ def _skip_test_dependency(dependency):
     ("jax", "skipjax", "JAX is not installed"),
     ("matplotlib", "skipplot", "Matplotlib is not installed"),
     ("netgen", "skipnetgen", "Netgen and ngsPETSc are not installed"),
+    ("vtk", "skipvtk", "VTK is not installed"),
+
 )
 
 
@@ -118,6 +128,10 @@ def pytest_configure(config):
         "markers",
         "skipslepc: mark as skipped if slepc4py is not installed"
     )
+    config.addinivalue_line(
+        "markers",
+        "skipvtk: mark as skipped if vtk is not installed"
+    )
     config.addinivalue_line(
         "markers",
         "skiptorch: mark as skipped if PyTorch is not installed"

tests/firedrake/output/test_pvd_output.py

@@ -42,15 +42,13 @@ def pvd(dumpdir):
     return VTKFile(f)
 
 
+@pytest.mark.skipvtk
+@pytest.mark.parallel([1, 3])
 def test_can_save_coordinates(mesh, pvd):
     pvd.write(mesh.coordinates)
 
 
-@pytest.mark.parallel
-def test_can_save_coordinates_parallel(mesh, pvd):
-    pvd.write(mesh.coordinates)
-
-
+@pytest.mark.skipvtk
 @pytest.mark.parametrize("typ",
                          ["vector", "tensor", "tensor-3"])
 def test_bad_shape(typ, mesh, pvd):
@@ -67,11 +65,13 @@ def test_bad_shape(typ, mesh, pvd):
         pvd.write(f)
 
 
+@pytest.mark.skipvtk
 def test_bad_file_name(tmpdir):
     with pytest.raises(ValueError):
         VTKFile(str(tmpdir.join("foo.vtu")))
 
 
+@pytest.mark.skipvtk
 @pytest.mark.parametrize("space",
                          ["primal", "dual"])
 def test_different_functions(mesh, pvd, space):
@@ -89,6 +89,7 @@ def test_different_functions(mesh, pvd, space):
         pvd.write(g)
 
 
+@pytest.mark.skipvtk
 def test_multiple_functions(mesh, pvd):
     V = FunctionSpace(mesh, "DG", 0)
     P = FunctionSpace(mesh, "CG", 1)
@@ -104,6 +105,7 @@ def test_multiple_functions(mesh, pvd):
         pvd.write(g, f)
 
 
+@pytest.mark.skipvtk
 def test_different_meshes(mesh, pvd):
     V = VectorFunctionSpace(mesh, "DG", 1)
     f = Function(V)
@@ -114,6 +116,7 @@ def test_different_meshes(mesh, pvd):
         pvd.write(mesh.coordinates, mesh2.coordinates)
 
 
+@pytest.mark.skipvtk
 def test_bad_cell(pvd):
     mesh = UnitCubeMesh(1, 1, 1)
     mesh = ExtrudedMesh(mesh, layers=1)
@@ -122,6 +125,7 @@ def test_bad_cell(pvd):
         pvd.write(mesh.coordinates)
 
 
+@pytest.mark.skipvtk
 def test_not_function(mesh, pvd):
     c = Constant(1)
     with pytest.raises(ValueError):
@@ -134,6 +138,7 @@ def test_not_function(mesh, pvd):
         pvd.write(grad(f))
 
 
+@pytest.mark.skipvtk
 @pytest.mark.parametrize("space",
                          ["primal", "dual"])
 def test_append(mesh, tmpdir, space):