make import lazy for asdf extension

gwcs/__init__.py

@@ -65,8 +65,12 @@
     # package is not installed
     pass  # pragma: no cover
 
-
-from .wcs import *   # noqa
-from .wcstools import *   # noqa
-from .coordinate_frames import *  # noqa
-from .selector import *   # noqa
+def __getattr__(name):
+    for sub_module in ("wcs", "wcstools", "coordinate_frames", "selector"):
+        module = __import__(f"gwcs.{sub_module}", fromlist=['*'])
+        globals().update({name: getattr(module, name) for name in module.__all__})
+
+    try:
+        return globals()[name]
+    except KeyError:
+        raise AttributeError(f"module 'gwcs' has no attribute '{name}'") from None

gwcs/converters/selector.py

@@ -2,12 +2,7 @@
 # -*- coding: utf-8 -*-
 
 from collections import OrderedDict
-import numpy as np
-from astropy.modeling import models
-from astropy.modeling.core import Model
-from astropy.utils.misc import isiterable
 
-from asdf.tags.core.ndarray import NDArrayType
 from asdf_astropy.converters.transform.core import TransformConverterBase
 
 
@@ -20,6 +15,12 @@ class LabelMapperConverter(TransformConverterBase):
              "gwcs.selector.LabelMapperRange", "gwcs.selector.LabelMapper"]
 
     def from_yaml_tree_transform(self, node, tag, ctx):
+        from asdf.tags.core.ndarray import NDArrayType
+        from astropy.modeling import models
+        from astropy.modeling.core import Model
+        from astropy.utils.misc import isiterable
+        import numpy as np
+
         from ..selector import (LabelMapperArray, LabelMapperDict,
                                 LabelMapperRange, LabelMapper)
         inputs_mapping = node.get('inputs_mapping', None)
@@ -52,6 +53,7 @@ def from_yaml_tree_transform(self, node, tag, ctx):
                 return LabelMapperDict(inputs, dict_mapper, inputs_mapping, atol=atol)
 
     def to_yaml_tree_transform(self, model, tag, ctx):
+        from astropy.utils.misc import isiterable
         from ..selector import (LabelMapperArray, LabelMapperDict,
                                 LabelMapperRange, LabelMapper)
         node = OrderedDict()
@@ -91,6 +93,7 @@ class RegionsSelectorConverter(TransformConverterBase):
 
     def from_yaml_tree_transform(self, node, tag, ctx):
         from ..selector import RegionsSelector
+
         inputs = node['inputs']
         outputs = node['outputs']
         label_mapper = node['label_mapper']

gwcs/converters/spectroscopy.py

@@ -2,7 +2,6 @@
 ASDF tags for spectroscopy related models.
 
 """
-from astropy import units as u
 from asdf_astropy.converters.transform.core import (
     TransformConverterBase, parameter_to_value
 )
@@ -84,6 +83,7 @@ def from_yaml_tree_transform(self, node, tag, ctx):
         return model
 
     def to_yaml_tree_transform(self, model, tag, ctx):
+        from astropy import units as u
         from ..spectroscopy import (AnglesFromGratingEquation3D,
                                     WavelengthFromGratingEquation)
         if model.groove_density.unit is not None: