Fix astra import

hotopy.tomo sub-module can now be imported with reduced function set if ASTRA toolbox is not available on the given Python installation.

src/hotopy/tomo/__init__.py

@@ -38,43 +38,67 @@ Consistency checks
     hlcc
 """
 
+from logging import getLogger
+
 from ._alignment import hlcc, find_sample_rotaxis_shift, sample_rotaxis_shift_correlation
-from ._astra import (
-    AstraWrapper,
-    AstraWrapper2D,
-    AstraWrapper3D,
-    iradon_parallel,
-    fdk,
-    fanflat,
-    parallel3d,
-    sirt3d,
-    geom_postalignment,
-    geom_rotate_detector,
-    geom_2vec,
-    reproj_alignment,
-)
 from ._ringremove import ringremove, ringremove_wavelet, ringremove_additive
 
+
+logger = getLogger(__name__)
+
+# only load astra-depended methods if astra is available
+try:
+    from ._astra import (
+        AstraWrapper,
+        AstraWrapper2D,
+        AstraWrapper3D,
+        iradon_parallel,
+        fdk,
+        fanflat,
+        parallel3d,
+        sirt3d,
+        geom_postalignment,
+        geom_rotate_detector,
+        geom_2vec,
+        reproj_alignment,
+    )
+
+    __has_astra__ = True
+    logger.info(f"Loaded {__name__} with ASTRA sub-module")
+except ModuleNotFoundError as err:
+    __has_astra__ = False
+    logger.warning(
+        f"Could not import 'astra' package for tomographic operations. See https://astra-toolbox.com for installation "
+        f"guidance."
+        f"Loaded {__name__} without ASTRA sub-module. ASTRA-depended functions are not available."
+    )
+    logger.debug(err)
+
+
 __all__ = [
     # alignment
     "hlcc",
     "find_sample_rotaxis_shift",
     "sample_rotaxis_shift_correlation",
-    # astra
-    "AstraWrapper",
-    "AstraWrapper2D",
-    "AstraWrapper3D",
-    "iradon_parallel",
-    "fdk",
-    "fanflat",
-    "parallel3d",
-    "sirt3d",
-    "geom_postalignment",
-    "geom_rotate_detector",
-    "geom_2vec",
-    "reproj_alignment",
     # ringremove
     "ringremove",
     "ringremove_additive",
     "ringremove_wavelet",
 ]
+
+if __has_astra__:
+    __all__ += [
+        # astra
+        "AstraWrapper",
+        "AstraWrapper2D",
+        "AstraWrapper3D",
+        "iradon_parallel",
+        "fdk",
+        "fanflat",
+        "parallel3d",
+        "sirt3d",
+        "geom_postalignment",
+        "geom_rotate_detector",
+        "geom_2vec",
+        "reproj_alignment",
+    ]