Defer/fix some minor import issues

docs/releasehistory.md

@@ -13,6 +13,7 @@ Releases follow the `major.minor.micro` scheme recommended by [PEP440](https://w
 ### Behavior changes
 
 - [PR #1954](https://github.com/openforcefield/openff-toolkit/pull/1954): `Topology.from_openmm` no longer casts residue numbers to int.
+- [PR #1959](https://github.com/openforcefield/openff-toolkit/pull/1959): Speeds up import times.
 
 ### Bugfixes
 

openff/toolkit/topology/topology.py

@@ -31,7 +31,6 @@
 
 import numpy as np
 from numpy.typing import NDArray
-from openff.units import ensure_quantity
 from typing_extensions import TypeAlias
 
 from openff.toolkit import Quantity, unit
@@ -618,6 +617,8 @@ def box_vectors(self, box_vectors):
             return
         if not hasattr(box_vectors, "units"):
             if hasattr(box_vectors, "unit"):
+                from openff.units import ensure_quantity
+
                 # this is probably an openmm.unit.Quantity; we should gracefully import OpenMM but
                 # the chances of this being an object with the two previous conditions met is low
                 box_vectors = ensure_quantity(box_vectors, "openff")
@@ -1538,6 +1539,8 @@ def from_openmm(
             topology.box_vectors = from_openmm(openmm_topology.getPeriodicBoxVectors())
 
         if positions is not None:
+            from openff.units import ensure_quantity
+
             topology.set_positions(ensure_quantity(positions, "openff"))
 
         # TODO: How can we preserve metadata from the openMM topology when creating the OFF topology?

openff/toolkit/utils/_viz.py

@@ -1,15 +1,16 @@
 import uuid
 from io import StringIO
+from typing import TYPE_CHECKING
 
 from nglview.base_adaptor import Structure, Trajectory
 
-from openff.toolkit import Molecule, Topology, unit
+if TYPE_CHECKING:
+    from openff.toolkit import Molecule, Topology
 
 MOLECULE_DEFAULT_REPS = [
     dict(type="licorice", params=dict(radius=0.25, multipleBond=True))
 ]
 
-
 class MoleculeNGLViewTrajectory(Structure, Trajectory):
     """
     OpenFF Molecule adaptor.
@@ -40,7 +41,7 @@ class MoleculeNGLViewTrajectory(Structure, Trajectory):
 
     def __init__(
         self,
-        molecule: Molecule,
+        molecule: "Molecule",
         ext: str = "MOL2",
     ):
         if not molecule.conformers:
@@ -53,7 +54,7 @@ def __init__(
         self.id = str(uuid.uuid4())
 
     def get_coordinates(self, index: int = 0):
-        return self.molecule.conformers[index].m_as(unit.angstrom)
+        return self.molecule.conformers[index].m_as("angstrom")
 
     @property
     def n_frames(self):
@@ -91,7 +92,7 @@ class TopologyNGLViewStructure(Structure):
 
     def __init__(
         self,
-        topology: Topology,
+        topology: "Topology",
         ext: str = "PDB",
     ):
         self.topology = topology

openff/toolkit/utils/ambertools_wrapper.py

@@ -11,7 +11,6 @@
 from typing import TYPE_CHECKING, Optional
 
 import numpy as np
-from openff.utilities.provenance import get_ambertools_version
 
 from openff.toolkit import Quantity, unit
 from openff.toolkit.utils import base_wrapper, rdkit_wrapper
@@ -62,6 +61,8 @@ class AmberToolsToolkitWrapper(base_wrapper.ToolkitWrapper):
     SUPPORTED_CHARGE_METHODS = _supported_charge_methods
 
     def __init__(self):
+        from openff.utilities.provenance import get_ambertools_version
+
         super().__init__()
 
         if not self.is_available():

openff/toolkit/utils/rdkit_wrapper.py

@@ -18,6 +18,7 @@
 
 import numpy as np
 from cachetools import LRUCache, cached
+from numpy.typing import NDArray
 from openff.units.elements import SYMBOLS
 
 from openff.toolkit import Quantity, unit
@@ -1985,7 +1986,7 @@ def _elf_compute_electrostatic_energy(
         return 0.5 * interaction_energies.sum()
 
     @classmethod
-    def _elf_compute_rms_matrix(cls, molecule: "Molecule") -> np.ndarray:
+    def _elf_compute_rms_matrix(cls, molecule: "Molecule") -> NDArray:
         """Computes the symmetric RMS matrix of all conformers in a molecule taking
         only heavy atoms into account.
 

openff/toolkit/utils/utils.py

@@ -28,9 +28,9 @@
 import logging
 from typing import TYPE_CHECKING, Any, Iterable, TypeVar, Union, overload
 
-import numpy
 import numpy as np
 import pint
+from numpy.typing import NDArray
 from openff.units import Quantity, Unit, unit
 from openff.utilities import requires_package
 
@@ -397,8 +397,6 @@ def serialize_numpy(np_array) -> tuple[bytes, tuple[int]]:
     shape
         The shape of the serialized array
     """
-    import numpy as np
-
     bigendian_float = np.dtype(float).newbyteorder(">")
     bigendian_array = np_array.astype(bigendian_float)
     serialized = bigendian_array.tobytes()
@@ -409,7 +407,7 @@ def serialize_numpy(np_array) -> tuple[bytes, tuple[int]]:
 def deserialize_numpy(
     serialized_np: Union[bytes, list],
     shape: tuple[int, ...],
-) -> numpy.ndarray:
+) -> NDArray:
     """
     Deserializes a numpy array from a bytestring or list. The input, if a bytestring, is
     assumed to be in big-endian byte order.
@@ -426,9 +424,6 @@ def deserialize_numpy(
     np_array
         The deserialized numpy array
     """
-
-    import numpy as np
-
     if isinstance(serialized_np, list):
         np_array = np.array(serialized_np)
     if isinstance(serialized_np, bytes):