Defer scipy imports so import spatialmath is faster (#244)

Part of #234 (the SciPy half; SymPy is not changed here).

`spatialmath/base/quaternions.py` and `spatialmath/spline.py` imported
`scipy.interpolate` / `scipy.spatial.transform` at module level, so
`import spatialmath` pulled in scipy even though only `qrand` (with a
theta range), `SplineSE3` and `BSplineSE3` use it. These imports now
happen inside those functions. The `interpolate.interp1d` return
annotation stays as a string under `TYPE_CHECKING`.

Measured with `python -X importtime -c "import spatialmath"` on one
machine, single run each: 1.42 s before, 0.40 s after. `scipy` is no
longer in `sys.modules` after `import spatialmath`. `sympy` still is, so
the remaining part of #234 (`issymbol` / `argcheck`) is untouched.

Added `tests/test_import.py`, which imports spatialmath in a subprocess
and checks that no scipy module is loaded. It fails on master and passes
here.

Run: full `tests/` suite with `MPLBACKEND=Agg`, 348 passed, 3 skipped;
black 23.10.0 clean on the changed files; `qrand(theta_range=...)` and
importing `SplineSE3`/`BSplineSE3` smoke-checked. Not run: the spline
classes were not exercised by hand beyond what the existing tests cover,
and only Python 3 on Linux was tried.

Author: RoboX2020

spatialmath/base/quaternions.py

@@ -16,11 +16,13 @@
 import spatialmath.base as smb
 from spatialmath.base.argcheck import getunit
 from spatialmath.base.types import *
-import scipy.interpolate as interpolate
-from typing import Optional
+from typing import TYPE_CHECKING, Optional
 from functools import lru_cache
 import warnings
 
+if TYPE_CHECKING:
+    import scipy.interpolate as interpolate
+
 _eps = np.finfo(np.float64).eps
 
 
@@ -920,7 +922,7 @@ def _compute_cdf_sin_squared(theta: float):
 @lru_cache(maxsize=1)
 def _generate_inv_cdf_sin_squared_interp(
     num_interpolation_points: int = 256,
-) -> interpolate.interp1d:
+) -> "interpolate.interp1d":
     """
     Computes an interpolation function for the inverse CDF of the distribution of angular magnitude.
 
@@ -933,6 +935,8 @@ def _generate_inv_cdf_sin_squared_interp(
     This function returns interpolation function for the inverse of the integral of the
     pdf of angular magnitudes (2/pi * sin^2(theta/2)), which is not analytically defined.
     """
+    import scipy.interpolate as interpolate  # deferred, it is slow to import
+
     cdf_sin_squared_interp_angles = np.linspace(0, np.pi, num_interpolation_points)
     cdf_sin_squared_interp_values = _compute_cdf_sin_squared(
         cdf_sin_squared_interp_angles

spatialmath/spline.py

@@ -11,8 +11,6 @@
 from typing import TYPE_CHECKING, List, Optional, Tuple
 
 import numpy as np
-from scipy.interpolate import BSpline, CubicSpline
-from scipy.spatial.transform import Rotation, RotationSpline
 
 from spatialmath import SE3, SO3, Twist3
 from spatialmath.base.transforms3d import tranimate
@@ -122,6 +120,9 @@ def __init__(
             self.timepoints = self.timepoints - self.timepoints[0]
             self.timepoints = self.timepoints / self.timepoints[-1]
 
+        from scipy.interpolate import CubicSpline  # deferred, scipy is slow to import
+        from scipy.spatial.transform import Rotation, RotationSpline
+
         self.spline_xyz = CubicSpline(
             self.timepoints,
             np.array([pose.t for pose in self.control_poses]),
@@ -292,6 +293,8 @@ def __init__(
             )  # ensures the curve ends on the last control pose
         self.knots = knots
 
+        from scipy.interpolate import BSpline  # deferred, scipy is slow to import
+
         self.splines = [
             BSpline(knots, self.control_pose_matrix[:, i], degree)
             for i in range(0, 6)  # twists are length 6

tests/test_import.py

@@ -0,0 +1,16 @@
+import subprocess
+import sys
+import unittest
+
+
+class TestImport(unittest.TestCase):
+    def test_import_does_not_load_scipy(self):
+        # scipy is slow to import and only a few functions need it, so it is
+        # imported by those functions and not when spatialmath is imported
+        code = "import sys, spatialmath; sys.exit(any(m.split('.')[0] == 'scipy' for m in sys.modules))"
+        result = subprocess.run([sys.executable, "-c", code])
+        self.assertEqual(result.returncode, 0, "import spatialmath loaded scipy")
+
+
+if __name__ == "__main__":  # pragma nocover
+    unittest.main()