Make pyssht and healpy optional dependencies

notebooks/JAX_HEALPix_frontend.ipynb

@@ -19,9 +19,9 @@
     "import sys\n",
     "IN_COLAB = 'google.colab' in sys.modules\n",
     "\n",
-    "# Install s2fft and data if running on google colab.\n",
+    "# Install s2fft and healpy if running on google colab.\n",
     "if IN_COLAB:\n",
-    "    !pip install s2fft &> /dev/null"
+    "    !pip install s2fft healpy &> /dev/null"
    ]
   },
   {

notebooks/JAX_SSHT_frontend.ipynb

@@ -19,9 +19,9 @@
     "import sys\n",
     "IN_COLAB = 'google.colab' in sys.modules\n",
     "\n",
-    "# Install s2fft and data if running on google colab.\n",
+    "# Install s2fft and pyssht if running on google colab.\n",
     "if IN_COLAB:\n",
-    "    !pip install s2fft &> /dev/null"
+    "    !pip install s2fft pyssht &> /dev/null"
    ]
   },
   {

pyproject.toml

@@ -27,15 +27,12 @@ classifiers = [
 ]
 description = "Differentiable and accelerated spherical transforms with JAX"
 dependencies = [
-    "numpy>=1.20,<2",
+    "numpy>=1.20",
     "colorlog",
     "pyyaml",
     "jax>=0.3.13",
     "jaxlib",
     "torch",
-    "pyssht",
-    "healpy",
-    "ducc0",
 ]
 dynamic = [
     "version",
@@ -71,9 +68,13 @@ plotting = [
     "ipywidgets",
 ]
 tests = [
+    "ducc0",
+    "healpy",
+    "numpy<2",  # Required currently due to lack of Numpy v2 compatible pyssht release
     "pytest",
     "pytest-cov",
     "so3",
+    "pyssht",
 ]
 
 [tool.scikit-build]

s2fft/transforms/c_backend_spherical.py

@@ -1,18 +1,33 @@
+"""C backend functions for which to provide JAX frontends."""
+
+import importlib
 from functools import partial
+from types import ModuleType
 
-import healpy
 import jax.numpy as jnp
 import numpy as np
-
-# C backend functions for which to provide JAX frontend.
-import pyssht
 from jax import core, custom_vjp
 from jax.interpreters import ad
 
 from s2fft.sampling import reindex
 from s2fft.utils import iterative_refinement, quadrature_jax
 
 
+class MissingWrapperDependencyError(Exception):
+    """Exception raised when a dependency for a wrapper function is missing."""
+
+
+def _try_import_module(module_name: str) -> ModuleType:
+    """Try to import a named module which may not be installed."""
+    try:
+        module = importlib.import_module(module_name)
+    except ImportError as e:
+        raise MissingWrapperDependencyError(
+            "Wrapper function requires {module_name} to be installed"
+        ) from e
+    return module
+
+
 @custom_vjp
 def ssht_inverse(
     flm: jnp.ndarray,
@@ -30,6 +45,8 @@ def ssht_inverse(
     custom JAX frontends, hence providing support for automatic differentiation. Currently
     these transforms can only be deployed on CPU, which is a limitation of the SSHT C package.
 
+    Requires `pyssht` package to be installed.
+
     Args:
         flm (jnp.ndarray): Spherical harmonic coefficients.
 
@@ -56,6 +73,7 @@ def ssht_inverse(
             IEEE Transactions on Signal Processing 59 (2011): 5876-5887.
 
     """
+    pyssht = _try_import_module("pyssht")
     sampling_str = ["MW", "MWSS", "DH", "GL"]
     flm_1d = reindex.flm_2d_to_1d_fast(flm, L)
     _backend = "SSHT" if _ssht_backend == 0 else "ducc0"
@@ -86,6 +104,7 @@ def _ssht_inverse_fwd(
 
 def _ssht_inverse_bwd(res, f):
     """Private function which implements the backward pass for inverse jax_ssht."""
+    pyssht = _try_import_module("pyssht")
     _, L, spin, reality, ssht_sampling, _ssht_backend = res
     sampling_str = ["MW", "MWSS", "DH", "GL"]
     _backend = "SSHT" if _ssht_backend == 0 else "ducc0"
@@ -149,6 +168,8 @@ def ssht_forward(
     custom JAX frontends, hence providing support for automatic differentiation. Currently
     these transforms can only be deployed on CPU, which is a limitation of the SSHT C package.
 
+    Requires `pyssht` package to be installed.
+
     Args:
         f (jnp.ndarray): Signal on the sphere.
 
@@ -175,6 +196,7 @@ def ssht_forward(
             IEEE Transactions on Signal Processing 59 (2011): 5876-5887.
 
     """
+    pyssht = _try_import_module("pyssht")
     sampling_str = ["MW", "MWSS", "DH", "GL"]
     _backend = "SSHT" if _ssht_backend == 0 else "ducc0"
     flm = jnp.array(
@@ -205,6 +227,7 @@ def _ssht_forward_fwd(
 
 def _ssht_forward_bwd(res, flm):
     """Private function which implements the backward pass for forward jax_ssht."""
+    pyssht = _try_import_module("pyssht")
     _, L, spin, reality, ssht_sampling, _ssht_backend = res
     sampling_str = ["MW", "MWSS", "DH", "GL"]
     _backend = "SSHT" if _ssht_backend == 0 else "ducc0"
@@ -300,6 +323,7 @@ def _real_dtype(complex_dtype):
 
 
 def _healpy_map2alm_impl(f: jnp.ndarray, L: int, nside: int) -> jnp.ndarray:
+    healpy = _try_import_module("healpy")
     return jnp.array(healpy.map2alm(np.array(f), lmax=L - 1, iter=0))
 
 
@@ -332,6 +356,8 @@ def healpy_map2alm(f: jnp.ndarray, L: int, nside: int) -> jnp.ndarray:
     array using HEALPix (ring-ordered) indexing. To instead return a two-dimensional
     array of harmonic coefficients use :py:func:`healpy_forward`.
 
+    Requires `healpy` package to be installed.
+
     Args:
         f (jnp.ndarray): Signal on the sphere.
 
@@ -347,6 +373,7 @@ def healpy_map2alm(f: jnp.ndarray, L: int, nside: int) -> jnp.ndarray:
 
 
 def _healpy_alm2map_impl(flm: jnp.ndarray, L: int, nside: int) -> jnp.ndarray:
+    healpy = _try_import_module("healpy")
     return jnp.array(healpy.alm2map(np.array(flm), lmax=L - 1, nside=nside))
 
 
@@ -384,6 +411,8 @@ def healpy_alm2map(flm: jnp.ndarray, L: int, nside: int) -> jnp.ndarray:
     dimensional array using HEALPix (ring-ordered) indexing. To instead pass a
     two-dimensional array of harmonic coefficients use :py:func:`healpy_inverse`.
 
+    Requires `healpy` package to be installed.
+
     Args:
         flm (jnp.ndarray): Spherical harmonic coefficients.
 
@@ -408,6 +437,8 @@ def healpy_forward(f: jnp.ndarray, L: int, nside: int, iter: int = 3) -> jnp.nda
     Currently these transforms can only be deployed on CPU, which is a limitation of the
     C++ library.
 
+    Requires `healpy` package to be installed.
+
     Args:
         f (jnp.ndarray): Signal on the sphere.
 
@@ -448,6 +479,8 @@ def healpy_inverse(flm: jnp.ndarray, L: int, nside: int) -> jnp.ndarray:
     Currently these transforms can only be deployed on CPU, which is a limitation of the
     C++ library.
 
+    Requires `healpy` package to be installed.
+
     Args:
         flm (jnp.ndarray): Spherical harmonic coefficients.
 

tests/test_utils.py

@@ -6,6 +6,10 @@
 from jax.test_util import check_grads
 
 from s2fft.sampling import s2_samples as samples
+from s2fft.transforms.c_backend_spherical import (
+    MissingWrapperDependencyError,
+    _try_import_module,
+)
 from s2fft.utils.rotation import generate_rotate_dls, rotate_flms
 
 jax.config.update("jax_enable_x64", True)
@@ -120,3 +124,12 @@ def func(flm):
         return jnp.sum(jnp.abs(flm_rot - flm_target))
 
     check_grads(func, (flm_start,), order=1, modes=("rev"))
+
+
+def test_try_import_module():
+    # Use an intentionally long and unlikely to clash module name
+    module_name = "_a_random_module_name_that_should_not_exist"
+    with pytest.raises(
+        MissingWrapperDependencyError, match="requires {module_name} to be installed"
+    ):
+        _try_import_module(module_name)