Fix dependencies

pyproject.toml

@@ -30,6 +30,7 @@ classifiers = [
 requires-python = ">=3.8"
 dependencies = [
     "numpy>=1.0",
+    "packaging",
     "spglib>=1.14.1",
 ]
 license = {text = "The MIT license"}

seekpath/__init__.py

@@ -45,7 +45,19 @@ class SupercellWarning(UserWarning):
 )
 
 from .hpkot import EdgeCaseWarning, SymmetryDetectionError
-from .brillouinzone import brillouinzone
+
+
+def __getattr__(name):
+    """Import the Brillouin zone subpackage only when it is used.
+
+    It needs the optional ``scipy`` dependency, so importing it eagerly would
+    make ``import seekpath`` fail without it.
+    """
+    if name == 'brillouinzone':
+        import importlib
+
+        return importlib.import_module(f'{__name__}.brillouinzone')
+    raise AttributeError(f'module {__name__!r} has no attribute {name!r}')
 
 
 __all__ = (

seekpath/brillouinzone/__init__.py

@@ -0,0 +1,9 @@
+"""Compute the Brillouin zone of a crystal.
+
+This subpackage needs the optional ``scipy`` dependency, installed with
+``pip install seekpath[bz]``.
+"""
+
+from .brillouinzone import BZ, get_BZ
+
+__all__ = ('BZ', 'get_BZ')

seekpath/brillouinzone/brillouinzone.py

@@ -5,7 +5,14 @@
 from typing import Union
 
 import numpy as np
-from scipy.spatial import Voronoi, ConvexHull, Delaunay
+
+try:
+    from scipy.spatial import ConvexHull, Delaunay, Voronoi
+except ImportError as exc:
+    raise ImportError(
+        'The Brillouin zone module requires scipy, install it with '
+        '`pip install seekpath[bz]`'
+    ) from exc
 
 
 def get_BZ(

tests/test_optional_dependencies.py

@@ -0,0 +1,62 @@
+"""Test that seekpath works without its optional dependencies."""
+
+import subprocess
+import sys
+import unittest
+
+# Make every import of scipy fail, as if it were not installed
+BLOCK_SCIPY = "import sys; sys.modules['scipy'] = None; "
+
+
+def run_python(code):
+    """Run ``code`` in a fresh interpreter, so no module is imported yet."""
+    return subprocess.run(
+        [sys.executable, '-c', code], capture_output=True, text=True, check=False
+    )
+
+
+class TestWithoutScipy(unittest.TestCase):
+    """seekpath only needs scipy for the Brillouin zone subpackage."""
+
+    def test_get_path_without_scipy(self):
+        """``import seekpath`` and ``get_path`` must not need scipy."""
+        result = run_python(
+            BLOCK_SCIPY + 'import seekpath; '
+            'print(seekpath.get_path(([[4.0, 0, 0], [0, 4.0, 0], [0, 0, 4.0]], '
+            "[[0, 0, 0]], [1]))['bravais_lattice'])"
+        )
+        self.assertEqual(result.returncode, 0, result.stderr)
+        self.assertEqual(result.stdout.strip(), 'cP')
+
+    def test_brillouinzone_without_scipy(self):
+        """The Brillouin zone subpackage explains how to install scipy."""
+        result = run_python(BLOCK_SCIPY + 'import seekpath; seekpath.brillouinzone')
+        self.assertNotEqual(result.returncode, 0)
+        self.assertIn('ImportError', result.stderr)
+        self.assertIn('pip install seekpath[bz]', result.stderr)
+
+
+class TestBrillouinzoneAccess(unittest.TestCase):
+    """The Brillouin zone subpackage is reachable in all the supported ways."""
+
+    def test_access_paths_agree(self):
+        """``seekpath.brillouinzone.BZ`` is the same class however it is reached."""
+        result = run_python(
+            'import seekpath; '
+            'from seekpath.brillouinzone import brillouinzone; '
+            'from seekpath.brillouinzone import BZ; '
+            'print(seekpath.brillouinzone.BZ is brillouinzone.BZ is BZ)'
+        )
+        self.assertEqual(result.returncode, 0, result.stderr)
+        self.assertEqual(result.stdout.strip(), 'True')
+
+    def test_unknown_attribute(self):
+        """Other missing attributes still raise ``AttributeError``."""
+        import seekpath
+
+        with self.assertRaises(AttributeError):
+            seekpath.not_an_attribute  # noqa: B018
+
+
+if __name__ == '__main__':
+    unittest.main()